Linking DRM Connectors to Backlight Devices

Use static initializers instead of setting up global variables during runtime. This reduces code size and execution time.

Signed-off-by David Herrmann <[email protected]>

---
 drivers/video/backlight/backlight.c | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

drivers/video/backlight/backlight.c +3 -6

index bddc8b1..726c6c6 100644
@@ -21,9 +21,9 @@
 #include <asm/backlight.h>
 #endif
 
-static struct list_head backlight_dev_list;
-static struct mutex backlight_dev_list_mutex;
-static struct blocking_notifier_head backlight_notifier;
+static LIST_HEAD(backlight_dev_list);
+static DEFINE_MUTEX(backlight_dev_list_mutex);
+static BLOCKING_NOTIFIER_HEAD(backlight_notifier);
 
 static const char *const backlight_types[] = {
 	[BACKLIGHT_RAW] = "raw",

@@ -582,9 +582,6 @@ static int __init backlight_class_init(void)
 
 	backlight_class->dev_groups = bl_device_groups;
 	backlight_class->pm = &backlight_class_dev_pm_ops;
-	INIT_LIST_HEAD(&backlight_dev_list);
-	mutex_init(&backlight_dev_list_mutex);
-	BLOCKING_INIT_NOTIFIER_HEAD(&backlight_notifier);
 
 	return 0;
 }

Signature · 6 lines

2.1.0

_______________________________________________
dri-devel mailing list
[email protected]
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There is really no reason to use a mutex to protect a simple list. Convert the list-lock to a simple spinlock instead.

The spin-locks prepare for a backlight_find() helper, which should preferably be usable from atomic context. A mutex would prevent that, so use an irq-save spinlock instead.

Signed-off-by David Herrmann <[email protected]>

---
 drivers/video/backlight/backlight.c | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

drivers/video/backlight/backlight.c +9 -7

index 726c6c6..33b64be 100644
@@ -16,13 +16,14 @@
 #include <linux/err.h>
 #include <linux/fb.h>
 #include <linux/slab.h>
+#include <linux/spinlock.h>
 
 #ifdef CONFIG_PMAC_BACKLIGHT
 #include <asm/backlight.h>
 #endif
 
 static LIST_HEAD(backlight_dev_list);
-static DEFINE_MUTEX(backlight_dev_list_mutex);
+static DEFINE_SPINLOCK(backlight_dev_list_lock);
 static BLOCKING_NOTIFIER_HEAD(backlight_notifier);
 
 static const char *const backlight_types[] = {

@@ -369,9 +370,9 @@ struct backlight_device *backlight_device_register(const char *name,
 	mutex_unlock(&pmac_backlight_mutex);
 #endif
 
-	mutex_lock(&backlight_dev_list_mutex);
+	spin_lock_irq(&backlight_dev_list_lock);
 	list_add(&new_bd->entry, &backlight_dev_list);
-	mutex_unlock(&backlight_dev_list_mutex);
+	spin_unlock_irq(&backlight_dev_list_lock);
 
 	blocking_notifier_call_chain(&backlight_notifier,
 				     BACKLIGHT_REGISTERED, new_bd);

@@ -384,15 +385,16 @@ bool backlight_device_registered(enum backlight_type type)
 {
 	bool found = false;
 	struct backlight_device *bd;
+	unsigned long flags;
 
-	mutex_lock(&backlight_dev_list_mutex);
+	spin_lock_irqsave(&backlight_dev_list_lock, flags);
 	list_for_each_entry(bd, &backlight_dev_list, entry) {
 		if (bd->props.type == type) {
 			found = true;
 			break;
 		}
 	}
-	mutex_unlock(&backlight_dev_list_mutex);
+	spin_unlock_irqrestore(&backlight_dev_list_lock, flags);
 
 	return found;
 }

@@ -409,9 +411,9 @@ void backlight_device_unregister(struct backlight_device *bd)
 	if (!bd)
 		return;
 
-	mutex_lock(&backlight_dev_list_mutex);
+	spin_lock_irq(&backlight_dev_list_lock);
 	list_del(&bd->entry);
-	mutex_unlock(&backlight_dev_list_mutex);
+	spin_unlock_irq(&backlight_dev_list_lock);
 
 #ifdef CONFIG_PMAC_BACKLIGHT
 	mutex_lock(&pmac_backlight_mutex);

Signature · 6 lines

2.1.0

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include/linux/backlight.h +0 -0

index adb14a8..bcc0dec 100644
@@ -141,6 +141,22 @@ extern bool backlight_device_registered(enum backlight_type type);
 extern int backlight_register_notifier(struct notifier_block *nb);
 extern int backlight_unregister_notifier(struct notifier_block *nb);

On Wed, Sep 10, 2014 at 05:54:22PM +0200, David Herrmann wrote:
[...]

drivers/video/backlight/backlight.c int backlight_unregister_notifier(struct notifier_block *nb) · line 537

Quoted 6 lines

drivers/video/backlight/backlight.c +6 -0

+void backlight_set_brightness(struct backlight_device *bd, unsigned int value,
+                           enum backlight_update_reason reason)
+{
+     mutex_lock(&bd->ops_lock);
+     if (bd->ops) {
+             value = clamp(value, 0U, (unsigned)bd->props.max_brightness);

max_brightness should really be unsigned to begin with...

Quoted 1 line

+ pr_debug("set brightness to %u\n", value);

dev_dbg(&bd->dev, ...)?

Quoted 7 lines

include/linux/backlight.h +0 -0

index adb14a8..bcc0dec 100644
@@ -141,6 +141,22 @@ extern bool backlight_device_registered(enum backlight_type type);
 extern int backlight_register_notifier(struct notifier_block *nb);
 extern int backlight_unregister_notifier(struct notifier_block *nb);

include/linux/backlight.h extern bool backlight_device_registered(enum backlight_type type); · line 152

Quoted 10 lines

include/linux/backlight.h +9 -0

 
+struct backlight_device *backlight_device_lookup(const char *name);
+void backlight_set_brightness(struct backlight_device *bd, unsigned int value,
+                           enum backlight_update_reason reason);
+
+static inline void backlight_device_ref(struct backlight_device *bd)
+{
+     if (bd)
+             get_device(&bd->dev);
+}

Perhaps for consistency with get_device() this should return bd? That way you can chain things like so:

        priv->backlight = backlight_device_ref(bd);

Hi

Quoted 11 lines · Thierry Reding

On Wed, Sep 10, 2014 at 05:54:22PM +0200, David Herrmann wrote:
[...]

drivers/video/backlight/backlight.c int backlight_unregister_notifier(struct notifier_block *nb) · line 537

Quoted 6 lines

drivers/video/backlight/backlight.c +6 -0

+void backlight_set_brightness(struct backlight_device *bd, unsigned int value,
+                           enum backlight_update_reason reason)
+{
+     mutex_lock(&bd->ops_lock);
+     if (bd->ops) {
+             value = clamp(value, 0U, (unsigned)bd->props.max_brightness);

max_brightness should really be unsigned to begin with...

Quoted 3 lines

Quoted 1 line

+ pr_debug("set brightness to %u\n", value);

dev_dbg(&bd->dev, ...)?

I agree with both comments, but I tried to be consistent with what brightness_store() does.

Quoted 21 lines

Quoted 7 lines

include/linux/backlight.h +0 -0

index adb14a8..bcc0dec 100644
@@ -141,6 +141,22 @@ extern bool backlight_device_registered(enum backlight_type type);
 extern int backlight_register_notifier(struct notifier_block *nb);
 extern int backlight_unregister_notifier(struct notifier_block *nb);

include/linux/backlight.h extern bool backlight_device_registered(enum backlight_type type); · line 152

Quoted 10 lines

include/linux/backlight.h +9 -0

 
+struct backlight_device *backlight_device_lookup(const char *name);
+void backlight_set_brightness(struct backlight_device *bd, unsigned int value,
+                           enum backlight_update_reason reason);
+
+static inline void backlight_device_ref(struct backlight_device *bd)
+{
+     if (bd)
+             get_device(&bd->dev);
+}

Perhaps for consistency with get_device() this should return bd? That way you can chain things like so:

Quoted 1 line

        priv->backlight = backlight_device_ref(bd);

Makes sense, will change it. Same is actually true for _unref(), which should return NULL unconditionally. This way, you can use:
priv->backlight = backlight_device_unref(priv->backlight);
to release a reference and reset the pointer at the same time.

Backlight devices have always been managed independently of display controllers. They're often controlled via different hardware interfaces and their relationship to display-controllers varies vastly between different boards. However, display brightness is obviously a property of a display, and thus of a DRM connector. Therefore, it'd be really appreciated if user-space APIs would highlight this relationship.

