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

Add bindings for DisplayPort aux backlight driver.

Changes in v2:
- New

Signed-off-by Rajeev Nandan <[email protected]>

---
 .../bindings/leds/backlight/dp-aux-backlight.yaml  | 49 ++++++++++++++++++++++
 1 file changed, 49 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml

Add bindings for DisplayPort aux backlight driver.

Changes in v2:
- New

Signed-off-by Rajeev Nandan <[email protected]>

---
 .../bindings/leds/backlight/dp-aux-backlight.yaml  | 49 ++++++++++++++++++++++
 1 file changed, 49 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml

Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml new file +35 -0

new file mode 100644
index 00000000..0fa8bf0
@@ -0,0 +1,49 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/leds/backlight/dp-aux-backlight.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: DisplayPort aux backlight driver bindings
+
+maintainers:
+  - Rajeev Nandan <[email protected]>
+
+description:
+  Backlight driver to control the brightness over DisplayPort aux channel.
+
+allOf:
+  - $ref: common.yaml#
+
+properties:
+  compatible:
+    const: dp-aux-backlight
+
+  ddc-i2c-bus:
+    $ref: /schemas/types.yaml#/definitions/phandle
+    description:
+      A phandle to the system I2C controller connected to the DDC bus used
+      for the DisplayPort AUX channel.
+
+  enable-gpios:
+    maxItems: 1
+    description: GPIO specifier for backlight enable pin.
+
+  max-brightness: true
+
+required:
+  - compatible

Quoted 52 lines · Rajeev Nandan

Add bindings for DisplayPort aux backlight driver.

Changes in v2:
- New

Signed-off-by Rajeev Nandan <[email protected]>

---
 .../bindings/leds/backlight/dp-aux-backlight.yaml  | 49 ++++++++++++++++++++++
 1 file changed, 49 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml

Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml new file +35 -0

new file mode 100644
index 00000000..0fa8bf0
@@ -0,0 +1,49 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/leds/backlight/dp-aux-backlight.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: DisplayPort aux backlight driver bindings
+
+maintainers:
+  - Rajeev Nandan <[email protected]>
+
+description:
+  Backlight driver to control the brightness over DisplayPort aux channel.
+
+allOf:
+  - $ref: common.yaml#
+
+properties:
+  compatible:
+    const: dp-aux-backlight
+
+  ddc-i2c-bus:
+    $ref: /schemas/types.yaml#/definitions/phandle
+    description:
+      A phandle to the system I2C controller connected to the DDC bus used
+      for the DisplayPort AUX channel.
+
+  enable-gpios:
+    maxItems: 1
+    description: GPIO specifier for backlight enable pin.
+
+  max-brightness: true
+
+required:
+  - compatible

Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml line 45

Quoted 10 lines

Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml new file +10 -0

+  - ddc-i2c-bus
+
+additionalProperties: false
+
+examples:
+  - |
+    backlight {
+        compatible = "dp-aux-backlight";
+        ddc-i2c-bus = <&sn65dsi86_bridge>;
+        enable-gpios = <&tlmm 12 GPIO_ACTIVE_HIGH>;

So the DDC bus is connected to a backlight and also a panel? This binding is not reflecting the h/w, but rather what you want for some driver.

There's only one thing here and that's an eDP panel which supports backlight control via DP aux channel. You can figure all that out from the panel's compatible and/or reading the EDID.

You might also be interested in this thread:

https://lore.kernel.org/lkml/[email protected]/

Hi,

Quoted 69 lines · Rob Herring

Quoted 52 lines · Rajeev Nandan

Add bindings for DisplayPort aux backlight driver.

Changes in v2:
- New

Signed-off-by Rajeev Nandan <[email protected]>

---
 .../bindings/leds/backlight/dp-aux-backlight.yaml  | 49 ++++++++++++++++++++++
 1 file changed, 49 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml

Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml new file +35 -0

new file mode 100644
index 00000000..0fa8bf0
@@ -0,0 +1,49 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/leds/backlight/dp-aux-backlight.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: DisplayPort aux backlight driver bindings
+
+maintainers:
+  - Rajeev Nandan <[email protected]>
+
+description:
+  Backlight driver to control the brightness over DisplayPort aux channel.
+
+allOf:
+  - $ref: common.yaml#
+
+properties:
+  compatible:
+    const: dp-aux-backlight
+
+  ddc-i2c-bus:
+    $ref: /schemas/types.yaml#/definitions/phandle
+    description:
+      A phandle to the system I2C controller connected to the DDC bus used
+      for the DisplayPort AUX channel.
+
+  enable-gpios:
+    maxItems: 1
+    description: GPIO specifier for backlight enable pin.
+
+  max-brightness: true
+
+required:
+  - compatible

Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml line 45

Quoted 10 lines

Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml new file +10 -0

+  - ddc-i2c-bus
+
+additionalProperties: false
+
+examples:
+  - |
+    backlight {
+        compatible = "dp-aux-backlight";
+        ddc-i2c-bus = <&sn65dsi86_bridge>;
+        enable-gpios = <&tlmm 12 GPIO_ACTIVE_HIGH>;

So the DDC bus is connected to a backlight and also a panel? This binding is not reflecting the h/w, but rather what you want for some driver.

Quoted 7 lines

There's only one thing here and that's an eDP panel which supports backlight control via DP aux channel. You can figure all that out from the panel's compatible and/or reading the EDID.

You might also be interested in this thread:

https://lore.kernel.org/lkml/[email protected]/

I think Rajeev needs to rework everything anyway as per:

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

...but you're right that it makes sense not to model the backlight as a separate node in the device tree. The panel driver can handle setting up the backlight.

-Doug

Quoted 82 lines · Doug Anderson

Hi,

Quoted 69 lines · Rob Herring

Quoted 52 lines · Rajeev Nandan

Add bindings for DisplayPort aux backlight driver.

Changes in v2:
- New

Signed-off-by Rajeev Nandan <[email protected]>

---
 .../bindings/leds/backlight/dp-aux-backlight.yaml  | 49 ++++++++++++++++++++++
 1 file changed, 49 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml

Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml new file +35 -0

new file mode 100644
index 00000000..0fa8bf0
@@ -0,0 +1,49 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/leds/backlight/dp-aux-backlight.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: DisplayPort aux backlight driver bindings
+
+maintainers:
+  - Rajeev Nandan <[email protected]>
+
+description:
+  Backlight driver to control the brightness over DisplayPort aux channel.
+
+allOf:
+  - $ref: common.yaml#
+
+properties:
+  compatible:
+    const: dp-aux-backlight
+
+  ddc-i2c-bus:
+    $ref: /schemas/types.yaml#/definitions/phandle
+    description:
+      A phandle to the system I2C controller connected to the DDC bus used
+      for the DisplayPort AUX channel.
+
+  enable-gpios:
+    maxItems: 1
+    description: GPIO specifier for backlight enable pin.
+
+  max-brightness: true
+
+required:
+  - compatible

Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml line 45

Quoted 10 lines

Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml new file +10 -0

+  - ddc-i2c-bus
+
+additionalProperties: false
+
+examples:
+  - |
+    backlight {
+        compatible = "dp-aux-backlight";
+        ddc-i2c-bus = <&sn65dsi86_bridge>;
+        enable-gpios = <&tlmm 12 GPIO_ACTIVE_HIGH>;

So the DDC bus is connected to a backlight and also a panel? This binding is not reflecting the h/w, but rather what you want for some driver.

Quoted 7 lines

There's only one thing here and that's an eDP panel which supports backlight control via DP aux channel. You can figure all that out from the panel's compatible and/or reading the EDID.

You might also be interested in this thread:

https://lore.kernel.org/lkml/[email protected]/

I think Rajeev needs to rework everything anyway as per:

Quoted 7 lines

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

...but you're right that it makes sense not to model the backlight as a separate node in the device tree. The panel driver can handle setting up the backlight.

-Doug

It was not a good idea to create a separate backlight driver and use ddc-i2c-bus to get access to DP aux. I am working to move the code to the panel driver and to utilize the new DRM helper functions (drm_edp_backlight_*) Lyude has added [1].

To use these helper functions, the panel driver should have access to the
"struct drm_dp_aux *". The simple-panel has a "ddc-i2c-bus" property to give the panel access to the DDC bus and is currently being used to get the EDID from the panel. Can I use the same ddc bus i2c_adapter to get
the "struct drm_dp_aux *"?

As per the suggestion [2], I get the "struct drm_dp_aux *" from the i2c_adapter of ddc bus (maybe I didn't understand the suggestion correctly),
and, it turned out, the way I have implemented is not the right way [3]. So, I am afraid to use the same method in the panel driver.

