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

[Lyude Paul]

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