The main runtime users of backlight interfaces are user-space compositors. But currently they have to jump through hoops to find the correct backlight device for a given connector. Furthermore, they need root privileges to write to sysfs. sysfs has never been designed as run-time non-root API. It does not provide file-contexts, run-time management or any kind of API control. There is no way to control access to sysfs via different links (in that case: mounts). Char-devs provide all this!

So far, backlight APIs have been fairly trivial, so adding char-devs to backlights is rather heavy-weight. Therefore, this patch introduces a new API interface to modify backlight brightness via DRM: A "BRIGHTNESS" property on DRM connectors.

Instead of adding backlight hardware support to DRM, we rely on the backlight-class and simply add a new API. Each DRM Connector can optionally be linked to a backlight class device. Modifying the connector property will have the same effect as writing into the "brightness" sysfs file of the linked backlight class device. However, we now can manage access to backlight devices via the same interface as access to mode-setting on the underlying display. Furthermore, the connection between displays and their backlight devices are visible in user-space.

Obviously, matching backlights to displays cannot be solved magically with this link. Therefore, we also add a user-space attribute to DRM connectors called 'backlight'. If a DRM driver is incapable of matching existing backlights to a connector, or if a given board has just crappy backlight drivers, udev can write the name of a backlight-device into this attribute and the connector-property will be re-linked to this backlight device. The udev hwdb can be easily employed to track such quirks and fixups for different board+GPU combinations.
Note that the name written into the 'backlight' attribute is saved on the connector, so in case the real backlight device is probed after the DRM card, the backlight will still get properly attached once probed.

Signed-off-by David Herrmann <[email protected]>

---
 drivers/gpu/drm/Kconfig             |   1 +
 drivers/gpu/drm/Makefile            |   2 +-
 drivers/gpu/drm/drm_backlight.c     | 387 ++++++++++++++++++++++++++++++++++++
 drivers/gpu/drm/drm_crtc.c          |  45 +++--
 drivers/gpu/drm/drm_drv.c           |  11 +
 drivers/gpu/drm/drm_sysfs.c         |  53 +++++
 drivers/video/backlight/backlight.c |   3 +
 include/drm/drm_backlight.h         |  44 ++++
 include/drm/drm_crtc.h              |   3 +
 include/linux/backlight.h           |   1 +
 10 files changed, 530 insertions(+), 20 deletions(-)
 create mode 100644 drivers/gpu/drm/drm_backlight.c
 create mode 100644 include/drm/drm_backlight.h

drivers/gpu/drm/Kconfig +1 -0

index e3b4b0f..46bca34 100644
@@ -12,6 +12,7 @@ menuconfig DRM
 	select I2C
 	select I2C_ALGOBIT
 	select DMA_SHARED_BUFFER
+	select BACKLIGHT_CLASS_DEVICE
 	help
 	  Kernel-level support for the Direct Rendering Infrastructure (DRI)
 	  introduced in XFree86 4.0. If you say Y here, you need to select

drivers/gpu/drm/Makefile +1 -1

index 9292a76..224544d 100644
@@ -14,7 +14,7 @@ drm-y       :=	drm_auth.o drm_bufs.o drm_cache.o \
 		drm_info.o drm_debugfs.o drm_encoder_slave.o \
 		drm_trace_points.o drm_global.o drm_prime.o \
 		drm_rect.o drm_vma_manager.o drm_flip_work.o \
-		drm_modeset_lock.o
+		drm_modeset_lock.o drm_backlight.o
 
 drm-$(CONFIG_COMPAT) += drm_ioc32.o
 drm-$(CONFIG_DRM_GEM_CMA_HELPER) += drm_gem_cma_helper.o