[1] https://lore.kernel.org/dri-devel/[email protected]/
[2] https://www.spinics.net/lists/dri-devel/msg295429.html
[3]
https://lore.kernel.org/dri-devel/[email protected]/

Hi,

Quoted 112 lines · On Fri, Apr 30, 2021 at 8:10 AM <[email protected]> wrote:

Quoted 82 lines · Doug Anderson

Hi,

Quoted 69 lines · Rob Herring

Quoted 52 lines · Rajeev Nandan

Add bindings for DisplayPort aux backlight driver.

Changes in v2:
- New

Signed-off-by Rajeev Nandan <[email protected]>

---
 .../bindings/leds/backlight/dp-aux-backlight.yaml  | 49 ++++++++++++++++++++++
 1 file changed, 49 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml

Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml new file +35 -0

new file mode 100644
index 00000000..0fa8bf0
@@ -0,0 +1,49 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/leds/backlight/dp-aux-backlight.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: DisplayPort aux backlight driver bindings
+
+maintainers:
+  - Rajeev Nandan <[email protected]>
+
+description:
+  Backlight driver to control the brightness over DisplayPort aux channel.
+
+allOf:
+  - $ref: common.yaml#
+
+properties:
+  compatible:
+    const: dp-aux-backlight
+
+  ddc-i2c-bus:
+    $ref: /schemas/types.yaml#/definitions/phandle
+    description:
+      A phandle to the system I2C controller connected to the DDC bus used
+      for the DisplayPort AUX channel.
+
+  enable-gpios:
+    maxItems: 1
+    description: GPIO specifier for backlight enable pin.
+
+  max-brightness: true
+
+required:
+  - compatible

Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml line 45

Quoted 10 lines

Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml new file +10 -0

+  - ddc-i2c-bus
+
+additionalProperties: false
+
+examples:
+  - |
+    backlight {
+        compatible = "dp-aux-backlight";
+        ddc-i2c-bus = <&sn65dsi86_bridge>;
+        enable-gpios = <&tlmm 12 GPIO_ACTIVE_HIGH>;

So the DDC bus is connected to a backlight and also a panel? This binding is not reflecting the h/w, but rather what you want for some driver.

Quoted 7 lines

There's only one thing here and that's an eDP panel which supports backlight control via DP aux channel. You can figure all that out from the panel's compatible and/or reading the EDID.

You might also be interested in this thread:

https://lore.kernel.org/lkml/[email protected]/

I think Rajeev needs to rework everything anyway as per:

Quoted 7 lines

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

...but you're right that it makes sense not to model the backlight as a separate node in the device tree. The panel driver can handle setting up the backlight.

-Doug

It was not a good idea to create a separate backlight driver and use ddc-i2c-bus to get access to DP aux. I am working to move the code to the panel driver and to utilize the new DRM helper functions (drm_edp_backlight_*) Lyude has added [1].

To use these helper functions, the panel driver should have access to the
"struct drm_dp_aux *". The simple-panel has a "ddc-i2c-bus" property to give the panel access to the DDC bus and is currently being used to get the EDID from the panel. Can I use the same ddc bus i2c_adapter to get
the "struct drm_dp_aux *"?

As per the suggestion [2], I get the "struct drm_dp_aux *" from the i2c_adapter of ddc bus (maybe I didn't understand the suggestion correctly),
and, it turned out, the way I have implemented is not the right way [3]. So, I am afraid to use the same method in the panel driver.

Quoted 4 lines

[1] https://lore.kernel.org/dri-devel/[email protected]/
[2] https://www.spinics.net/lists/dri-devel/msg295429.html
[3]
https://lore.kernel.org/dri-devel/[email protected]/

So it's definitely up to maintainers, not me. ...but I guess I would have expected something like a new property called "ddc-aux-bus". Then you'd have to create a new API call called something like "of_find_ddc_aux_adapter_by_node()" that would allow you to find it.

I guess an alternate way to solve this (I'm not totally sure whether it's better or worse) would be to add a function that would walk up the chain of parent bridges and ask them for a pointer to the aux bus. I definitely haven't thought it all the way through, but I'd imagine something like drm_bridge_chain_get_ddc_aux(). This is _probably_ better than adding the "ddc-aux-bus" property but it assumes that the aux bus is provided by one of our parents. Hrm, looking at this briefly, though, I'm not sure how to do it. It doesn't seem possible to get the parent bridges from the panel structure. Even if you assume that your parent is wrapping you with a panel_bridge it still doesn't seem possible?

This probably needs more drm-expertise.

-Doug

Quoted 109 lines · Doug Anderson

Quoted 103 lines · On Fri, Apr 30, 2021 at 8:10 AM <[email protected]> wrote:

Quoted 77 lines · Doug Anderson

Quoted 68 lines · Rob Herring

Quoted 52 lines · Rajeev Nandan

Add bindings for DisplayPort aux backlight driver.

Changes in v2:
- New

Signed-off-by Rajeev Nandan <[email protected]>

---
 .../bindings/leds/backlight/dp-aux-backlight.yaml  | 49 ++++++++++++++++++++++
 1 file changed, 49 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml

Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml new file +35 -0

new file mode 100644
index 00000000..0fa8bf0
@@ -0,0 +1,49 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/leds/backlight/dp-aux-backlight.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: DisplayPort aux backlight driver bindings
+
+maintainers:
+  - Rajeev Nandan <[email protected]>
+
+description:
+  Backlight driver to control the brightness over DisplayPort aux channel.
+
+allOf:
+  - $ref: common.yaml#
+
+properties:
+  compatible:
+    const: dp-aux-backlight
+
+  ddc-i2c-bus:
+    $ref: /schemas/types.yaml#/definitions/phandle
+    description:
+      A phandle to the system I2C controller connected to the DDC bus used
+      for the DisplayPort AUX channel.
+
+  enable-gpios:
+    maxItems: 1
+    description: GPIO specifier for backlight enable pin.
+
+  max-brightness: true
+
+required:
+  - compatible

Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml line 45

Quoted 10 lines

Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml new file +10 -0

+  - ddc-i2c-bus
+
+additionalProperties: false
+
+examples:
+  - |
+    backlight {
+        compatible = "dp-aux-backlight";
+        ddc-i2c-bus = <&sn65dsi86_bridge>;
+        enable-gpios = <&tlmm 12 GPIO_ACTIVE_HIGH>;

So the DDC bus is connected to a backlight and also a panel? This binding is not reflecting the h/w, but rather what you want for some driver.

Quoted 7 lines

There's only one thing here and that's an eDP panel which supports backlight control via DP aux channel. You can figure all that out from the panel's compatible and/or reading the EDID.

You might also be interested in this thread:

https://lore.kernel.org/lkml/[email protected]/

Quoted 7 lines

I think Rajeev needs to rework everything anyway as per:

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

...but you're right that it makes sense not to model the backlight as a separate node in the device tree. The panel driver can handle setting up the backlight.

It was not a good idea to create a separate backlight driver and use ddc-i2c-bus to get access to DP aux. I am working to move the code to the panel driver and to utilize the new DRM helper functions (drm_edp_backlight_*) Lyude has added [1].

To use these helper functions, the panel driver should have access to the "struct drm_dp_aux *". The simple-panel has a "ddc-i2c-bus" property to give the panel access to the DDC bus and is currently being used to get the EDID from the panel. Can I use the same ddc bus i2c_adapter to get the "struct drm_dp_aux *"?

As per the suggestion [2], I get the "struct drm_dp_aux *" from the i2c_adapter of ddc bus (maybe I didn't understand the suggestion correctly), and, it turned out, the way I have implemented is not the right way [3]. So, I am afraid to use the same method in the panel driver.

Quoted 4 lines

[1] https://lore.kernel.org/dri-devel/[email protected]/
[2] https://www.spinics.net/lists/dri-devel/msg295429.html
[3]
https://lore.kernel.org/dri-devel/[email protected]/

Quoted 4 lines

So it's definitely up to maintainers, not me. ...but I guess I would have expected something like a new property called "ddc-aux-bus". Then you'd have to create a new API call called something like "of_find_ddc_aux_adapter_by_node()" that would allow you to find it.

To implement the first suggestion, I can think of the following way to get the "struct drm_dp_aux" in the panel_simple_probe function:

- Create a new panel-simple DT property "ddc-aux-bus", a phandle to the platform device that implements the AUX channel.

- Create a global list of drm_dp_aux in drm_dp_helper.c. Initialize list head
in drm_dp_aux_init(), add the drm_dp_aux onto the list in drm_dp_aux_register().
Similarly, remove the drm_dp_aux from list in drm_dp_aux_unregister().

