Nouveau - Jun 2022 - [PATCH 12/14] drm/nouveau: Register ACPI video backlight when nv_backlight registration fails

Typically the acpi_video driver will initialize before nouveau, which
used to cause /sys/class/backlight/acpi_video0 to get registered and then
nouveau would register its own nv_backlight device later. After which
the drivers/acpi/video_detect.c code unregistered the acpi_video0 device
to avoid there being 2 backlight devices.

This means that userspace used to briefly see 2 devices and the
disappearing of acpi_video0 after a brief time confuses the systemd
backlight level save/restore code, see e.g.:
https://bbs.archlinux.org/viewtopic.php?id=269920

To fix this the ACPI video code has been modified to make backlight class
device registration a separate step, relying on the drm/kms driver to
ask for the acpi_video backlight registration after it is done setting up
its native backlight device.

Add a call to the new acpi_video_register_backlight() when native backlight
device registration has failed / was skipped to ensure that there is a
backlight device available before the drm_device gets registered with
userspace.

Signed-off-by: Hans de Goede <hdegoede at redhat.com>
---
 drivers/gpu/drm/nouveau/nouveau_backlight.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/gpu/drm/nouveau/nouveau_backlight.c
b/drivers/gpu/drm/nouveau/nouveau_backlight.c
index f56ff797c78c..0ae8793357a4 100644
--- a/drivers/gpu/drm/nouveau/nouveau_backlight.c
+++ b/drivers/gpu/drm/nouveau/nouveau_backlight.c
@@ -436,6 +436,13 @@ nouveau_backlight_init(struct drm_connector *connector)
 
 fail_alloc:
 	kfree(bl);
+	/*
+	 * If we get here we have an internal panel, but no nv_backlight,
+	 * try registering an ACPI video backlight device instead.
+	 */
+	if (ret == 0)
+		acpi_video_register_backlight();
+
 	return ret;
 }
 
-- 
2.36.0

On Tue, 2022-05-17 at 17:23 +0200, Hans de Goede wrote:
> Typically the acpi_video driver will initialize before nouveau, which
> used to cause /sys/class/backlight/acpi_video0 to get registered and then
> nouveau would register its own nv_backlight device later. After which
> the drivers/acpi/video_detect.c code unregistered the acpi_video0 device
> to avoid there being 2 backlight devices.
> 
> This means that userspace used to briefly see 2 devices and the
> disappearing of acpi_video0 after a brief time confuses the systemd
> backlight level save/restore code, see e.g.:
> https://bbs.archlinux.org/viewtopic.php?id=269920
> 
> To fix this the ACPI video code has been modified to make backlight class
> device registration a separate step, relying on the drm/kms driver to
> ask for the acpi_video backlight registration after it is done setting up
> its native backlight device.
> 
> Add a call to the new acpi_video_register_backlight() when native backlight
> device registration has failed / was skipped to ensure that there is a
> backlight device available before the drm_device gets registered with
> userspace.
> 
> Signed-off-by: Hans de Goede <hdegoede at redhat.com>
> ---
> ?drivers/gpu/drm/nouveau/nouveau_backlight.c | 7 +++++++
> ?1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/gpu/drm/nouveau/nouveau_backlight.c
> b/drivers/gpu/drm/nouveau/nouveau_backlight.c
> index f56ff797c78c..0ae8793357a4 100644
> --- a/drivers/gpu/drm/nouveau/nouveau_backlight.c
> +++ b/drivers/gpu/drm/nouveau/nouveau_backlight.c
> @@ -436,6 +436,13 @@ nouveau_backlight_init(struct drm_connector
*connector)
> ?
> ?fail_alloc:
> ????????kfree(bl);
> +???????/*
> +??????? * If we get here we have an internal panel, but no nv_backlight,
> +??????? * try registering an ACPI video backlight device instead.
> +??????? */
> +???????if (ret == 0)
> +???????????????acpi_video_register_backlight();
Assuming we don't need to return the value of
acpi_video_register_backlight()
here:

Reviewed-by: Lyude Paul <lyude at redhat.com>
> +
> ????????return ret;
> ?}
> ?
-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat

Hi Lyude,

Thank you for the reviews.

On 5/18/22 19:39, Lyude Paul wrote:
> On Tue, 2022-05-17 at 17:23 +0200, Hans de Goede wrote:
>> Typically the acpi_video driver will initialize before nouveau, which
>> used to cause /sys/class/backlight/acpi_video0 to get registered and
then
>> nouveau would register its own nv_backlight device later. After which
>> the drivers/acpi/video_detect.c code unregistered the acpi_video0
device
>> to avoid there being 2 backlight devices.
>>
>> This means that userspace used to briefly see 2 devices and the
>> disappearing of acpi_video0 after a brief time confuses the systemd
>> backlight level save/restore code, see e.g.:
>> https://bbs.archlinux.org/viewtopic.php?id=269920
>>
>> To fix this the ACPI video code has been modified to make backlight
class
>> device registration a separate step, relying on the drm/kms driver to
>> ask for the acpi_video backlight registration after it is done setting
up
>> its native backlight device.
>>
>> Add a call to the new acpi_video_register_backlight() when native
backlight
>> device registration has failed / was skipped to ensure that there is a
>> backlight device available before the drm_device gets registered with
>> userspace.
>>
>> Signed-off-by: Hans de Goede <hdegoede at redhat.com>
>> ---
>> ?drivers/gpu/drm/nouveau/nouveau_backlight.c | 7 +++++++
>> ?1 file changed, 7 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/nouveau/nouveau_backlight.c
>> b/drivers/gpu/drm/nouveau/nouveau_backlight.c
>> index f56ff797c78c..0ae8793357a4 100644
>> --- a/drivers/gpu/drm/nouveau/nouveau_backlight.c
>> +++ b/drivers/gpu/drm/nouveau/nouveau_backlight.c
>> @@ -436,6 +436,13 @@ nouveau_backlight_init(struct drm_connector
*connector)
>> ?
>> ?fail_alloc:
>> ????????kfree(bl);
>> +???????/*
>> +??????? * If we get here we have an internal panel, but no
nv_backlight,
>> +??????? * try registering an ACPI video backlight device instead.
>> +??????? */
>> +???????if (ret == 0)
>> +???????????????acpi_video_register_backlight();
> 
> Assuming we don't need to return the value of
acpi_video_register_backlight()
> here:
The function return type is void, so no return value to check :)
> 
> Reviewed-by: Lyude Paul <lyude at redhat.com>
> 
>> +
>> ????????return ret;
>> ?}
>> ?
>