drivers/gpu/drm/drm_backlight.c new file +387 -0

new file mode 100644
index 0000000..92d231a
@@ -0,0 +1,387 @@
+/*
+ * DRM Backlight Helpers
+ * Copyright (c) 2014 David Herrmann
+ */
+
+#include <linux/backlight.h>
+#include <linux/fs.h>
+#include <linux/list.h>
+#include <linux/math64.h>
+#include <linux/module.h>
+#include <linux/mutex.h>
+#include <linux/notifier.h>
+#include <linux/slab.h>
+#include <linux/spinlock.h>
+#include <drm/drmP.h>
+#include <drm/drm_backlight.h>
+
+/**
+ * DOC: Backlight Devices
+ *
+ * Backlight devices have always been managed as a separate subsystem,
+ * independent of DRM. They are usually controlled via separate hardware
+ * interfaces than the display controller, so the split works out fine.
+ * However, backlight brightness is a property of a display, and thus a
+ * property of a DRM connector. We already manage DPMS states via connector
+ * properties, so it is natural to keep brightness control at the same place.
+ *
+ * This DRM backlight interface implements generic backlight properties on
+ * connectors. It does not handle any hardware backends but simply forwards
+ * the requests to an available and linked backlight device. The links between
+ * connectors and backlight devices have to be established by DRM drivers and
+ * can be modified by user-space via sysfs (and udev rules). The name of the
+ * backlight device can be written to a sysfs attribute called 'backlight'.
+ * The device is looked up and linked to the connector (replacing a possible
+ * previous backlight device). A 'change' uevent is sent whenever a link is
+ * modified.
+ *
+ * Drivers have to call drm_backlight_alloc() after allocating a connector via
+ * drm_connector_init(). This will automatically add a backlight device to the
+ * given connector. No hardware device is linked to the connector by default.
+ * Drivers can set up a default device via drm_backlight_set_name(), but are
+ * free to leave it empty. User-space will then have to set up the link.
+ */
+
+struct drm_backlight {
+	struct list_head list;
+	struct drm_connector *connector;
+	char *link_name;
+	struct backlight_device *link;
+	struct work_struct work;
+	unsigned int set_value;
+	bool changed : 1;
+};
+
+static LIST_HEAD(drm_backlight_list);
+static DEFINE_SPINLOCK(drm_backlight_lock);
+
+/* caller must hold @drm_backlight_lock */
+static bool __drm_backlight_is_registered(struct drm_backlight *b)
+{
+	/* a device is live if it is linked to @drm_backlight_list */
+	return !list_empty(&b->list);
+}
+
+/* caller must hold @drm_backlight_lock */
+static void __drm_backlight_schedule(struct drm_backlight *b)
+{
+	if (__drm_backlight_is_registered(b))
+		schedule_work(&b->work);
+}
+
+static void __drm_backlight_worker(struct work_struct *w)
+{
+	struct drm_backlight *b = container_of(w, struct drm_backlight, work);
+	static char *ep[] = { "BACKLIGHT=1", NULL };
+	struct backlight_device *bd;
+	bool send_uevent;
+	unsigned int v;
+
+	spin_lock(&drm_backlight_lock);
+	send_uevent = b->changed;
+	b->changed = false;
+	v = b->set_value;
+	bd = b->link;
+	backlight_device_ref(bd);
+	spin_unlock(&drm_backlight_lock);
+
+	if (bd) {
+		backlight_set_brightness(bd, v, BACKLIGHT_UPDATE_DRM);
+		backlight_device_unref(bd);
+	}
+
+	if (send_uevent)
+		kobject_uevent_env(&b->connector->kdev->kobj, KOBJ_CHANGE, ep);
+}
+
+/* caller must hold @drm_backlight_lock */
+static void __drm_backlight_prop_changed(struct drm_backlight *b, uint64_t v)
+{
+	uint64_t max;
+
+	if (!b->link)
+		return;
+
+	max = b->link->props.max_brightness;
+	if (v >= U16_MAX)
+		b->set_value = max;
+	else
+		b->set_value = (v * max) >> 16;
+	__drm_backlight_schedule(b);
+}
+
+/* caller must hold @drm_backlight_lock */
+static void __drm_backlight_real_changed(struct drm_backlight *b, uint64_t v)
+{
+	struct drm_mode_config *config = &b->connector->dev->mode_config;
+	unsigned int max, set;
+
+	if (!b->link)
+		return;
+
+	set = v;
+	max = b->link->props.max_brightness;
+	if (max < 1)
+		return;
+
+	if (set >= max)
+		set = U16_MAX;
+	else if (max <= U16_MAX)
+		set = (set << 16) / max;
+	else
+		set = div_u64(v << 16, max);
+
+	drm_object_property_set_value(&b->connector->base,
+				      config->brightness_property, v);
+}
+
+/* caller must hold @drm_backlight_lock */
+static void __drm_backlight_link(struct drm_backlight *b,
+				 struct backlight_device *bd)
+{
+	if (bd != b->link) {
+		backlight_device_unref(b->link);
+		b->link = bd;
+		backlight_device_ref(b->link);
+		if (bd)
+			__drm_backlight_real_changed(b, bd->props.brightness);
+		b->changed = true;
+		__drm_backlight_schedule(b);
+	}
+}
+
+/* caller must hold @drm_backlight_lock */
+static void __drm_backlight_lookup(struct drm_backlight *b)
+{
+	struct backlight_device *bd;
+
+	if (b->link_name)
+		bd = backlight_device_lookup(b->link_name);
+	else
+		bd = NULL;
+
+	__drm_backlight_link(b, bd);
+	backlight_device_unref(bd);
+}
+
+/**
+ * drm_backlight_alloc - add backlight capability to a connector
+ * @connector: connector to add backlight to
+ *
+ * This allocates a new DRM-backlight device and links it to @connector. This
+ * *must* be called before registering the connector. The backlight device will
+ * be automatically registered in sync with the connector. It will also get
+ * removed once the connector is removed.
+ *
+ * The connector will not have any hardware backlight linked by default. You
+ * need to call drm_backlight_set_name() if you want to set a default
+ * backlight. User-space can overwrite those via sysfs.
+ *
+ * Returns: 0 on success, negative error code on failure.
+ */
+int drm_backlight_alloc(struct drm_connector *connector)
+{
+	struct drm_mode_config *config = &connector->dev->mode_config;
+	struct drm_backlight *b;
+
+	b = kzalloc(sizeof(*b), GFP_KERNEL);
+	if (!b)
+		return -ENOMEM;
+
+	INIT_LIST_HEAD(&b->list);
+	INIT_WORK(&b->work, __drm_backlight_worker);
+	b->connector = connector;
+	connector->backlight = b;
+
+	drm_object_attach_property(&connector->base,
+				   config->brightness_property, U16_MAX);
+
+	return 0;
+}
+EXPORT_SYMBOL(drm_backlight_alloc);
+
+void drm_backlight_free(struct drm_connector *connector)
+{
+	struct drm_backlight *b = connector->backlight;
+
+	if (b) {
+		WARN_ON(__drm_backlight_is_registered(b));
+		WARN_ON(b->link);
+
+		kfree(b->link_name);
+		kfree(b);
+		connector->backlight = NULL;
+	}
+}
+
+void drm_backlight_register(struct drm_backlight *b)
+{
+	if (!b)
+		return;
+
+	WARN_ON(__drm_backlight_is_registered(b));
+
+	spin_lock(&drm_backlight_lock);
+	list_add(&b->list, &drm_backlight_list);
+	__drm_backlight_lookup(b);
+	spin_unlock(&drm_backlight_lock);
+}
+
+void drm_backlight_unregister(struct drm_backlight *b)
+{
+	if (!b)
+		return;
+
+	WARN_ON(!__drm_backlight_is_registered(b));
+
+	spin_lock(&drm_backlight_lock);
+	list_del_init(&b->list);
+	__drm_backlight_link(b, NULL);
+	spin_unlock(&drm_backlight_lock);
+
+	cancel_work_sync(&b->work);
+}
+
+/**
+ * drm_backlight_get_name - retrieve name of linked backlight device
+ * @b: DRM backlight to retrieve name of
+ * @buf: target buffer for name
+ * @max: size of the target buffer
+ *
+ * This retrieves the name of the backlight device linked to @b and writes it
+ * into @buf. If @buf is NULL or @max is 0, no name will be retrieved, but this
+ * function only tests whether a link is set.
+ * Otherwise, the name will always be written into @buf and will always be
+ * zero-terminated (truncated if too long).
+ *
+ * If no backlight device is linked to @b, this returns -ENOENT. Otherwise, the
+ * length of the written name (excluding the terminating 0 character) is
+ * returned.
+ * Note that if a device name has been set but the underlying backlight device
+ * does not exist, this will still return the linked name. -ENOENT is only
+ * returned if no device name has been set, yet (or has been cleared).
+ *
+ * Returns: On success the length of the written name, on failure a negative
+ *          error code.
+ */
+int drm_backlight_get_name(struct drm_backlight *b, char *buf, size_t max)
+{
+	int r;
+
+	spin_lock(&drm_backlight_lock);
+
+	if (!b->link_name) {
+		r = -ENOENT;
+		goto unlock;
+	}
+
+	r = 0;
+	if (buf && max > 0) {
+		r = strlen(b->link_name);
+		if (r + 1 > max)
+			r = max - 1;
+		buf[r] = 0;
+		memcpy(buf, b->link_name, r);
+	}
+
+unlock:
+	spin_unlock(&drm_backlight_lock);
+	return r;
+}
+EXPORT_SYMBOL(drm_backlight_get_name);
+
+/**
+ * drm_backlight_set_name - Change the device link of a DRM backlight
+ * @b: DRM backlight to modify
+ * @name: name of backlight device
+ *
+ * This changes the backlight device-link on @b to the hardware device with
+ * name @name. @name is stored on the backlight device, even if no such
+ * hardware device is registered, yet. If a backlight device appears later on,
+ * it will be automatically linked to all matching DRM backlight devices. If a
+ * real hardware backlight device already exists with such a name, it is linked
+ * with immediate effect.
+ *
+ * Whenever a real hardware backlight is linked or unlinked from a DRM connector
+ * an uevent with "BACKLIGHT=1" is generated on the connector.
+ *
+ * Returns: 0 on success, negative error code on failure.
+ */
+int drm_backlight_set_name(struct drm_backlight *b, const char *name)
+{
+	char *namecopy;
+
+	if (name && *name) {
+		namecopy = kstrdup(name, GFP_KERNEL);
+		if (!namecopy)
+			return -ENOMEM;
+	} else {
+		namecopy = NULL;
+	}
+
+	spin_lock(&drm_backlight_lock);
+
+	kfree(b->link_name);
+	b->link_name = namecopy;
+	if (__drm_backlight_is_registered(b))
+		__drm_backlight_lookup(b);
+
+	spin_unlock(&drm_backlight_lock);
+
+	return 0;
+}
+EXPORT_SYMBOL(drm_backlight_set_name);
+
+void drm_backlight_set_brightness(struct drm_backlight *b, uint64_t value)
+{
+	spin_lock(&drm_backlight_lock);
+	__drm_backlight_prop_changed(b, value);
+	spin_unlock(&drm_backlight_lock);
+}
+
+static int drm_backlight_notify(struct notifier_block *self,
+				unsigned long event, void *data)
+{
+	struct backlight_device *bd = data;
+	struct drm_backlight *b;
+	const char *name;
+
+	spin_lock(&drm_backlight_lock);
+
+	switch (event) {
+	case BACKLIGHT_REGISTERED:
+		name = dev_name(&bd->dev);
+		if (!name)
+			break;
+
+		list_for_each_entry(b, &drm_backlight_list, list)
+			if (!b->link && !strcmp(name, b->link_name))
+				__drm_backlight_link(b, bd);
+
+		break;
+	case BACKLIGHT_UNREGISTERED:
+		list_for_each_entry(b, &drm_backlight_list, list)
+			if (b->link == bd)
+				__drm_backlight_link(b, NULL);
+
+		break;
+	}
+
+	spin_unlock(&drm_backlight_lock);
+
+	return 0;
+}
+
+static struct notifier_block drm_backlight_notifier = {
+	.notifier_call = drm_backlight_notify,
+};
+
+int drm_backlight_init(void)
+{
+	return backlight_register_notifier(&drm_backlight_notifier);
+}
+
+void drm_backlight_exit(void)
+{
+	backlight_unregister_notifier(&drm_backlight_notifier);
+}

drivers/gpu/drm/drm_crtc.c +26 -19

index 7d7c1fd..1e8caa3 100644
@@ -34,6 +34,7 @@
 #include <linux/slab.h>
 #include <linux/export.h>
 #include <drm/drmP.h>
+#include <drm/drm_backlight.h>
 #include <drm/drm_crtc.h>
 #include <drm/drm_edid.h>
 #include <drm/drm_fourcc.h>

@@ -918,6 +919,7 @@ void drm_connector_cleanup(struct drm_connector *connector)
 	ida_remove(&drm_connector_enum_list[connector->connector_type].ida,
 		   connector->connector_type_id);
 
+	drm_backlight_free(connector);
 	drm_mode_object_put(dev, &connector->base);
 	kfree(connector->name);
 	connector->name = NULL;

@@ -963,6 +965,7 @@ int drm_connector_register(struct drm_connector *connector)
 	int ret;
 
 	drm_mode_object_register(connector->dev, &connector->base);
+	drm_backlight_register(connector->backlight);
 
 	ret = drm_sysfs_connector_add(connector);
 	if (ret)

@@ -986,6 +989,7 @@ EXPORT_SYMBOL(drm_connector_register);
  */
 void drm_connector_unregister(struct drm_connector *connector)
 {
+	drm_backlight_unregister(connector->backlight);
 	drm_sysfs_connector_remove(connector);
 	drm_debugfs_connector_remove(connector);
 }