- Create a new function of_drm_find_dp_aux_by_node() to get the expected drm_dp_aux from this global list.

Please let me know your views on this implementation.

Below is the summary of the changes in drm dp helper:

---

// drm_dp_helper.h

struct drm_dp_aux {
	...
	struct list_head list;
	...
}

// drm_dp_helper.c

static DEFINE_MUTEX(dp_aux_lock);
static LIST_HEAD(dp_aux_list);

static void drm_dp_aux_add(struct drm_dp_aux *aux)

{
     mutex_lock(&dp_aux_lock);
     list_add_tail(&aux->list, &dp_aux_list);
     mutex_unlock(&dp_aux_lock);
}

static void drm_dp_aux_remove(struct drm_dp_aux *aux)

{
     mutex_lock(&dp_aux_lock);
     list_del_init(&aux->list);
     mutex_unlock(&dp_aux_lock);
}

#ifdef CONFIG_OF
struct drm_dp_aux *of_drm_find_dp_aux_by_node(struct device_node *np)

{
     struct drm_dp_aux *aux;
     mutex_lock(&dp_aux_lock);

     list_for_each_entry(aux, &dp_aux_list, list) {
         if (aux->dev->of_node == np) {
             mutex_unlock(&dp_aux_lock);
             return aux;
         }
     }

     mutex_unlock(&dp_aux_lock);
     return NULL;
}

EXPORT_SYMBOL(of_drm_find_dp_aux_by_node);
#endif

int drm_dp_aux_init(struct drm_dp_aux *aux)

{
     INIT_LIST_HEAD(&aux->list);
     ...
}

int drm_dp_aux_register(struct drm_dp_aux *aux)

{
     ...
     drm_dp_aux_add(aux);

     return 0;
}

void drm_dp_aux_unregister(struct drm_dp_aux *aux)

{
     drm_dp_aux_remove(aux);
     ...
}

Quoted 13 lines

I guess an alternate way to solve this (I'm not totally sure whether it's better or worse) would be to add a function that would walk up the chain of parent bridges and ask them for a pointer to the aux bus. I definitely haven't thought it all the way through, but I'd imagine something like drm_bridge_chain_get_ddc_aux(). This is _probably_ better than adding the "ddc-aux-bus" property but it assumes that the aux bus is provided by one of our parents. Hrm, looking at this briefly, though, I'm not sure how to do it. It doesn't seem possible to get the parent bridges from the panel structure. Even if you assume that your parent is wrapping you with a panel_bridge it still doesn't seem possible?

This probably needs more drm-expertise.

Quoted 121 lines · Doug Anderson

Hi,

Quoted 113 lines · On Fri, Apr 30, 2021 at 8:10 AM <[email protected]> wrote:

Quoted 82 lines · Doug Anderson

Hi,

Quoted 69 lines · Rob Herring

Quoted 52 lines · Rajeev Nandan

Add bindings for DisplayPort aux backlight driver.

Changes in v2:
- New

Signed-off-by Rajeev Nandan <[email protected]>

---
 .../bindings/leds/backlight/dp-aux-backlight.yaml  | 49 ++++++++++++++++++++++
 1 file changed, 49 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml

Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml new file +35 -0

new file mode 100644
index 00000000..0fa8bf0
@@ -0,0 +1,49 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/leds/backlight/dp-aux-backlight.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: DisplayPort aux backlight driver bindings
+
+maintainers:
+  - Rajeev Nandan <[email protected]>
+
+description:
+  Backlight driver to control the brightness over DisplayPort aux channel.
+
+allOf:
+  - $ref: common.yaml#
+
+properties:
+  compatible:
+    const: dp-aux-backlight
+
+  ddc-i2c-bus:
+    $ref: /schemas/types.yaml#/definitions/phandle
+    description:
+      A phandle to the system I2C controller connected to the DDC bus used
+      for the DisplayPort AUX channel.
+
+  enable-gpios:
+    maxItems: 1
+    description: GPIO specifier for backlight enable pin.
+
+  max-brightness: true
+
+required:
+  - compatible

Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml line 45

Quoted 10 lines

Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml new file +10 -0

+  - ddc-i2c-bus
+
+additionalProperties: false
+
+examples:
+  - |
+    backlight {
+        compatible = "dp-aux-backlight";
+        ddc-i2c-bus = <&sn65dsi86_bridge>;
+        enable-gpios = <&tlmm 12 GPIO_ACTIVE_HIGH>;

So the DDC bus is connected to a backlight and also a panel? This binding is not reflecting the h/w, but rather what you want for some driver.

Quoted 7 lines

There's only one thing here and that's an eDP panel which supports backlight control via DP aux channel. You can figure all that out from the panel's compatible and/or reading the EDID.

You might also be interested in this thread:

https://lore.kernel.org/lkml/[email protected]/

I think Rajeev needs to rework everything anyway as per:

Quoted 7 lines

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

...but you're right that it makes sense not to model the backlight as a separate node in the device tree. The panel driver can handle setting up the backlight.

-Doug

It was not a good idea to create a separate backlight driver and use ddc-i2c-bus to get access to DP aux. I am working to move the code to the panel driver and to utilize the new DRM helper functions (drm_edp_backlight_*) Lyude has added [1].

To use these helper functions, the panel driver should have access to the
"struct drm_dp_aux *". The simple-panel has a "ddc-i2c-bus" property to give the panel access to the DDC bus and is currently being used to get the EDID from the panel. Can I use the same ddc bus i2c_adapter to get
the "struct drm_dp_aux *"?

As per the suggestion [2], I get the "struct drm_dp_aux *" from the i2c_adapter of ddc bus (maybe I didn't understand the suggestion correctly),
and, it turned out, the way I have implemented is not the right way [3].
So, I am afraid to use the same method in the panel driver.

Quoted 4 lines

[1] https://lore.kernel.org/dri-devel/[email protected]/
[2] https://www.spinics.net/lists/dri-devel/msg295429.html
[3]
https://lore.kernel.org/dri-devel/[email protected]/

Quoted 5 lines

So it's definitely up to maintainers, not me. ...but I guess I would have expected something like a new property called "ddc-aux-bus". Then you'd have to create a new API call called something like "of_find_ddc_aux_adapter_by_node()" that would allow you to find it.

To implement the first suggestion, I can think of the following way to get the "struct drm_dp_aux" in the panel_simple_probe function:

- Create a new panel-simple DT property "ddc-aux-bus", a phandle to the platform device that implements the AUX channel.

- Create a global list of drm_dp_aux in drm_dp_helper.c. Initialize list head
in drm_dp_aux_init(), add the drm_dp_aux onto the list in drm_dp_aux_register().
Similarly, remove the drm_dp_aux from list in drm_dp_aux_unregister().

- Create a new function of_drm_find_dp_aux_by_node() to get the expected drm_dp_aux from this global list.

Please let me know your views on this implementation.

Hi,

Quoted 141 lines · On Tue, May 11, 2021 at 11:12 AM <[email protected]> wrote:

Quoted 121 lines · Doug Anderson

Hi,

Quoted 113 lines · On Fri, Apr 30, 2021 at 8:10 AM <[email protected]> wrote:

Quoted 82 lines · Doug Anderson

Hi,

Quoted 69 lines · Rob Herring

Quoted 52 lines · Rajeev Nandan

Add bindings for DisplayPort aux backlight driver.

Changes in v2:
- New

Signed-off-by Rajeev Nandan <[email protected]>

---
 .../bindings/leds/backlight/dp-aux-backlight.yaml  | 49 ++++++++++++++++++++++
 1 file changed, 49 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml

Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml new file +35 -0

new file mode 100644
index 00000000..0fa8bf0
@@ -0,0 +1,49 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/leds/backlight/dp-aux-backlight.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: DisplayPort aux backlight driver bindings
+
+maintainers:
+  - Rajeev Nandan <[email protected]>
+
+description:
+  Backlight driver to control the brightness over DisplayPort aux channel.
+
+allOf:
+  - $ref: common.yaml#
+
+properties:
+  compatible:
+    const: dp-aux-backlight
+
+  ddc-i2c-bus:
+    $ref: /schemas/types.yaml#/definitions/phandle
+    description:
+      A phandle to the system I2C controller connected to the DDC bus used
+      for the DisplayPort AUX channel.
+
+  enable-gpios:
+    maxItems: 1
+    description: GPIO specifier for backlight enable pin.
+
+  max-brightness: true
+
+required:
+  - compatible

Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml line 45

Quoted 10 lines

Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml new file +10 -0

+  - ddc-i2c-bus
+
+additionalProperties: false
+
+examples:
+  - |
+    backlight {
+        compatible = "dp-aux-backlight";
+        ddc-i2c-bus = <&sn65dsi86_bridge>;
+        enable-gpios = <&tlmm 12 GPIO_ACTIVE_HIGH>;

So the DDC bus is connected to a backlight and also a panel? This binding is not reflecting the h/w, but rather what you want for some driver.

