drm: Add support for backlight control of eDP panel on ti-sn65dsi86 bridge

The backlight level of an eDP panel can be controlled through the AUX channel using DPCD registers of the panel.

The capability for the Source device to adjust backlight characteristics within the panel, using the Sink device DPCD registers is indicated by the TCON_BACKLIGHT_ADJUSTMENT_CAPABLE bit in the EDP_GENERAL_CAPABILITY_1 register (DPCD Address 701h, bit0). In this configuration, the eDP TCON receives the backlight level information from the host, through the AUX channel.

The changes in this patch series do the following:
- Add drm_dp_aux_backlight_ APIs to support backlight control using DPCD registers on the DisplayPort AUX channel.
The current version only supports backlight brightness control by the EDP_BACKLIGHT_BRIGHTNESS_MSB/LSB registers (DPCD Addresses 722h-723h).
- Add support for backlight control of the eDP panel connected to the ti-sn65dsi86 bridge.

Rajeev Nandan (3):
drm/dp: Add DisplayPort aux backlight control support
dt-bindings: drm/bridge: ti-sn65dsi86: Document use-aux-backlight drm/bridge: ti-sn65dsi86: Add DisplayPort aux backlight support

 .../bindings/display/bridge/ti,sn65dsi86.yaml      |   8 +
 drivers/gpu/drm/Kconfig                            |   8 +
 drivers/gpu/drm/Makefile                           |   1 +
 drivers/gpu/drm/bridge/Kconfig                     |   1 +
 drivers/gpu/drm/bridge/ti-sn65dsi86.c              |  26 +++
 drivers/gpu/drm/drm_dp_aux_backlight.c             | 191 +++++++++++++++++++++
 include/drm/drm_dp_aux_backlight.h                 |  29 ++++
 7 files changed, 264 insertions(+)
 create mode 100644 drivers/gpu/drm/drm_dp_aux_backlight.c
 create mode 100644 include/drm/drm_dp_aux_backlight.h

2.7.4

Add panel backlight control using DPCD registers on the DisplayPort aux channel.

---
 drivers/gpu/drm/Kconfig                |   8 ++
 drivers/gpu/drm/Makefile               |   1 +
 drivers/gpu/drm/drm_dp_aux_backlight.c | 191 +++++++++++++++++++++++++++++++++
 include/drm/drm_dp_aux_backlight.h     |  29 +++++
 4 files changed, 229 insertions(+)
 create mode 100644 drivers/gpu/drm/drm_dp_aux_backlight.c
 create mode 100644 include/drm/drm_dp_aux_backlight.h

index 85b79a7f..01f8a48 100644
@@ -177,6 +177,14 @@ config DRM_DP_CEC
 	  Note: not all adapters support this feature, and even for those
 	  that do support this they often do not hook up the CEC pin.
 
+config DRM_DP_AUX_BACKLIGHT
+	bool "Enable DisplayPort aux backlight control support"
+	depends on DRM
+	select DRM_KMS_HELPER
+	help
+	  Choose this option if you want to use panel backlight control
+	  using DPCD registers on the DisplayPort aux channel.
+
 config DRM_TTM
 	tristate
 	depends on DRM && MMU

index 926adef..e41e40f 100644
@@ -53,6 +53,7 @@ drm_kms_helper-$(CONFIG_DRM_FBDEV_EMULATION) += drm_fb_helper.o
 drm_kms_helper-$(CONFIG_DRM_KMS_CMA_HELPER) += drm_fb_cma_helper.o
 drm_kms_helper-$(CONFIG_DRM_DP_AUX_CHARDEV) += drm_dp_aux_dev.o
 drm_kms_helper-$(CONFIG_DRM_DP_CEC) += drm_dp_cec.o
+drm_kms_helper-$(CONFIG_DRM_DP_AUX_BACKLIGHT) += drm_dp_aux_backlight.o
 
 obj-$(CONFIG_DRM_KMS_HELPER) += drm_kms_helper.o
 obj-$(CONFIG_DRM_DEBUG_SELFTEST) += selftests/