@@ -1309,28 +1313,25 @@ EXPORT_SYMBOL(drm_plane_force_disable);
 
 static int drm_mode_create_standard_connector_properties(struct drm_device *dev)
 {
-	struct drm_property *edid;
-	struct drm_property *dpms;
-	struct drm_property *dev_path;
+	struct drm_property *p;
 
 	/*
 	 * Standard properties (apply to all connectors)
 	 */
-	edid = drm_property_create(dev, DRM_MODE_PROP_BLOB |
-				   DRM_MODE_PROP_IMMUTABLE,
-				   "EDID", 0);
-	dev->mode_config.edid_property = edid;
-
-	dpms = drm_property_create_enum(dev, 0,
-				   "DPMS", drm_dpms_enum_list,
-				   ARRAY_SIZE(drm_dpms_enum_list));
-	dev->mode_config.dpms_property = dpms;
-
-	dev_path = drm_property_create(dev,
-				       DRM_MODE_PROP_BLOB |
-				       DRM_MODE_PROP_IMMUTABLE,
-				       "PATH", 0);
-	dev->mode_config.path_property = dev_path;
+	p = drm_property_create(dev, DRM_MODE_PROP_BLOB |
+				DRM_MODE_PROP_IMMUTABLE, "EDID", 0);
+	dev->mode_config.edid_property = p;
+
+	p = drm_property_create_enum(dev, 0, "DPMS", drm_dpms_enum_list,
+				     ARRAY_SIZE(drm_dpms_enum_list));
+	dev->mode_config.dpms_property = p;
+
+	p = drm_property_create(dev, DRM_MODE_PROP_BLOB |
+				DRM_MODE_PROP_IMMUTABLE, "PATH", 0);
+	dev->mode_config.path_property = p;
+
+	p = drm_property_create_range(dev, 0, "BRIGHTNESS", 0, U16_MAX);
+	dev->mode_config.brightness_property = p;
 
 	return 0;
 }

@@ -4145,12 +4146,18 @@ static int drm_mode_connector_set_obj_prop(struct drm_mode_object *obj,
 {
 	int ret = -EINVAL;
 	struct drm_connector *connector = obj_to_connector(obj);
+	struct drm_mode_config *config = &connector->dev->mode_config;
 
 	/* Do DPMS ourselves */
-	if (property == connector->dev->mode_config.dpms_property) {
+	if (property == config->dpms_property) {
 		if (connector->funcs->dpms)
 			(*connector->funcs->dpms)(connector, (int)value);
 		ret = 0;
+	} else if (property == config->brightness_property) {
+		if (connector->backlight)
+			drm_backlight_set_brightness(connector->backlight,
+						     value);
+		ret = 0;
 	} else if (connector->funcs->set_property)
 		ret = connector->funcs->set_property(connector, property, value);
 

drivers/gpu/drm/drm_drv.c +11 -0

index 6645669..76c9401 100644
@@ -33,6 +33,7 @@
 #include <linux/mount.h>
 #include <linux/slab.h>
 #include <drm/drmP.h>
+#include <drm/drm_backlight.h>
 #include <drm/drm_core.h>
 #include "drm_legacy.h"
 

@@ -890,9 +891,18 @@ static int __init drm_core_init(void)
 		goto err_p3;
 	}
 
+	ret = drm_backlight_init();
+	if (ret < 0) {
+		DRM_ERROR("Cannot initialize backlight interface\n");
+		goto err_p4;
+	}
+
 	DRM_INFO("Initialized %s %d.%d.%d %s\n",
 		 CORE_NAME, CORE_MAJOR, CORE_MINOR, CORE_PATCHLEVEL, CORE_DATE);
 	return 0;
+
+err_p4:
+	debugfs_remove(drm_debugfs_root);
 err_p3:
 	drm_sysfs_destroy();
 err_p2:

@@ -905,6 +915,7 @@ err_p1:
 
 static void __exit drm_core_exit(void)
 {
+	drm_backlight_exit();
 	debugfs_remove(drm_debugfs_root);
 	drm_sysfs_destroy();
 

drivers/gpu/drm/drm_sysfs.c +53 -0

index ab1a5f6..bc5d7f3 100644
@@ -18,6 +18,7 @@
 #include <linux/err.h>
 #include <linux/export.h>
 
+#include <drm/drm_backlight.h>
 #include <drm/drm_sysfs.h>
 #include <drm/drm_core.h>
 #include <drm/drmP.h>

@@ -256,6 +257,57 @@ static ssize_t modes_show(struct device *device,
 	return written;
 }
 
+static ssize_t backlight_show(struct device *device,
+			      struct device_attribute *attr,
+			      char *buf)
+{
+	struct drm_connector *connector = to_drm_connector(device);
+	int r;
+
+	if (!connector->backlight)
+		return -ENOTSUPP;
+
+	r = drm_backlight_get_name(connector->backlight, buf, PAGE_SIZE);
+	if (r < 0)
+		return r;
+
+	if (r + 1 < PAGE_SIZE) {
+		buf[r++] = '\n';
+		buf[r] = 0;
+	}
+
+	return r;
+}
+
+static ssize_t backlight_store(struct device *device,
+			       struct device_attribute *attr,
+			       const char *buf, size_t size)
+{
+	struct drm_connector *connector = to_drm_connector(device);
+	const char *t;
+	char *name;
+	size_t len;
+	int r;
+
+	if (!connector->backlight)
+		return -ENOTSUPP;
+
+	t = strchrnul(buf, '\n');
+	len = t - buf;
+
+	name = kstrndup(buf, len, GFP_TEMPORARY);
+	if (!name)
+		return -ENOMEM;
+
+	r = drm_backlight_set_name(connector->backlight, name);
+	kfree(name);
+
+	if (r < 0)
+		return r;
+
+	return len;
+}
+
 static ssize_t subconnector_show(struct device *device,
 			   struct device_attribute *attr,
 			   char *buf)

@@ -343,6 +395,7 @@ static struct device_attribute connector_attrs[] = {
 	__ATTR_RO(enabled),
 	__ATTR_RO(dpms),
 	__ATTR_RO(modes),
+	__ATTR_RW(backlight),
 };
 
 /* These attributes are for both DVI-I connectors and all types of tv-out. */

drivers/video/backlight/backlight.c +3 -0

index 04f323b..a1bc533 100644
@@ -113,6 +113,9 @@ static void backlight_generate_event(struct backlight_device *bd,
 	case BACKLIGHT_UPDATE_HOTKEY:
 		envp[0] = "SOURCE=hotkey";
 		break;
+	case BACKLIGHT_UPDATE_DRM:
+		envp[0] = "SOURCE=drm";
+		break;
 	default:
 		envp[0] = "SOURCE=unknown";
 		break;

include/drm/drm_backlight.h new file +44 -0

new file mode 100644
index 0000000..9f0c31b
@@ -0,0 +1,44 @@
+#ifndef __DRM_BACKLIGHT_H__
+#define __DRM_BACKLIGHT_H__
+
+/*
+ * Copyright (c) 2014 David Herrmann <[email protected]>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
+ * THE COPYRIGHT HOLDER(S) OR AUTHOR(S) BE LIABLE FOR ANY CLAIM, DAMAGES OR
+ * OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
+ * ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+ * OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+#include <linux/kernel.h>
+#include <linux/types.h>
+
+struct drm_backlight;
+struct drm_connector;
+
+int drm_backlight_init(void);
+void drm_backlight_exit(void);
+
+int drm_backlight_alloc(struct drm_connector *connector);
+void drm_backlight_free(struct drm_connector *connector);
+void drm_backlight_register(struct drm_backlight *b);
+void drm_backlight_unregister(struct drm_backlight *b);
+
+int drm_backlight_get_name(struct drm_backlight *b, char *buf, size_t max);
+int drm_backlight_set_name(struct drm_backlight *b, const char *name);
+void drm_backlight_set_brightness(struct drm_backlight *b, uint64_t value);
+
+#endif /* __DRM_BACKLIGHT_H__ */

include/drm/drm_crtc.h +3 -0

index c40070a..ce3b2f0 100644
@@ -542,6 +542,8 @@ struct drm_connector {
 
 	/* requested DPMS state */
 	int dpms;
+	/* backlight link */
+	struct drm_backlight *backlight;
 
 	void *helper_private;
 

@@ -823,6 +825,7 @@ struct drm_mode_config {
 	struct list_head property_blob_list;
 	struct drm_property *edid_property;
 	struct drm_property *dpms_property;
+	struct drm_property *brightness_property;
 	struct drm_property *path_property;
 	struct drm_property *plane_type_property;
 	struct drm_property *rotation_property;

include/linux/backlight.h +1 -0

index bcc0dec..8615f54 100644
@@ -31,6 +31,7 @@
 enum backlight_update_reason {
 	BACKLIGHT_UPDATE_HOTKEY,
 	BACKLIGHT_UPDATE_SYSFS,
+	BACKLIGHT_UPDATE_DRM,
 };
 
 enum backlight_type {

Signature · 1 line

2.1.0

Nice you skid around all the pitfalls and trapdoors, I guess we've all been rather blind ;-)

Two high-level comments:
- We also want to forward "bl_power". cros was totally not happy when we stopped treating brightness == 0 as completely off (it upsets some panels terminally, so there's a vbt lower limit). Instead we expose this now through the bl_power knob.