Quoted 7 lines

There's only one thing here and that's an eDP panel which supports backlight control via DP aux channel. You can figure all that out from the panel's compatible and/or reading the EDID.

You might also be interested in this thread:

https://lore.kernel.org/lkml/[email protected]/

I think Rajeev needs to rework everything anyway as per:

Quoted 7 lines

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

...but you're right that it makes sense not to model the backlight as a separate node in the device tree. The panel driver can handle setting up the backlight.

-Doug

It was not a good idea to create a separate backlight driver and use ddc-i2c-bus to get access to DP aux. I am working to move the code to the panel driver and to utilize the new DRM helper functions (drm_edp_backlight_*) Lyude has added [1].

To use these helper functions, the panel driver should have access to the
"struct drm_dp_aux *". The simple-panel has a "ddc-i2c-bus" property to give the panel access to the DDC bus and is currently being used to get the EDID from the panel. Can I use the same ddc bus i2c_adapter to get
the "struct drm_dp_aux *"?

As per the suggestion [2], I get the "struct drm_dp_aux *" from the i2c_adapter of ddc bus (maybe I didn't understand the suggestion correctly),
and, it turned out, the way I have implemented is not the right way [3].
So, I am afraid to use the same method in the panel driver.

Quoted 4 lines

[1] https://lore.kernel.org/dri-devel/[email protected]/
[2] https://www.spinics.net/lists/dri-devel/msg295429.html
[3]
https://lore.kernel.org/dri-devel/[email protected]/

Quoted 5 lines

So it's definitely up to maintainers, not me. ...but I guess I would have expected something like a new property called "ddc-aux-bus". Then you'd have to create a new API call called something like "of_find_ddc_aux_adapter_by_node()" that would allow you to find it.

To implement the first suggestion, I can think of the following way to get the "struct drm_dp_aux" in the panel_simple_probe function:

- Create a new panel-simple DT property "ddc-aux-bus", a phandle to the platform device that implements the AUX channel.

- Create a global list of drm_dp_aux in drm_dp_helper.c. Initialize list head
in drm_dp_aux_init(), add the drm_dp_aux onto the list in drm_dp_aux_register().
Similarly, remove the drm_dp_aux from list in drm_dp_aux_unregister().

Quoted 4 lines

- Create a new function of_drm_find_dp_aux_by_node() to get the expected drm_dp_aux from this global list.

Please let me know your views on this implementation.

BTW: a bunch of discussion today on IRC:

https://people.freedesktop.org/~cbrill/dri-log/?channel=dri-devel&highlight_names=&date=2021-05-11&show_html=true

Hi,

Quoted 145 lines · Doug Anderson

Hi,

Quoted 141 lines · On Tue, May 11, 2021 at 11:12 AM <[email protected]> wrote:

Quoted 121 lines · Doug Anderson

Hi,

Quoted 113 lines · On Fri, Apr 30, 2021 at 8:10 AM <[email protected]> wrote:

Quoted 82 lines · Doug Anderson

Hi,

Quoted 69 lines · Rob Herring

Quoted 52 lines · Rajeev Nandan

Add bindings for DisplayPort aux backlight driver.

Changes in v2:
- New

Signed-off-by Rajeev Nandan <[email protected]>

---
 .../bindings/leds/backlight/dp-aux-backlight.yaml  | 49 ++++++++++++++++++++++
 1 file changed, 49 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml

Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml new file +35 -0

new file mode 100644
index 00000000..0fa8bf0
@@ -0,0 +1,49 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/leds/backlight/dp-aux-backlight.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: DisplayPort aux backlight driver bindings
+
+maintainers:
+  - Rajeev Nandan <[email protected]>
+
+description:
+  Backlight driver to control the brightness over DisplayPort aux channel.
+
+allOf:
+  - $ref: common.yaml#
+
+properties:
+  compatible:
+    const: dp-aux-backlight
+
+  ddc-i2c-bus:
+    $ref: /schemas/types.yaml#/definitions/phandle
+    description:
+      A phandle to the system I2C controller connected to the DDC bus used
+      for the DisplayPort AUX channel.
+
+  enable-gpios:
+    maxItems: 1
+    description: GPIO specifier for backlight enable pin.
+
+  max-brightness: true
+
+required:
+  - compatible

Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml line 45

Quoted 10 lines

Documentation/devicetree/bindings/leds/backlight/dp-aux-backlight.yaml new file +10 -0

+  - ddc-i2c-bus
+
+additionalProperties: false
+
+examples:
+  - |
+    backlight {
+        compatible = "dp-aux-backlight";
+        ddc-i2c-bus = <&sn65dsi86_bridge>;
+        enable-gpios = <&tlmm 12 GPIO_ACTIVE_HIGH>;

So the DDC bus is connected to a backlight and also a panel? This binding is not reflecting the h/w, but rather what you want for some driver.

Quoted 7 lines

There's only one thing here and that's an eDP panel which supports backlight control via DP aux channel. You can figure all that out from the panel's compatible and/or reading the EDID.

You might also be interested in this thread:

https://lore.kernel.org/lkml/[email protected]/

I think Rajeev needs to rework everything anyway as per:

Quoted 7 lines

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

...but you're right that it makes sense not to model the backlight as a separate node in the device tree. The panel driver can handle setting up the backlight.

-Doug

It was not a good idea to create a separate backlight driver and use ddc-i2c-bus to get access to DP aux. I am working to move the code to the panel driver and to utilize the new DRM helper functions (drm_edp_backlight_*) Lyude has added [1].

To use these helper functions, the panel driver should have access to the
"struct drm_dp_aux *". The simple-panel has a "ddc-i2c-bus" property to give the panel access to the DDC bus and is currently being used to get the EDID from the panel. Can I use the same ddc bus i2c_adapter to get
the "struct drm_dp_aux *"?

As per the suggestion [2], I get the "struct drm_dp_aux *" from the i2c_adapter of ddc bus (maybe I didn't understand the suggestion correctly),
and, it turned out, the way I have implemented is not the right way [3].
So, I am afraid to use the same method in the panel driver.

Quoted 4 lines

[1] https://lore.kernel.org/dri-devel/[email protected]/
[2] https://www.spinics.net/lists/dri-devel/msg295429.html
[3]
https://lore.kernel.org/dri-devel/[email protected]/

Quoted 5 lines

So it's definitely up to maintainers, not me. ...but I guess I would have expected something like a new property called "ddc-aux-bus". Then you'd have to create a new API call called something like "of_find_ddc_aux_adapter_by_node()" that would allow you to find it.

To implement the first suggestion, I can think of the following way to get the "struct drm_dp_aux" in the panel_simple_probe function:

- Create a new panel-simple DT property "ddc-aux-bus", a phandle to the platform device that implements the AUX channel.

- Create a global list of drm_dp_aux in drm_dp_helper.c. Initialize list head
in drm_dp_aux_init(), add the drm_dp_aux onto the list in drm_dp_aux_register().
Similarly, remove the drm_dp_aux from list in drm_dp_aux_unregister().

Quoted 8 lines

Quoted 4 lines

- Create a new function of_drm_find_dp_aux_by_node() to get the expected drm_dp_aux from this global list.

Please let me know your views on this implementation.

BTW: a bunch of discussion today on IRC:

https://people.freedesktop.org/~cbrill/dri-log/?channel=dri-devel&highlight_names=&date=2021-05-11&show_html=true

To finish adding breadcrumbs to those playing at home, v7 of my patch series should get the AUX channel to the panel device:

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

Assuming that people don't hate that solution it may be a good things for you to build your patches atop. :-)

-Doug

Add backlight driver for the panels supporting backlight control using DPCD registers on the DisplayPort aux channel.

Changes in v2:
- New (most of the code reused from drm_dp_aux_backlight.c of v1)

Changes in v3:
- Add missing ';' to fix module compilation (kernel test bot)

Signed-off-by Rajeev Nandan <[email protected]>

---
 drivers/video/backlight/Kconfig            |   7 +
 drivers/video/backlight/Makefile           |   1 +
 drivers/video/backlight/dp_aux_backlight.c | 245 +++++++++++++++++++++++++++++
 3 files changed, 253 insertions(+)
 create mode 100644 drivers/video/backlight/dp_aux_backlight.c

drivers/video/backlight/Kconfig +7 -0

index d83c87b..82c88f0 100644
@@ -456,6 +456,13 @@ config BACKLIGHT_LED
 	  If you have a LCD backlight adjustable by LED class driver, say Y
 	  to enable this driver.
 
