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

[Hans de Goede]

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;
>>  }
>>  
>