While at it I think we should expose all the other backlight properties too (read-only ofc for actual/max_brightness).

- How does udev match on the drm connector name? They are not terribly stable atm, and if you reload your drm driver, or much more likely, have two gpus with two drm drivers they change. We probably should change the name allocation scheme to be per device instance instead of global first. Within a driver probe order is hopefully deterministic on a given platform, since even with super dynamic setups (based on dt/acpi) the firmware tables should change really.

Hi

Quoted 11 lines · Daniel Vetter

Nice you skid around all the pitfalls and trapdoors, I guess we've all been rather blind ;-)

Two high-level comments:
- We also want to forward "bl_power". cros was totally not happy when we stopped treating brightness == 0 as completely off (it upsets some panels terminally, so there's a vbt lower limit). Instead we expose this now through the bl_power knob.

While at it I think we should expose all the other backlight properties too (read-only ofc for actual/max_brightness).

bl_power is easy to add. I guess v2 will have:
"BACKLIGHT-POWER" (range 0-4)

actual-brightness is a bit more tricky. Currently, DRM caches property values, so there is no read_property() hook. We'd have to add this. But it'll be quite nasty as we have to call into the backlight driver. So I think we want to run an async-interruptible worker on the backlight, drop the locks in the ioctl and wait for the job to finish. Not sure whether it's worth it.. maybe we can add this later.

Quoted 8 lines

- How does udev match on the drm connector name? They are not terribly stable atm, and if you reload your drm driver, or much more likely, have two gpus with two drm drivers they change. We probably should change the name allocation scheme to be per device instance instead of global first. Within a driver probe order is hopefully deterministic on a given platform, since even with super dynamic setups (based on dt/acpi) the firmware tables should change really.

You can match on EDID attributes. Ok, so far this is pretty ugly as the EDID property is binary. But we can add rather trivial udev extensions to make EDID binary against text matching possible.

Quoted 6 lines · David Herrmann

actual-brightness is a bit more tricky. Currently, DRM caches property values, so there is no read_property() hook. We'd have to add this. But it'll be quite nasty as we have to call into the backlight driver. So I think we want to run an async-interruptible worker on the backlight, drop the locks in the ioctl and wait for the job to finish. Not sure whether it's worth it.. maybe we can add this later.

See Jani's reply - we probably don't need it, at least not in version 1.

Quoted 9 lines

Quoted 8 lines

- How does udev match on the drm connector name? They are not terribly stable atm, and if you reload your drm driver, or much more likely, have two gpus with two drm drivers they change. We probably should change the name allocation scheme to be per device instance instead of global first. Within a driver probe order is hopefully deterministic on a given platform, since even with super dynamic setups (based on dt/acpi) the firmware tables should change really.

Quoted 3 lines

You can match on EDID attributes. Ok, so far this is pretty ugly as the EDID property is binary. But we can add rather trivial udev extensions to make EDID binary against text matching possible.

Why EDID? This is purely about the drm connector name, e.g. if I have 2 gpus, both with an eDP connector (optimus, so just one panel) then the first driver gets eDP-1 as the name of it and the 2nd one eDP-2. Which is ok if both should control backlight brightness through the same driver, but a total mess if not just gpus get switched, but also backlight controllers.

And if you reload you get then eDP-2 and eDP-3. Well at least in the past, that hilarity at least was fixed in

commit b21e3afe2357c0f49348a5fb61247012bf8262ec

Author: Ilia Mirkin <[email protected]>
Date:   Wed Aug 7 22:34:48 2013 -0400

drm: use ida to allocate connector id

What I think we need to do is to make these ida allocators per-device, so that both drivers have an eDP-1 connector. Otherwise you need to either match both or do funny tricks like "the first eDP connector, no matter which one on this gpu". After all we can now support more than one eDP (and more than one LVDS since a long time actually).

Or how exactly is the udev hw db supposed to match this stuff for special cases. In general we need to duplicate the existing logic from libbacklight, like Matthew suggested.

Quoted 41 lines · David Herrmann

Backlight devices have always been managed independently of display controllers. They're often controlled via different hardware interfaces and their relationship to display-controllers varies vastly between different boards. However, display brightness is obviously a property of a display, and thus of a DRM connector. Therefore, it'd be really appreciated if user-space APIs would highlight this relationship.

The main runtime users of backlight interfaces are user-space compositors. But currently they have to jump through hoops to find the correct backlight device for a given connector. Furthermore, they need root privileges to write to sysfs. sysfs has never been designed as run-time non-root API. It does not provide file-contexts, run-time management or any kind of API control. There is no way to control access to sysfs via different links (in that case: mounts). Char-devs provide all this!

So far, backlight APIs have been fairly trivial, so adding char-devs to backlights is rather heavy-weight. Therefore, this patch introduces a new API interface to modify backlight brightness via DRM: A "BRIGHTNESS" property on DRM connectors.

Instead of adding backlight hardware support to DRM, we rely on the backlight-class and simply add a new API. Each DRM Connector can optionally be linked to a backlight class device. Modifying the connector property will have the same effect as writing into the "brightness" sysfs file of the linked backlight class device. However, we now can manage access to backlight devices via the same interface as access to mode-setting on the underlying display. Furthermore, the connection between displays and their backlight devices are visible in user-space.

Obviously, matching backlights to displays cannot be solved magically with this link. Therefore, we also add a user-space attribute to DRM connectors called 'backlight'. If a DRM driver is incapable of matching existing backlights to a connector, or if a given board has just crappy backlight drivers, udev can write the name of a backlight-device into this attribute and the connector-property will be re-linked to this backlight device. The udev hwdb can be easily employed to track such quirks and fixups for different board+GPU combinations.
Note that the name written into the 'backlight' attribute is saved on the connector, so in case the real backlight device is probed after the DRM card, the backlight will still get properly attached once probed.

Quoted 1 line

Signed-off-by David Herrmann <[email protected]>

Nice you skid around all the pitfalls and trapdoors, I guess we've all been rather blind ;-)

Two high-level comments:
- We also want to forward "bl_power". cros was totally not happy when we stopped treating brightness == 0 as completely off (it upsets some panels terminally, so there's a vbt lower limit). Instead we expose this now through the bl_power knob.

While at it I think we should expose all the other backlight properties too (read-only ofc for actual/max_brightness).

- How does udev match on the drm connector name? They are not terribly stable atm, and if you reload your drm driver, or much more likely, have two gpus with two drm drivers they change. We probably should change the name allocation scheme to be per device instance instead of global first. Within a driver probe order is hopefully deterministic on a given platform, since even with super dynamic setups (based on dt/acpi) the firmware tables should change really.

Cheers, Daniel

Quoted 750 lines

---
 drivers/gpu/drm/Kconfig             |   1 +
 drivers/gpu/drm/Makefile            |   2 +-
 drivers/gpu/drm/drm_backlight.c     | 387 ++++++++++++++++++++++++++++++++++++
 drivers/gpu/drm/drm_crtc.c          |  45 +++--
 drivers/gpu/drm/drm_drv.c           |  11 +
 drivers/gpu/drm/drm_sysfs.c         |  53 +++++
 drivers/video/backlight/backlight.c |   3 +
 include/drm/drm_backlight.h         |  44 ++++
 include/drm/drm_crtc.h              |   3 +
 include/linux/backlight.h           |   1 +
 10 files changed, 530 insertions(+), 20 deletions(-)
 create mode 100644 drivers/gpu/drm/drm_backlight.c
 create mode 100644 include/drm/drm_backlight.h

drivers/gpu/drm/Kconfig +1 -0

index e3b4b0f..46bca34 100644
@@ -12,6 +12,7 @@ menuconfig DRM
 	select I2C
 	select I2C_ALGOBIT
 	select DMA_SHARED_BUFFER
+	select BACKLIGHT_CLASS_DEVICE
 	help
 	  Kernel-level support for the Direct Rendering Infrastructure (DRI)
 	  introduced in XFree86 4.0. If you say Y here, you need to select

drivers/gpu/drm/Makefile +1 -1

index 9292a76..224544d 100644
@@ -14,7 +14,7 @@ drm-y       :=	drm_auth.o drm_bufs.o drm_cache.o \
 		drm_info.o drm_debugfs.o drm_encoder_slave.o \
 		drm_trace_points.o drm_global.o drm_prime.o \
 		drm_rect.o drm_vma_manager.o drm_flip_work.o \
-		drm_modeset_lock.o
+		drm_modeset_lock.o drm_backlight.o
 