+config BACKLIGHT_DP_AUX
+       tristate "DisplayPort aux backlight driver"
+       depends on DRM && DRM_KMS_HELPER
+       help
+         If you have a panel backlight controlled by DPCD registers
+         on the DisplayPort aux channel, say Y to enable this driver.
+
 endif # BACKLIGHT_CLASS_DEVICE
 
 endmenu

drivers/video/backlight/Makefile +1 -0

index 685f3f1..ba23c7c 100644
@@ -57,3 +57,4 @@ obj-$(CONFIG_BACKLIGHT_WM831X)		+= wm831x_bl.o
 obj-$(CONFIG_BACKLIGHT_ARCXCNN) 	+= arcxcnn_bl.o
 obj-$(CONFIG_BACKLIGHT_RAVE_SP)		+= rave-sp-backlight.o
 obj-$(CONFIG_BACKLIGHT_LED)		+= led_bl.o
+obj-$(CONFIG_BACKLIGHT_DP_AUX)		+= dp_aux_backlight.o

drivers/video/backlight/dp_aux_backlight.c new file +245 -0

new file mode 100644
index 00000000..3398383
@@ -0,0 +1,245 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Backlight driver to control the brightness over DisplayPort aux channel.
+ */
+
+#include <linux/backlight.h>
+#include <linux/err.h>
+#include <linux/gpio/consumer.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/platform_device.h>
+#include <drm/drm_dp_helper.h>
+
+#define DP_AUX_MAX_BRIGHTNESS		0xffff
+
+/**
+ * struct dp_aux_backlight - DisplayPort aux backlight data
+ * @dev: pointer to our device.
+ * @aux: the DisplayPort aux channel.
+ * @enable_gpio: the backlight enable gpio.
+ * @enabled: true if backlight is enabled else false.
+ */
+struct dp_aux_backlight {
+	struct device *dev;
+	struct drm_dp_aux *aux;
+	struct gpio_desc *enable_gpio;
+	bool enabled;
+};
+
+static struct drm_dp_aux *i2c_to_aux(struct i2c_adapter *i2c)
+{
+	return container_of(i2c, struct drm_dp_aux, ddc);
+}
+
+static int dp_aux_backlight_enable(struct dp_aux_backlight *aux_bl)
+{
+	u8 val = 0;
+	int ret;
+
+	if (aux_bl->enabled)
+		return 0;
+
+	/* Set backlight control mode */
+	ret = drm_dp_dpcd_readb(aux_bl->aux, DP_EDP_BACKLIGHT_MODE_SET_REGISTER,
+				&val);
+	if (ret < 0)
+		return ret;
+
+	val &= ~DP_EDP_BACKLIGHT_CONTROL_MODE_MASK;
+	val |= DP_EDP_BACKLIGHT_CONTROL_MODE_DPCD;
+	ret = drm_dp_dpcd_writeb(aux_bl->aux, DP_EDP_BACKLIGHT_MODE_SET_REGISTER,
+				 val);
+	if (ret < 0)
+		return ret;
+
+	/* Enable backlight */
+	ret = drm_dp_dpcd_readb(aux_bl->aux, DP_EDP_DISPLAY_CONTROL_REGISTER,
+				&val);
+	if (ret < 0)
+		return ret;
+
+	val |= DP_EDP_BACKLIGHT_ENABLE;
+	ret = drm_dp_dpcd_writeb(aux_bl->aux, DP_EDP_DISPLAY_CONTROL_REGISTER,
+				 val);
+	if (ret < 0)
+		return ret;
+
+	if (aux_bl->enable_gpio)
+		gpiod_set_value(aux_bl->enable_gpio, 1);
+
+	aux_bl->enabled = true;
+
+	return 0;
+}
+
+static int dp_aux_backlight_disable(struct dp_aux_backlight *aux_bl)
+{
+	u8 val = 0;
+	int ret;
+
+	if (!aux_bl->enabled)
+		return 0;
+
+	if (aux_bl->enable_gpio)
+		gpiod_set_value(aux_bl->enable_gpio, 0);
+
+	ret = drm_dp_dpcd_readb(aux_bl->aux, DP_EDP_DISPLAY_CONTROL_REGISTER,
+				&val);
+	if (ret < 0)
+		return ret;
+
+	val &= ~DP_EDP_BACKLIGHT_ENABLE;
+	ret = drm_dp_dpcd_writeb(aux_bl->aux, DP_EDP_DISPLAY_CONTROL_REGISTER,
+				 val);
+	if (ret < 0)
+		return ret;
+
+	aux_bl->enabled = false;
+
+	return 0;
+}
+
+static int dp_aux_backlight_update_status(struct backlight_device *bd)
+{
+	struct dp_aux_backlight *aux_bl = bl_get_data(bd);
+	u16 brightness = backlight_get_brightness(bd);
+	u8 val[2] = { 0x0 };
+	int ret = 0;
+
+	if (brightness > 0) {
+		val[0] = brightness >> 8;
+		val[1] = brightness & 0xff;
+		ret = drm_dp_dpcd_write(aux_bl->aux, DP_EDP_BACKLIGHT_BRIGHTNESS_MSB,
+					val, sizeof(val));
+		if (ret < 0)
+			return ret;
+
+		dp_aux_backlight_enable(aux_bl);
+	} else {
+		dp_aux_backlight_disable(aux_bl);
+	}
+
+	return 0;
+}
+
+static int dp_aux_backlight_get_brightness(struct backlight_device *bd)
+{
+	struct dp_aux_backlight *aux_bl = bl_get_data(bd);
+	u8 val[2] = { 0x0 };
+	int ret = 0;
+
+	if (backlight_is_blank(bd))
+		return 0;
+
+	ret = drm_dp_dpcd_read(aux_bl->aux, DP_EDP_BACKLIGHT_BRIGHTNESS_MSB,
+			       &val, sizeof(val));
+	if (ret < 0)
+		return ret;
+
+	return (val[0] << 8 | val[1]);
+}
+
+static const struct backlight_ops aux_bl_ops = {
+	.update_status = dp_aux_backlight_update_status,
+	.get_brightness = dp_aux_backlight_get_brightness,
+};
+
+
+static int dp_aux_backlight_probe(struct platform_device *pdev)
+{
+	struct dp_aux_backlight *aux_bl;
+	struct backlight_device *bd;
+	struct backlight_properties bl_props = { 0 };
+	struct device_node *np;
+	struct i2c_adapter *ddc;
+	int ret = 0;
+	u32 val;
+
+	aux_bl = devm_kzalloc(&pdev->dev, sizeof(*aux_bl), GFP_KERNEL);
+	if (!aux_bl)
+		return -ENOMEM;
+
+	aux_bl->dev = &pdev->dev;
+
+	np = of_parse_phandle(pdev->dev.of_node, "ddc-i2c-bus", 0);
+	if (!np) {
+		dev_err(&pdev->dev, "failed to get aux ddc I2C bus\n");
+		return -ENODEV;
+	}
+
+	ddc = of_find_i2c_adapter_by_node(np);
+	of_node_put(np);
+	if (!ddc)
+		return -EPROBE_DEFER;
+
+	aux_bl->aux = i2c_to_aux(ddc);
+	dev_dbg(&pdev->dev, "using dp aux %s\n", aux_bl->aux->name);
+
+	aux_bl->enable_gpio = devm_gpiod_get_optional(&pdev->dev, "enable",
+					     GPIOD_OUT_LOW);
+	if (IS_ERR(aux_bl->enable_gpio)) {
+		ret = PTR_ERR(aux_bl->enable_gpio);
+		goto free_ddc;
+	}
+
+	val = DP_AUX_MAX_BRIGHTNESS;
+	of_property_read_u32(pdev->dev.of_node, "max-brightness", &val);
+	if (val > DP_AUX_MAX_BRIGHTNESS)
+		val = DP_AUX_MAX_BRIGHTNESS;
+
+	bl_props.max_brightness = val;
+	bl_props.brightness = val;
+	bl_props.type = BACKLIGHT_RAW;
+	bd = devm_backlight_device_register(&pdev->dev, dev_name(&pdev->dev),
+					    &pdev->dev, aux_bl,
+					    &aux_bl_ops, &bl_props);
+	if (IS_ERR(bd)) {
+		ret = PTR_ERR(bd);
+		dev_err(&pdev->dev,
+			      "failed to register backlight (%d)\n", ret);
+		goto free_ddc;
+	}
+
+	platform_set_drvdata(pdev, bd);
+
+	return 0;
+
+free_ddc:
+	if (ddc)
+		put_device(&ddc->dev);
+
+	return ret;
+}
+
+static int dp_aux_backlight_remove(struct platform_device *pdev)
+{
+	struct backlight_device *bd = platform_get_drvdata(pdev);
+	struct dp_aux_backlight *aux_bl = bl_get_data(bd);
+	struct i2c_adapter *ddc = &aux_bl->aux->ddc;
+
+	if (ddc)
+		put_device(&ddc->dev);
+
+	return 0;
+}
+
+static const struct of_device_id dp_aux_bl_of_match_table[] = {
+	{ .compatible = "dp-aux-backlight"},
+	{},
+};
+MODULE_DEVICE_TABLE(of, dp_aux_bl_of_match_table);
+
+static struct platform_driver dp_aux_backlight_driver = {
+	.driver = {
+		.name = "dp-aux-backlight",
+		.of_match_table = dp_aux_bl_of_match_table,
+	},
+	.probe = dp_aux_backlight_probe,
+	.remove = dp_aux_backlight_remove,
+
+};
+module_platform_driver(dp_aux_backlight_driver);
+
+MODULE_DESCRIPTION("DisplayPort aux backlight driver");
+MODULE_LICENSE("GPL v2");