new file mode 100644
index 0000000..23cc554
@@ -0,0 +1,29 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#ifndef _DRM_DP_AUX_BACKLIGHT_H_
+#define _DRM_DP_AUX_BACKLIGHT_H_
+
+#include <linux/backlight.h>
+#include <drm/drm_dp_helper.h>
+
+/**
+ * struct drm_dp_aux_backlight - DisplayPort aux backlight
+ * @dev: the device to register
+ * @aux: the DisplayPort aux channel
+ * @bd: the backlight device
+ * @enabled: true if backlight is enabled else false.
+ */
+struct drm_dp_aux_backlight {
+	struct device *dev;
+	struct drm_dp_aux *aux;
+	struct backlight_device *bd;
+	bool enabled;
+};
+
+int drm_dp_aux_backlight_enable(struct drm_dp_aux_backlight *aux_bl);
+int drm_dp_aux_backlight_disable(struct drm_dp_aux_backlight *aux_bl);
+
+int drm_dp_aux_backlight_register(const char *name,
+				struct drm_dp_aux_backlight *aux_bl);
+
+#endif /* _DRM_DP_AUX_BACKLIGHT_H_ */

2.7.4

If the panel connected to the bridge supports backlight control using DPCD registers on the DisplayPort aux channel, setting "use-aux-backlight" property in the bridge node will enable the registration of a DP aux backlight device from the bridge driver.

---
 .../devicetree/bindings/display/bridge/ti,sn65dsi86.yaml          | 8 ++++++++
 1 file changed, 8 insertions(+)

index 26932d2..c8d8c00 100644
@@ -58,6 +58,12 @@ properties:
   clock-names:
     const: refclk
 
+  use-aux-backlight:
+    type: boolean
+    description:
+      The panel backlight to be controlled using DPCD registers on
+      the DP aux channel.
+
   gpio-controller: true
   '#gpio-cells':
     const: 2

@@ -188,6 +194,8 @@ examples:
 
         no-hpd;
 
+        use-aux-backlight;
+
         ports {
           #address-cells = <1>;
           #size-cells = <0>;

2.7.4

Add support to control the backlight of the eDP panel connected to the ti-sn65dsi86 bridge through aux channel.

---
 drivers/gpu/drm/bridge/Kconfig        |  1 +
 drivers/gpu/drm/bridge/ti-sn65dsi86.c | 26 ++++++++++++++++++++++++++
 2 files changed, 27 insertions(+)

index e4110d6..e556ec22 100644
@@ -239,6 +239,7 @@ config DRM_TI_SN65DSI86
 	select REGMAP_I2C
 	select DRM_PANEL
 	select DRM_MIPI_DSI
+	select DRM_DP_AUX_BACKLIGHT
 	help
 	  Texas Instruments SN65DSI86 DSI to eDP Bridge driver
 

index f27306c..813d067 100644
@@ -22,6 +22,7 @@
 #include <drm/drm_atomic.h>
 #include <drm/drm_atomic_helper.h>
 #include <drm/drm_bridge.h>
+#include <drm/drm_dp_aux_backlight.h>
 #include <drm/drm_dp_helper.h>
 #include <drm/drm_mipi_dsi.h>
 #include <drm/drm_of.h>

@@ -126,6 +127,7 @@
  * @panel:        Our panel.
  * @enable_gpio:  The GPIO we toggle to enable the bridge.
  * @supplies:     Data for bulk enabling/disabling our regulators.
+ * @backlight:    The DP aux backlight device.
  * @dp_lanes:     Count of dp_lanes we're using.
  * @ln_assign:    Value to program to the LN_ASSIGN register.
  * @ln_polrs:     Value for the 4-bit LN_POLRS field of SN_ENH_FRAME_REG.