 drm-$(CONFIG_COMPAT) += drm_ioc32.o
 drm-$(CONFIG_DRM_GEM_CMA_HELPER) += drm_gem_cma_helper.o

drivers/gpu/drm/drm_backlight.c new file +387 -0

new file mode 100644
index 0000000..92d231a
@@ -0,0 +1,387 @@
+/*
+ * DRM Backlight Helpers
+ * Copyright (c) 2014 David Herrmann
+ */
+
+#include <linux/backlight.h>
+#include <linux/fs.h>
+#include <linux/list.h>
+#include <linux/math64.h>
+#include <linux/module.h>
+#include <linux/mutex.h>
+#include <linux/notifier.h>
+#include <linux/slab.h>
+#include <linux/spinlock.h>
+#include <drm/drmP.h>
+#include <drm/drm_backlight.h>
+
+/**
+ * DOC: Backlight Devices
+ *
+ * Backlight devices have always been managed as a separate subsystem,
+ * independent of DRM. They are usually controlled via separate hardware
+ * interfaces than the display controller, so the split works out fine.
+ * However, backlight brightness is a property of a display, and thus a
+ * property of a DRM connector. We already manage DPMS states via connector
+ * properties, so it is natural to keep brightness control at the same place.
+ *
+ * This DRM backlight interface implements generic backlight properties on
+ * connectors. It does not handle any hardware backends but simply forwards
+ * the requests to an available and linked backlight device. The links between
+ * connectors and backlight devices have to be established by DRM drivers and
+ * can be modified by user-space via sysfs (and udev rules). The name of the
+ * backlight device can be written to a sysfs attribute called 'backlight'.
+ * The device is looked up and linked to the connector (replacing a possible
+ * previous backlight device). A 'change' uevent is sent whenever a link is
+ * modified.
+ *
+ * Drivers have to call drm_backlight_alloc() after allocating a connector via
+ * drm_connector_init(). This will automatically add a backlight device to the
+ * given connector. No hardware device is linked to the connector by default.
+ * Drivers can set up a default device via drm_backlight_set_name(), but are
+ * free to leave it empty. User-space will then have to set up the link.
+ */
+
+struct drm_backlight {
+	struct list_head list;
+	struct drm_connector *connector;
+	char *link_name;
+	struct backlight_device *link;
+	struct work_struct work;
+	unsigned int set_value;
+	bool changed : 1;
+};
+
+static LIST_HEAD(drm_backlight_list);
+static DEFINE_SPINLOCK(drm_backlight_lock);
+
+/* caller must hold @drm_backlight_lock */
+static bool __drm_backlight_is_registered(struct drm_backlight *b)
+{
+	/* a device is live if it is linked to @drm_backlight_list */
+	return !list_empty(&b->list);
+}
+
+/* caller must hold @drm_backlight_lock */
+static void __drm_backlight_schedule(struct drm_backlight *b)
+{
+	if (__drm_backlight_is_registered(b))
+		schedule_work(&b->work);
+}
+
+static void __drm_backlight_worker(struct work_struct *w)
+{
+	struct drm_backlight *b = container_of(w, struct drm_backlight, work);
+	static char *ep[] = { "BACKLIGHT=1", NULL };
+	struct backlight_device *bd;
+	bool send_uevent;
+	unsigned int v;
+
+	spin_lock(&drm_backlight_lock);
+	send_uevent = b->changed;
+	b->changed = false;
+	v = b->set_value;
+	bd = b->link;
+	backlight_device_ref(bd);
+	spin_unlock(&drm_backlight_lock);
+
+	if (bd) {
+		backlight_set_brightness(bd, v, BACKLIGHT_UPDATE_DRM);
+		backlight_device_unref(bd);
+	}
+
+	if (send_uevent)
+		kobject_uevent_env(&b->connector->kdev->kobj, KOBJ_CHANGE, ep);
+}
+
+/* caller must hold @drm_backlight_lock */
+static void __drm_backlight_prop_changed(struct drm_backlight *b, uint64_t v)
+{
+	uint64_t max;
+
+	if (!b->link)
+		return;
+
+	max = b->link->props.max_brightness;
+	if (v >= U16_MAX)
+		b->set_value = max;
+	else
+		b->set_value = (v * max) >> 16;
+	__drm_backlight_schedule(b);
+}
+
+/* caller must hold @drm_backlight_lock */
+static void __drm_backlight_real_changed(struct drm_backlight *b, uint64_t v)
+{
+	struct drm_mode_config *config = &b->connector->dev->mode_config;
+	unsigned int max, set;
+
+	if (!b->link)
+		return;
+
+	set = v;
+	max = b->link->props.max_brightness;
+	if (max < 1)
+		return;
+
+	if (set >= max)
+		set = U16_MAX;
+	else if (max <= U16_MAX)
+		set = (set << 16) / max;
+	else
+		set = div_u64(v << 16, max);
+
+	drm_object_property_set_value(&b->connector->base,
+				      config->brightness_property, v);
+}
+
+/* caller must hold @drm_backlight_lock */
+static void __drm_backlight_link(struct drm_backlight *b,
+				 struct backlight_device *bd)
+{
+	if (bd != b->link) {
+		backlight_device_unref(b->link);
+		b->link = bd;
+		backlight_device_ref(b->link);
+		if (bd)
+			__drm_backlight_real_changed(b, bd->props.brightness);
+		b->changed = true;
+		__drm_backlight_schedule(b);
+	}
+}
+
+/* caller must hold @drm_backlight_lock */
+static void __drm_backlight_lookup(struct drm_backlight *b)
+{
+	struct backlight_device *bd;
+
+	if (b->link_name)
+		bd = backlight_device_lookup(b->link_name);
+	else
+		bd = NULL;
+
+	__drm_backlight_link(b, bd);
+	backlight_device_unref(bd);
+}
+
+/**
+ * drm_backlight_alloc - add backlight capability to a connector
+ * @connector: connector to add backlight to
+ *
+ * This allocates a new DRM-backlight device and links it to @connector. This
+ * *must* be called before registering the connector. The backlight device will
+ * be automatically registered in sync with the connector. It will also get
+ * removed once the connector is removed.
+ *
+ * The connector will not have any hardware backlight linked by default. You
+ * need to call drm_backlight_set_name() if you want to set a default
+ * backlight. User-space can overwrite those via sysfs.
+ *
+ * Returns: 0 on success, negative error code on failure.
+ */
+int drm_backlight_alloc(struct drm_connector *connector)
+{
+	struct drm_mode_config *config = &connector->dev->mode_config;
+	struct drm_backlight *b;
+
+	b = kzalloc(sizeof(*b), GFP_KERNEL);
+	if (!b)
+		return -ENOMEM;
+
+	INIT_LIST_HEAD(&b->list);
+	INIT_WORK(&b->work, __drm_backlight_worker);
+	b->connector = connector;
+	connector->backlight = b;
+
+	drm_object_attach_property(&connector->base,
+				   config->brightness_property, U16_MAX);
+
+	return 0;
+}
+EXPORT_SYMBOL(drm_backlight_alloc);
+
+void drm_backlight_free(struct drm_connector *connector)
+{
+	struct drm_backlight *b = connector->backlight;
+
+	if (b) {
+		WARN_ON(__drm_backlight_is_registered(b));
+		WARN_ON(b->link);
+
+		kfree(b->link_name);
+		kfree(b);
+		connector->backlight = NULL;
+	}
+}
+
+void drm_backlight_register(struct drm_backlight *b)
+{
+	if (!b)
+		return;
+
+	WARN_ON(__drm_backlight_is_registered(b));
+
+	spin_lock(&drm_backlight_lock);
+	list_add(&b->list, &drm_backlight_list);
+	__drm_backlight_lookup(b);
+	spin_unlock(&drm_backlight_lock);
+}
+
+void drm_backlight_unregister(struct drm_backlight *b)
+{
+	if (!b)
+		return;
+
+	WARN_ON(!__drm_backlight_is_registered(b));
+
+	spin_lock(&drm_backlight_lock);
+	list_del_init(&b->list);
+	__drm_backlight_link(b, NULL);
+	spin_unlock(&drm_backlight_lock);
+
+	cancel_work_sync(&b->work);
+}
+
+/**
+ * drm_backlight_get_name - retrieve name of linked backlight device
+ * @b: DRM backlight to retrieve name of
+ * @buf: target buffer for name
+ * @max: size of the target buffer
+ *
+ * This retrieves the name of the backlight device linked to @b and writes it
+ * into @buf. If @buf is NULL or @max is 0, no name will be retrieved, but this
+ * function only tests whether a link is set.
+ * Otherwise, the name will always be written into @buf and will always be
+ * zero-terminated (truncated if too long).
+ *
+ * If no backlight device is linked to @b, this returns -ENOENT. Otherwise, the
+ * length of the written name (excluding the terminating 0 character) is
+ * returned.
+ * Note that if a device name has been set but the underlying backlight device
+ * does not exist, this will still return the linked name. -ENOENT is only
+ * returned if no device name has been set, yet (or has been cleared).
+ *
+ * Returns: On success the length of the written name, on failure a negative
+ *          error code.
+ */
+int drm_backlight_get_name(struct drm_backlight *b, char *buf, size_t max)
+{
+	int r;
+
+	spin_lock(&drm_backlight_lock);
+
+	if (!b->link_name) {
+		r = -ENOENT;
+		goto unlock;
+	}
+
+	r = 0;
+	if (buf && max > 0) {
+		r = strlen(b->link_name);
+		if (r + 1 > max)
+			r = max - 1;
+		buf[r] = 0;
+		memcpy(buf, b->link_name, r);
+	}
+
+unlock:
+	spin_unlock(&drm_backlight_lock);
+	return r;
+}
+EXPORT_SYMBOL(drm_backlight_get_name);
+
+/**
+ * drm_backlight_set_name - Change the device link of a DRM backlight
+ * @b: DRM backlight to modify
+ * @name: name of backlight device
+ *
+ * This changes the backlight device-link on @b to the hardware device with
+ * name @name. @name is stored on the backlight device, even if no such
+ * hardware device is registered, yet. If a backlight device appears later on,
+ * it will be automatically linked to all matching DRM backlight devices. If a
+ * real hardware backlight device already exists with such a name, it is linked
+ * with immediate effect.
+ *
+ * Whenever a real hardware backlight is linked or unlinked from a DRM connector
+ * an uevent with "BACKLIGHT=1" is generated on the connector.
+ *
+ * Returns: 0 on success, negative error code on failure.
+ */
+int drm_backlight_set_name(struct drm_backlight *b, const char *name)
+{
+	char *namecopy;
+
+	if (name && *name) {
+		namecopy = kstrdup(name, GFP_KERNEL);
+		if (!namecopy)
+			return -ENOMEM;
+	} else {
+		namecopy = NULL;
+	}
+
+	spin_lock(&drm_backlight_lock);
+
+	kfree(b->link_name);
+	b->link_name = namecopy;
+	if (__drm_backlight_is_registered(b))
+		__drm_backlight_lookup(b);
+
+	spin_unlock(&drm_backlight_lock);
+
+	return 0;
+}
+EXPORT_SYMBOL(drm_backlight_set_name);
+
+void drm_backlight_set_brightness(struct drm_backlight *b, uint64_t value)
+{
+	spin_lock(&drm_backlight_lock);
+	__drm_backlight_prop_changed(b, value);
+	spin_unlock(&drm_backlight_lock);
+}
+
+static int drm_backlight_notify(struct notifier_block *self,
+				unsigned long event, void *data)
+{
+	struct backlight_device *bd = data;
+	struct drm_backlight *b;
+	const char *name;
+
+	spin_lock(&drm_backlight_lock);
+
+	switch (event) {
+	case BACKLIGHT_REGISTERED:
+		name = dev_name(&bd->dev);
+		if (!name)
+			break;
+
+		list_for_each_entry(b, &drm_backlight_list, list)
+			if (!b->link && !strcmp(name, b->link_name))
+				__drm_backlight_link(b, bd);
+
+		break;
+	case BACKLIGHT_UNREGISTERED:
+		list_for_each_entry(b, &drm_backlight_list, list)
+			if (b->link == bd)
+				__drm_backlight_link(b, NULL);
+
+		break;
+	}
+
+	spin_unlock(&drm_backlight_lock);
+
+	return 0;
+}
+
+static struct notifier_block drm_backlight_notifier = {
+	.notifier_call = drm_backlight_notify,
+};
+
+int drm_backlight_init(void)
+{
+	return backlight_register_notifier(&drm_backlight_notifier);
+}
+
+void drm_backlight_exit(void)
+{
+	backlight_unregister_notifier(&drm_backlight_notifier);
+}