Quoted 2 lines · Rajeev Nandan

Add backlight driver for the panels supporting backlight control using DPCD registers on the DisplayPort aux channel.

No, please don't do this.

I wrote you last week in reply to v1 why I thought merging this would not be a good idea [1]. Why have you sent two versions since then without replying to me, or Cc'ing me or Lyude?

I think it's an even worse idea to merge this to drivers/video/backlight. With DP AUX backlight you can't pretend it's just an independent aux interface for backlight without everything else around it. It's not independent of eDP, and exposing it as a direct backlight sysfs interface bypasses the encoder.

And it still remains that there is existing DP AUX backlight code in use, in the tree, with more features than this, with plans and previously submitted patches to lift from one driver to drm core, and with patches to add support to another driver.

I don't say this lightly, or very often at all, but,

NAK.

BR,
Jani.

[1] https://lore.kernel.org/dri-devel/[email protected]/

Quoted 297 lines

Changes in v2:
- New (most of the code reused from drm_dp_aux_backlight.c of v1)

Changes in v3:
- Add missing ';' to fix module compilation (kernel test bot)

Signed-off-by Rajeev Nandan <[email protected]>

---
 drivers/video/backlight/Kconfig            |   7 +
 drivers/video/backlight/Makefile           |   1 +
 drivers/video/backlight/dp_aux_backlight.c | 245 
+++++++++++++++++++++++++++++
 3 files changed, 253 insertions(+)
 create mode 100644 drivers/video/backlight/dp_aux_backlight.c

drivers/video/backlight/Kconfig +7 -0

index d83c87b..82c88f0 100644
@@ -456,6 +456,13 @@ config BACKLIGHT_LED
 	  If you have a LCD backlight adjustable by LED class driver, say Y
 	  to enable this driver.
 
+config BACKLIGHT_DP_AUX
+       tristate "DisplayPort aux backlight driver"
+       depends on DRM && DRM_KMS_HELPER
+       help
+         If you have a panel backlight controlled by DPCD registers
+         on the DisplayPort aux channel, say Y to enable this driver.
+
 endif # BACKLIGHT_CLASS_DEVICE
 
 endmenu

drivers/video/backlight/Makefile +1 -0

index 685f3f1..ba23c7c 100644
@@ -57,3 +57,4 @@ obj-$(CONFIG_BACKLIGHT_WM831X)		+= wm831x_bl.o
 obj-$(CONFIG_BACKLIGHT_ARCXCNN) 	+= arcxcnn_bl.o
 obj-$(CONFIG_BACKLIGHT_RAVE_SP)		+= rave-sp-backlight.o
 obj-$(CONFIG_BACKLIGHT_LED)		+= led_bl.o
+obj-$(CONFIG_BACKLIGHT_DP_AUX)		+= dp_aux_backlight.o

drivers/video/backlight/dp_aux_backlight.c new file +3 -0

new file mode 100644
index 00000000..3398383
@@ -0,0 +1,245 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Backlight driver to control the brightness over DisplayPort aux 

channel.
+ */

drivers/video/backlight/dp_aux_backlight.c new file +40 -0

+
+#include <linux/backlight.h>
+#include <linux/err.h>
+#include <linux/gpio/consumer.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/platform_device.h>
+#include <drm/drm_dp_helper.h>
+
+#define DP_AUX_MAX_BRIGHTNESS		0xffff
+
+/**
+ * struct dp_aux_backlight - DisplayPort aux backlight data
+ * @dev: pointer to our device.
+ * @aux: the DisplayPort aux channel.
+ * @enable_gpio: the backlight enable gpio.
+ * @enabled: true if backlight is enabled else false.
+ */
+struct dp_aux_backlight {
+	struct device *dev;
+	struct drm_dp_aux *aux;
+	struct gpio_desc *enable_gpio;
+	bool enabled;
+};
+
+static struct drm_dp_aux *i2c_to_aux(struct i2c_adapter *i2c)
+{
+	return container_of(i2c, struct drm_dp_aux, ddc);
+}
+
+static int dp_aux_backlight_enable(struct dp_aux_backlight *aux_bl)
+{
+	u8 val = 0;
+	int ret;
+
+	if (aux_bl->enabled)
+		return 0;
+
+	/* Set backlight control mode */
+	ret = drm_dp_dpcd_readb(aux_bl->aux, 

DP_EDP_BACKLIGHT_MODE_SET_REGISTER,

drivers/video/backlight/dp_aux_backlight.c new file +7 -0

+				&val);
+	if (ret < 0)
+		return ret;
+
+	val &= ~DP_EDP_BACKLIGHT_CONTROL_MODE_MASK;
+	val |= DP_EDP_BACKLIGHT_CONTROL_MODE_DPCD;
+	ret = drm_dp_dpcd_writeb(aux_bl->aux, 

DP_EDP_BACKLIGHT_MODE_SET_REGISTER,

drivers/video/backlight/dp_aux_backlight.c new file +6 -0

+				 val);
+	if (ret < 0)
+		return ret;
+
+	/* Enable backlight */
+	ret = drm_dp_dpcd_readb(aux_bl->aux, 

DP_EDP_DISPLAY_CONTROL_REGISTER,

drivers/video/backlight/dp_aux_backlight.c new file +6 -0

+				&val);
+	if (ret < 0)
+		return ret;
+
+	val |= DP_EDP_BACKLIGHT_ENABLE;
+	ret = drm_dp_dpcd_writeb(aux_bl->aux, 

DP_EDP_DISPLAY_CONTROL_REGISTER,

drivers/video/backlight/dp_aux_backlight.c new file +24 -0

+				 val);
+	if (ret < 0)
+		return ret;
+
+	if (aux_bl->enable_gpio)
+		gpiod_set_value(aux_bl->enable_gpio, 1);
+
+	aux_bl->enabled = true;
+
+	return 0;
+}
+
+static int dp_aux_backlight_disable(struct dp_aux_backlight *aux_bl)
+{
+	u8 val = 0;
+	int ret;
+
+	if (!aux_bl->enabled)
+		return 0;
+
+	if (aux_bl->enable_gpio)
+		gpiod_set_value(aux_bl->enable_gpio, 0);
+
+	ret = drm_dp_dpcd_readb(aux_bl->aux, 

DP_EDP_DISPLAY_CONTROL_REGISTER,

drivers/video/backlight/dp_aux_backlight.c new file +6 -0

+				&val);
+	if (ret < 0)
+		return ret;
+
+	val &= ~DP_EDP_BACKLIGHT_ENABLE;
+	ret = drm_dp_dpcd_writeb(aux_bl->aux, 

DP_EDP_DISPLAY_CONTROL_REGISTER,

drivers/video/backlight/dp_aux_backlight.c new file +10 -0

+				 val);
+	if (ret < 0)
+		return ret;
+
+	aux_bl->enabled = false;
+
+	return 0;
+}
+
+static int dp_aux_backlight_update_status(struct backlight_device 

*bd)

drivers/video/backlight/dp_aux_backlight.c new file +10 -0

+{
+	struct dp_aux_backlight *aux_bl = bl_get_data(bd);
+	u16 brightness = backlight_get_brightness(bd);
+	u8 val[2] = { 0x0 };
+	int ret = 0;
+
+	if (brightness > 0) {
+		val[0] = brightness >> 8;
+		val[1] = brightness & 0xff;
+		ret = drm_dp_dpcd_write(aux_bl->aux, 