@@ -154,6 +156,7 @@ struct ti_sn_bridge {
 	struct drm_panel		*panel;
 	struct gpio_desc		*enable_gpio;
 	struct regulator_bulk_data	supplies[SN_REGULATOR_SUPPLY_NUM];
+	struct drm_dp_aux_backlight	*backlight;
 	int				dp_lanes;
 	u8				ln_assign;
 	u8				ln_polrs;

@@ -431,6 +434,8 @@ static void ti_sn_bridge_disable(struct drm_bridge *bridge)
 {
 	struct ti_sn_bridge *pdata = bridge_to_ti_sn_bridge(bridge);
 
+	drm_dp_aux_backlight_disable(pdata->backlight);
+
 	drm_panel_disable(pdata->panel);
 
 	/* disable video stream */

@@ -819,6 +824,8 @@ static void ti_sn_bridge_enable(struct drm_bridge *bridge)
 			   VSTREAM_ENABLE);
 
 	drm_panel_enable(pdata->panel);
+
+	drm_dp_aux_backlight_enable(pdata->backlight);
 }
 
 static void ti_sn_bridge_pre_enable(struct drm_bridge *bridge)

@@ -1215,6 +1222,7 @@ static int ti_sn_bridge_probe(struct i2c_client *client,
 			      const struct i2c_device_id *id)
 {
 	struct ti_sn_bridge *pdata;
+	struct drm_dp_aux_backlight *aux_bl;
 	int ret;
 
 	if (!i2c_check_functionality(client->adapter, I2C_FUNC_I2C)) {

@@ -1294,9 +1302,27 @@ static int ti_sn_bridge_probe(struct i2c_client *client,
 
 	drm_bridge_add(&pdata->bridge);
 
+	if (of_find_property(pdata->dev->of_node, "use-aux-backlight", NULL)) {
+		aux_bl = devm_kzalloc(pdata->dev, sizeof(*aux_bl), GFP_KERNEL);
+		if (!aux_bl) {
+			ret = -ENOMEM;
+			goto out;
+		}
+		aux_bl->dev = pdata->dev;
+		aux_bl->aux = &pdata->aux;
+		ret = drm_dp_aux_backlight_register("ti-sn-aux-backlight", aux_bl);
+		if (ret)
+			goto out;
+		pdata->backlight = aux_bl;
+	}
+
 	ti_sn_debugfs_init(pdata);
 
 	return 0;
+
+out:
+	pm_runtime_disable(pdata->dev);
+	return ret;
 }
 
 static int ti_sn_bridge_remove(struct i2c_client *client)

2.7.4

Hi,

So I haven't looked in massive detail at this patch series, but the fact that it's touching "ti-sn65dsi86.c" is a red flag. I know in out-of-band communications you said you weren't sure how to do better. ...but, perhaps, if folks don't hate my recent series [1] there may be a way forward.

I wonder if perhaps now that the AUX channel can be registered early if it gets around the circular dependency problems and now you can put your code in some combination of the panel code and (maybe?) a new backlight driver if it's generic enough.

It's possible that you might need to add some code to be able to look up a "struct drm_dp_aux *" from a device tree node and you might need to add a new device tree property like "ddc-aux-bus" in order to do this, but I don't _think_ that would be controversial?

[1] https://lore.kernel.org/r/[email protected]

-Doug
_______________________________________________
dri-devel mailing list
[email protected]
https://lists.freedesktop.org/mailman/listinfo/dri-devel

i915 has had this capability for some years now, and work is in progress to extract the DP AUX backlight code to drm core as helpers [1]. There's much more to it than what's proposed here. Adding incompatible DP AUX code at this point would be a pretty bad outcome.

For example, we can't tie backlight device register to DP AUX backlight, because there are modes where *both* the eDP PWM pin based backlight control and DP AUX backlight control are used *simultaneously*. The backlight device register needs to be in code that is aware of both.

Granted, it was a mistake way back when to add this in i915 only, and it should've been lifted to drm much earlier. It would've been done by Lyude by now, but people were not happy about not using drm device based logging. And that has unfortunately lead to a pretty massive prep series [2].

Please look into the code added to drm helpers in [1], and see how that would work for you.

BR,
Jani.

[1] http://lore.kernel.org/r/[email protected]
[2] http://lore.kernel.org/r/[email protected]

Jani Nikula, Intel Open Source Graphics Center