drivers/gpu/drm/drm_crtc.c +26 -19

index 7d7c1fd..1e8caa3 100644
@@ -34,6 +34,7 @@
 #include <linux/slab.h>
 #include <linux/export.h>
 #include <drm/drmP.h>
+#include <drm/drm_backlight.h>
 #include <drm/drm_crtc.h>
 #include <drm/drm_edid.h>
 #include <drm/drm_fourcc.h>

@@ -918,6 +919,7 @@ void drm_connector_cleanup(struct drm_connector *connector)
 	ida_remove(&drm_connector_enum_list[connector->connector_type].ida,
 		   connector->connector_type_id);
 
+	drm_backlight_free(connector);
 	drm_mode_object_put(dev, &connector->base);
 	kfree(connector->name);
 	connector->name = NULL;

@@ -963,6 +965,7 @@ int drm_connector_register(struct drm_connector *connector)
 	int ret;
 
 	drm_mode_object_register(connector->dev, &connector->base);
+	drm_backlight_register(connector->backlight);
 
 	ret = drm_sysfs_connector_add(connector);
 	if (ret)

@@ -986,6 +989,7 @@ EXPORT_SYMBOL(drm_connector_register);
  */
 void drm_connector_unregister(struct drm_connector *connector)
 {
+	drm_backlight_unregister(connector->backlight);
 	drm_sysfs_connector_remove(connector);
 	drm_debugfs_connector_remove(connector);
 }

@@ -1309,28 +1313,25 @@ EXPORT_SYMBOL(drm_plane_force_disable);
 
 static int drm_mode_create_standard_connector_properties(struct drm_device *dev)
 {
-	struct drm_property *edid;
-	struct drm_property *dpms;
-	struct drm_property *dev_path;
+	struct drm_property *p;
 
 	/*
 	 * Standard properties (apply to all connectors)
 	 */
-	edid = drm_property_create(dev, DRM_MODE_PROP_BLOB |
-				   DRM_MODE_PROP_IMMUTABLE,
-				   "EDID", 0);
-	dev->mode_config.edid_property = edid;
-
-	dpms = drm_property_create_enum(dev, 0,
-				   "DPMS", drm_dpms_enum_list,
-				   ARRAY_SIZE(drm_dpms_enum_list));
-	dev->mode_config.dpms_property = dpms;
-
-	dev_path = drm_property_create(dev,
-				       DRM_MODE_PROP_BLOB |
-				       DRM_MODE_PROP_IMMUTABLE,
-				       "PATH", 0);
-	dev->mode_config.path_property = dev_path;
+	p = drm_property_create(dev, DRM_MODE_PROP_BLOB |
+				DRM_MODE_PROP_IMMUTABLE, "EDID", 0);
+	dev->mode_config.edid_property = p;
+
+	p = drm_property_create_enum(dev, 0, "DPMS", drm_dpms_enum_list,
+				     ARRAY_SIZE(drm_dpms_enum_list));
+	dev->mode_config.dpms_property = p;
+
+	p = drm_property_create(dev, DRM_MODE_PROP_BLOB |
+				DRM_MODE_PROP_IMMUTABLE, "PATH", 0);
+	dev->mode_config.path_property = p;
+
+	p = drm_property_create_range(dev, 0, "BRIGHTNESS", 0, U16_MAX);
+	dev->mode_config.brightness_property = p;
 
 	return 0;
 }

@@ -4145,12 +4146,18 @@ static int drm_mode_connector_set_obj_prop(struct drm_mode_object *obj,
 {
 	int ret = -EINVAL;
 	struct drm_connector *connector = obj_to_connector(obj);
+	struct drm_mode_config *config = &connector->dev->mode_config;
 
 	/* Do DPMS ourselves */
-	if (property == connector->dev->mode_config.dpms_property) {
+	if (property == config->dpms_property) {
 		if (connector->funcs->dpms)
 			(*connector->funcs->dpms)(connector, (int)value);
 		ret = 0;
+	} else if (property == config->brightness_property) {
+		if (connector->backlight)
+			drm_backlight_set_brightness(connector->backlight,
+						     value);
+		ret = 0;
 	} else if (connector->funcs->set_property)
 		ret = connector->funcs->set_property(connector, property, value);
 

drivers/gpu/drm/drm_drv.c +11 -0

index 6645669..76c9401 100644
@@ -33,6 +33,7 @@
 #include <linux/mount.h>
 #include <linux/slab.h>
 #include <drm/drmP.h>
+#include <drm/drm_backlight.h>
 #include <drm/drm_core.h>
 #include "drm_legacy.h"
 