DP_EDP_BACKLIGHT_BRIGHTNESS_MSB,

drivers/video/backlight/dp_aux_backlight.c new file +13 -0

+					val, sizeof(val));
+		if (ret < 0)
+			return ret;
+
+		dp_aux_backlight_enable(aux_bl);
+	} else {
+		dp_aux_backlight_disable(aux_bl);
+	}
+
+	return 0;
+}
+
+static int dp_aux_backlight_get_brightness(struct backlight_device 

*bd)

drivers/video/backlight/dp_aux_backlight.c new file +68 -0

+{
+	struct dp_aux_backlight *aux_bl = bl_get_data(bd);
+	u8 val[2] = { 0x0 };
+	int ret = 0;
+
+	if (backlight_is_blank(bd))
+		return 0;
+
+	ret = drm_dp_dpcd_read(aux_bl->aux, DP_EDP_BACKLIGHT_BRIGHTNESS_MSB,
+			       &val, sizeof(val));
+	if (ret < 0)
+		return ret;
+
+	return (val[0] << 8 | val[1]);
+}
+
+static const struct backlight_ops aux_bl_ops = {
+	.update_status = dp_aux_backlight_update_status,
+	.get_brightness = dp_aux_backlight_get_brightness,
+};
+
+
+static int dp_aux_backlight_probe(struct platform_device *pdev)
+{
+	struct dp_aux_backlight *aux_bl;
+	struct backlight_device *bd;
+	struct backlight_properties bl_props = { 0 };
+	struct device_node *np;
+	struct i2c_adapter *ddc;
+	int ret = 0;
+	u32 val;
+
+	aux_bl = devm_kzalloc(&pdev->dev, sizeof(*aux_bl), GFP_KERNEL);
+	if (!aux_bl)
+		return -ENOMEM;
+
+	aux_bl->dev = &pdev->dev;
+
+	np = of_parse_phandle(pdev->dev.of_node, "ddc-i2c-bus", 0);
+	if (!np) {
+		dev_err(&pdev->dev, "failed to get aux ddc I2C bus\n");
+		return -ENODEV;
+	}
+
+	ddc = of_find_i2c_adapter_by_node(np);
+	of_node_put(np);
+	if (!ddc)
+		return -EPROBE_DEFER;
+
+	aux_bl->aux = i2c_to_aux(ddc);
+	dev_dbg(&pdev->dev, "using dp aux %s\n", aux_bl->aux->name);
+
+	aux_bl->enable_gpio = devm_gpiod_get_optional(&pdev->dev, "enable",
+					     GPIOD_OUT_LOW);
+	if (IS_ERR(aux_bl->enable_gpio)) {
+		ret = PTR_ERR(aux_bl->enable_gpio);
+		goto free_ddc;
+	}
+
+	val = DP_AUX_MAX_BRIGHTNESS;
+	of_property_read_u32(pdev->dev.of_node, "max-brightness", &val);
+	if (val > DP_AUX_MAX_BRIGHTNESS)
+		val = DP_AUX_MAX_BRIGHTNESS;
+
+	bl_props.max_brightness = val;
+	bl_props.brightness = val;
+	bl_props.type = BACKLIGHT_RAW;
+	bd = devm_backlight_device_register(&pdev->dev, 

dev_name(&pdev->dev),

drivers/video/backlight/dp_aux_backlight.c new file +43 -0

+					    &pdev->dev, aux_bl,
+					    &aux_bl_ops, &bl_props);
+	if (IS_ERR(bd)) {
+		ret = PTR_ERR(bd);
+		dev_err(&pdev->dev,
+			      "failed to register backlight (%d)\n", ret);
+		goto free_ddc;
+	}
+
+	platform_set_drvdata(pdev, bd);
+
+	return 0;
+
+free_ddc:
+	if (ddc)
+		put_device(&ddc->dev);
+
+	return ret;
+}
+
+static int dp_aux_backlight_remove(struct platform_device *pdev)
+{
+	struct backlight_device *bd = platform_get_drvdata(pdev);
+	struct dp_aux_backlight *aux_bl = bl_get_data(bd);
+	struct i2c_adapter *ddc = &aux_bl->aux->ddc;
+
+	if (ddc)
+		put_device(&ddc->dev);
+
+	return 0;
+}
+
+static const struct of_device_id dp_aux_bl_of_match_table[] = {
+	{ .compatible = "dp-aux-backlight"},
+	{},
+};
+MODULE_DEVICE_TABLE(of, dp_aux_bl_of_match_table);
+
+static struct platform_driver dp_aux_backlight_driver = {
+	.driver = {
+		.name = "dp-aux-backlight",
+		.of_match_table = dp_aux_bl_of_match_table,
+	},

drivers/video/backlight/dp_aux_backlight.c line 245

Quoted 8 lines

drivers/video/backlight/dp_aux_backlight.c new file +8 -0

+	.probe = dp_aux_backlight_probe,
+	.remove = dp_aux_backlight_remove,
+
+};
+module_platform_driver(dp_aux_backlight_driver);
+
+MODULE_DESCRIPTION("DisplayPort aux backlight driver");
+MODULE_LICENSE("GPL v2");

Quoted 2 lines · Rajeev Nandan

Add backlight driver for the panels supporting backlight control using DPCD registers on the DisplayPort aux channel.

No, please don't do this.

I wrote you last week in reply to v1 why I thought merging this would not be a good idea [1]. Why have you sent two versions since then without replying to me, or Cc'ing me or Lyude?

I think it's an even worse idea to merge this to drivers/video/backlight. With DP AUX backlight you can't pretend it's just an independent aux interface for backlight without everything else around it. It's not independent of eDP, and exposing it as a direct backlight sysfs interface bypasses the encoder.

And it still remains that there is existing DP AUX backlight code in use, in the tree, with more features than this, with plans and previously submitted patches to lift from one driver to drm core, and with patches to add support to another driver.

I don't say this lightly, or very often at all, but,

NAK.

BR,
Jani.

[1] https://lore.kernel.org/dri-devel/[email protected]/

Quoted 310 lines

Changes in v2:
- New (most of the code reused from drm_dp_aux_backlight.c of v1)

Changes in v3:
- Add missing ';' to fix module compilation (kernel test bot)

Signed-off-by Rajeev Nandan <[email protected]>

---
 drivers/video/backlight/Kconfig            |   7 + drivers/video/backlight/Makefile           |   1 + drivers/video/backlight/dp_aux_backlight.c | 245
+++++++++++++++++++++++++++++

 3 files changed, 253 insertions(+)

 create mode 100644 drivers/video/backlight/dp_aux_backlight.c

drivers/video/backlight/Kconfig +0 -0

index d83c87b..82c88f0 100644
@@ -456,6 +456,13 @@ config BACKLIGHT_LED

          If you have a LCD backlight adjustable by LED class driver, say Y
          to enable this driver.

drivers/video/backlight/Kconfig +7 -0

+config BACKLIGHT_DP_AUX
+       tristate "DisplayPort aux backlight driver"
+       depends on DRM && DRM_KMS_HELPER
+       help
+         If you have a panel backlight controlled by DPCD registers
+         on the DisplayPort aux channel, say Y to enable this driver.
+

 endif # BACKLIGHT_CLASS_DEVICE

 endmenu

drivers/video/backlight/Makefile +0 -0

index 685f3f1..ba23c7c 100644
@@ -57,3 +57,4 @@ obj-$(CONFIG_BACKLIGHT_WM831X)                +=

wm831x_bl.o
 obj-$(CONFIG_BACKLIGHT_ARCXCNN)        += arcxcnn_bl.o obj-$(CONFIG_BACKLIGHT_RAVE_SP)                += rave-sp-backlight.o obj-$(CONFIG_BACKLIGHT_LED)            += led_bl.o

drivers/video/backlight/Makefile +1 -0

+obj-$(CONFIG_BACKLIGHT_DP_AUX)         += dp_aux_backlight.o

drivers/video/backlight/dp_aux_backlight.c new file +3 -0

new file mode 100644
index 00000000..3398383
@@ -0,0 +1,245 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Backlight driver to control the brightness over DisplayPort aux

channel.
+ */

drivers/video/backlight/dp_aux_backlight.c new file +40 -0

+
+#include <linux/backlight.h>
+#include <linux/err.h>
+#include <linux/gpio/consumer.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/platform_device.h>
+#include <drm/drm_dp_helper.h>
+
+#define DP_AUX_MAX_BRIGHTNESS          0xffff
+
+/**
+ * struct dp_aux_backlight - DisplayPort aux backlight data
+ * @dev: pointer to our device.
+ * @aux: the DisplayPort aux channel.
+ * @enable_gpio: the backlight enable gpio.
+ * @enabled: true if backlight is enabled else false.
+ */
+struct dp_aux_backlight {
+       struct device *dev;
+       struct drm_dp_aux *aux;
+       struct gpio_desc *enable_gpio;
+       bool enabled;
+};
+
+static struct drm_dp_aux *i2c_to_aux(struct i2c_adapter *i2c)
+{
+       return container_of(i2c, struct drm_dp_aux, ddc);
+}
+
+static int dp_aux_backlight_enable(struct dp_aux_backlight *aux_bl)
+{
+       u8 val = 0;
+       int ret;
+
+       if (aux_bl->enabled)
+               return 0;
+
+       /* Set backlight control mode */
+       ret = drm_dp_dpcd_readb(aux_bl->aux,