@@ -890,9 +891,18 @@ static int __init drm_core_init(void)
 		goto err_p3;
 	}
 
+	ret = drm_backlight_init();
+	if (ret < 0) {
+		DRM_ERROR("Cannot initialize backlight interface\n");
+		goto err_p4;
+	}
+
 	DRM_INFO("Initialized %s %d.%d.%d %s\n",
 		 CORE_NAME, CORE_MAJOR, CORE_MINOR, CORE_PATCHLEVEL, CORE_DATE);
 	return 0;
+
+err_p4:
+	debugfs_remove(drm_debugfs_root);
 err_p3:
 	drm_sysfs_destroy();
 err_p2:

@@ -905,6 +915,7 @@ err_p1:
 
 static void __exit drm_core_exit(void)
 {
+	drm_backlight_exit();
 	debugfs_remove(drm_debugfs_root);
 	drm_sysfs_destroy();
 

drivers/gpu/drm/drm_sysfs.c +53 -0

index ab1a5f6..bc5d7f3 100644
@@ -18,6 +18,7 @@
 #include <linux/err.h>
 #include <linux/export.h>
 
+#include <drm/drm_backlight.h>
 #include <drm/drm_sysfs.h>
 #include <drm/drm_core.h>
 #include <drm/drmP.h>

@@ -256,6 +257,57 @@ static ssize_t modes_show(struct device *device,
 	return written;
 }
 
+static ssize_t backlight_show(struct device *device,
+			      struct device_attribute *attr,
+			      char *buf)
+{
+	struct drm_connector *connector = to_drm_connector(device);
+	int r;
+
+	if (!connector->backlight)
+		return -ENOTSUPP;
+
+	r = drm_backlight_get_name(connector->backlight, buf, PAGE_SIZE);
+	if (r < 0)
+		return r;
+
+	if (r + 1 < PAGE_SIZE) {
+		buf[r++] = '\n';
+		buf[r] = 0;
+	}
+
+	return r;
+}
+
+static ssize_t backlight_store(struct device *device,
+			       struct device_attribute *attr,
+			       const char *buf, size_t size)
+{
+	struct drm_connector *connector = to_drm_connector(device);
+	const char *t;
+	char *name;
+	size_t len;
+	int r;
+
+	if (!connector->backlight)
+		return -ENOTSUPP;
+
+	t = strchrnul(buf, '\n');
+	len = t - buf;
+
+	name = kstrndup(buf, len, GFP_TEMPORARY);
+	if (!name)
+		return -ENOMEM;
+
+	r = drm_backlight_set_name(connector->backlight, name);
+	kfree(name);
+
+	if (r < 0)
+		return r;
+
+	return len;
+}
+
 static ssize_t subconnector_show(struct device *device,
 			   struct device_attribute *attr,
 			   char *buf)

@@ -343,6 +395,7 @@ static struct device_attribute connector_attrs[] = {
 	__ATTR_RO(enabled),
 	__ATTR_RO(dpms),
 	__ATTR_RO(modes),
+	__ATTR_RW(backlight),
 };
 
 /* These attributes are for both DVI-I connectors and all types of tv-out. */

drivers/video/backlight/backlight.c +3 -0

index 04f323b..a1bc533 100644
@@ -113,6 +113,9 @@ static void backlight_generate_event(struct backlight_device *bd,
 	case BACKLIGHT_UPDATE_HOTKEY:
 		envp[0] = "SOURCE=hotkey";
 		break;
+	case BACKLIGHT_UPDATE_DRM:
+		envp[0] = "SOURCE=drm";
+		break;
 	default:
 		envp[0] = "SOURCE=unknown";
 		break;

include/drm/drm_backlight.h new file +44 -0

new file mode 100644
index 0000000..9f0c31b
@@ -0,0 +1,44 @@
+#ifndef __DRM_BACKLIGHT_H__
+#define __DRM_BACKLIGHT_H__
+
+/*
+ * Copyright (c) 2014 David Herrmann <[email protected]>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
+ * THE COPYRIGHT HOLDER(S) OR AUTHOR(S) BE LIABLE FOR ANY CLAIM, DAMAGES OR
+ * OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
+ * ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+ * OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+#include <linux/kernel.h>
+#include <linux/types.h>
+
+struct drm_backlight;
+struct drm_connector;
+
+int drm_backlight_init(void);
+void drm_backlight_exit(void);
+
+int drm_backlight_alloc(struct drm_connector *connector);
+void drm_backlight_free(struct drm_connector *connector);
+void drm_backlight_register(struct drm_backlight *b);
+void drm_backlight_unregister(struct drm_backlight *b);
+
+int drm_backlight_get_name(struct drm_backlight *b, char *buf, size_t max);
+int drm_backlight_set_name(struct drm_backlight *b, const char *name);
+void drm_backlight_set_brightness(struct drm_backlight *b, uint64_t value);
+
+#endif /* __DRM_BACKLIGHT_H__ */

include/drm/drm_crtc.h +3 -0

index c40070a..ce3b2f0 100644
@@ -542,6 +542,8 @@ struct drm_connector {
 
 	/* requested DPMS state */
 	int dpms;
+	/* backlight link */
+	struct drm_backlight *backlight;
 
 	void *helper_private;
 

@@ -823,6 +825,7 @@ struct drm_mode_config {
 	struct list_head property_blob_list;
 	struct drm_property *edid_property;
 	struct drm_property *dpms_property;
+	struct drm_property *brightness_property;
 	struct drm_property *path_property;
 	struct drm_property *plane_type_property;
 	struct drm_property *rotation_property;

include/linux/backlight.h +1 -0

index bcc0dec..8615f54 100644
@@ -31,6 +31,7 @@
 enum backlight_update_reason {
 	BACKLIGHT_UPDATE_HOTKEY,
 	BACKLIGHT_UPDATE_SYSFS,
+	BACKLIGHT_UPDATE_DRM,
 };
 
 enum backlight_type {

Signature · 1 line

2.1.0

Signature · 7 lines

Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
_______________________________________________
dri-devel mailing list
[email protected]
http://lists.freedesktop.org/mailman/listinfo/dri-devel

* User-space currently has a hard-time figuring out which backlight device to use, and which backlight device belongs to which display. So far, most systems only provide backlight-devices for internal displays, so figuring out the connection is easy, but that might change with more capable external connectors.

If multiple backlights are available, the easiest solution is to simply write to all of them and hope at least one of them works. This obviously fails if the devices interact badly, so it's not really a solution.

* User-space needs root privileges to write to sysfs. There are no char-devs that can be used to control access-management to the backlight API, so /sys is the only interface available. So far, udev policy has been "/sys is read-only for non-root" and it's not going to change. char-devs are *THE* standard way to provide non-root user-space APIs, so use them!

This series tries to solve this problem with a much simpler approach:
Instead of moving backlights into DRM, we simply link DRM properties to a backlight device. That is, the kernel manages a link between a connector and a backlight device (or n backlight devices) which can be modified by udev in case the kernel got it wrong (we don't want huge board-fixup-tables in the kernel). User-space can now use the simpl DRM API to manage backlights, and the kernel does not need any special driver code to make it work.

Quoted 5 lines · David Herrmann

* User-space currently has a hard-time figuring out which backlight device to use, and which backlight device belongs to which display. So far, most systems only provide backlight-devices for internal displays, so figuring out the connection is easy, but that might change with more capable external connectors.

The parent device of the backlight will be the correct display, if the kernel has a meaningful way to determine that. We could do a better job in the ACPI code than we currently do, but (unfortunately) that requires us to know the ACPI IDs that each GPU vendor uses.

Quoted 7 lines

This series tries to solve this problem with a much simpler approach:
Instead of moving backlights into DRM, we simply link DRM properties to a backlight device. That is, the kernel manages a link between a connector and a backlight device (or n backlight devices) which can be modified by udev in case the kernel got it wrong (we don't want huge board-fixup-tables in the kernel). User-space can now use the simpl DRM API to manage backlights, and the kernel does not need any special driver code to make it work.

This doesn't really simplify userspace significantly - something's still going to have to make the same policy decision as we do right now, and the kernel isn't really the right place to do that.

Hi David,

I am currently investigating:
https://bugs.freedesktop.org/show_bug.cgi?id=96572

Martin Peres suggested that your patches:
https://lists.freedesktop.org/archives/dri-devel/2014-September/thread.html#67984
could solve the xf86-video-modesetting backlight issues.

I have rebased your patches and I am working on an IGT test for the functionality. With my i915 implementation and the small included bug-fix, I can update the drm BACKLIGHT property and the value is updated in the backlight class device. However, if I set the brigness value through the sysfs file of the backlight class device the drm BRIGHTNESS property does not update which would be confusing to users.

My understanding is that DRM properties are cached and, by design, do not have the capability to read the status from the driver.

What do we want to do about this?