DP_EDP_BACKLIGHT_MODE_SET_REGISTER,
+                               &val);
+       if (ret < 0)
+               return ret;
+
+       val &= ~DP_EDP_BACKLIGHT_CONTROL_MODE_MASK; +       val |= DP_EDP_BACKLIGHT_CONTROL_MODE_DPCD; +       ret = drm_dp_dpcd_writeb(aux_bl->aux, DP_EDP_BACKLIGHT_MODE_SET_REGISTER,
+                                val);
+       if (ret < 0)
+               return ret;
+
+       /* Enable backlight */
+       ret = drm_dp_dpcd_readb(aux_bl->aux, DP_EDP_DISPLAY_CONTROL_REGISTER,
+                               &val);
+       if (ret < 0)
+               return ret;
+
+       val |= DP_EDP_BACKLIGHT_ENABLE;
+       ret = drm_dp_dpcd_writeb(aux_bl->aux, DP_EDP_DISPLAY_CONTROL_REGISTER,
+                                val);
+       if (ret < 0)
+               return ret;

drivers/video/backlight/dp_aux_backlight.c new file +21 -0

+
+       if (aux_bl->enable_gpio)
+               gpiod_set_value(aux_bl->enable_gpio, 1);
+
+       aux_bl->enabled = true;
+
+       return 0;
+}
+
+static int dp_aux_backlight_disable(struct dp_aux_backlight *aux_bl)
+{
+       u8 val = 0;
+       int ret;
+
+       if (!aux_bl->enabled)
+               return 0;
+
+       if (aux_bl->enable_gpio)
+               gpiod_set_value(aux_bl->enable_gpio, 0);
+
+       ret = drm_dp_dpcd_readb(aux_bl->aux,

DP_EDP_DISPLAY_CONTROL_REGISTER,
+                               &val);
+       if (ret < 0)
+               return ret;
+
+       val &= ~DP_EDP_BACKLIGHT_ENABLE;
+       ret = drm_dp_dpcd_writeb(aux_bl->aux,
DP_EDP_DISPLAY_CONTROL_REGISTER,
+                                val);
+       if (ret < 0)
+               return ret;

drivers/video/backlight/dp_aux_backlight.c new file +17 -0

+
+       aux_bl->enabled = false;
+
+       return 0;
+}
+
+static int dp_aux_backlight_update_status(struct backlight_device *bd)
+{
+       struct dp_aux_backlight *aux_bl = bl_get_data(bd);
+       u16 brightness = backlight_get_brightness(bd);
+       u8 val[2] = { 0x0 };
+       int ret = 0;
+
+       if (brightness > 0) {
+               val[0] = brightness >> 8;
+               val[1] = brightness & 0xff;
+               ret = drm_dp_dpcd_write(aux_bl->aux,

DP_EDP_BACKLIGHT_BRIGHTNESS_MSB,
+                                       val, sizeof(val)); +               if (ret < 0)
+                       return ret;

drivers/video/backlight/dp_aux_backlight.c new file +19 -0

+
+               dp_aux_backlight_enable(aux_bl);
+       } else {
+               dp_aux_backlight_disable(aux_bl);
+       }
+
+       return 0;
+}
+
+static int dp_aux_backlight_get_brightness(struct backlight_device *bd)
+{
+       struct dp_aux_backlight *aux_bl = bl_get_data(bd);
+       u8 val[2] = { 0x0 };
+       int ret = 0;
+
+       if (backlight_is_blank(bd))
+               return 0;
+
+       ret = drm_dp_dpcd_read(aux_bl->aux,

DP_EDP_BACKLIGHT_BRIGHTNESS_MSB,
+                              &val, sizeof(val)); +       if (ret < 0)
+               return ret;

drivers/video/backlight/dp_aux_backlight.c new file +41 -0

+
+       return (val[0] << 8 | val[1]);
+}
+
+static const struct backlight_ops aux_bl_ops = {
+       .update_status = dp_aux_backlight_update_status,
+       .get_brightness = dp_aux_backlight_get_brightness,
+};
+
+
+static int dp_aux_backlight_probe(struct platform_device *pdev)
+{
+       struct dp_aux_backlight *aux_bl;
+       struct backlight_device *bd;
+       struct backlight_properties bl_props = { 0 };
+       struct device_node *np;
+       struct i2c_adapter *ddc;
+       int ret = 0;
+       u32 val;
+
+       aux_bl = devm_kzalloc(&pdev->dev, sizeof(*aux_bl), GFP_KERNEL);
+       if (!aux_bl)
+               return -ENOMEM;
+
+       aux_bl->dev = &pdev->dev;
+
+       np = of_parse_phandle(pdev->dev.of_node, "ddc-i2c-bus", 0);
+       if (!np) {
+               dev_err(&pdev->dev, "failed to get aux ddc I2C bus\n");
+               return -ENODEV;
+       }
+
+       ddc = of_find_i2c_adapter_by_node(np);
+       of_node_put(np);
+       if (!ddc)
+               return -EPROBE_DEFER;
+
+       aux_bl->aux = i2c_to_aux(ddc);
+       dev_dbg(&pdev->dev, "using dp aux %s\n", aux_bl->aux->name);
+
+       aux_bl->enable_gpio = devm_gpiod_get_optional(&pdev->dev,

"enable",
+                                            GPIOD_OUT_LOW); +       if (IS_ERR(aux_bl->enable_gpio)) {
+               ret = PTR_ERR(aux_bl->enable_gpio); +               goto free_ddc;
+       }

drivers/video/backlight/dp_aux_backlight.c new file +10 -0

+
+       val = DP_AUX_MAX_BRIGHTNESS;
+       of_property_read_u32(pdev->dev.of_node, "max-brightness", &val);
+       if (val > DP_AUX_MAX_BRIGHTNESS)
+               val = DP_AUX_MAX_BRIGHTNESS;
+
+       bl_props.max_brightness = val;
+       bl_props.brightness = val;
+       bl_props.type = BACKLIGHT_RAW;
+       bd = devm_backlight_device_register(&pdev->dev, dev_name(&pdev-

Quoted 1 line

dev),

+                                           &pdev->dev, aux_bl, +                                           &aux_bl_ops, &bl_props); +       if (IS_ERR(bd)) {
+               ret = PTR_ERR(bd);
+               dev_err(&pdev->dev,
+                             "failed to register backlight (%d)\n", ret); +               goto free_ddc;
+       }

drivers/video/backlight/dp_aux_backlight.c new file +43 -0

+
+       platform_set_drvdata(pdev, bd);
+
+       return 0;
+
+free_ddc:
+       if (ddc)
+               put_device(&ddc->dev);
+
+       return ret;
+}
+
+static int dp_aux_backlight_remove(struct platform_device *pdev)
+{
+       struct backlight_device *bd = platform_get_drvdata(pdev);
+       struct dp_aux_backlight *aux_bl = bl_get_data(bd);
+       struct i2c_adapter *ddc = &aux_bl->aux->ddc;
+
+       if (ddc)
+               put_device(&ddc->dev);
+
+       return 0;
+}
+
+static const struct of_device_id dp_aux_bl_of_match_table[] = {
+       { .compatible = "dp-aux-backlight"},
+       {},
+};
+MODULE_DEVICE_TABLE(of, dp_aux_bl_of_match_table);
+
+static struct platform_driver dp_aux_backlight_driver = {
+       .driver = {
+               .name = "dp-aux-backlight",
+               .of_match_table = dp_aux_bl_of_match_table,
+       },
+       .probe = dp_aux_backlight_probe,
+       .remove = dp_aux_backlight_remove,
+
+};
+module_platform_driver(dp_aux_backlight_driver);
+
+MODULE_DESCRIPTION("DisplayPort aux backlight driver");
+MODULE_LICENSE("GPL v2");