Re: [PATCH 01/14] ACPI: video: Add a native function parameter to acpi_video_get_backlight_type()

[Jani Nikula <jani.nikula@linux.intel.com>]

On Tue, 17 May 2022, Hans de Goede <hdegoede@redhat.com> wrote:
> ATM on x86 laptops where we want userspace to use the acpi_video backlight
> device we often register both the GPU's native backlight device and
> acpi_video's firmware acpi_video# backlight device. This relies on
> userspace preferring firmware type backlight devices over native ones, but
> registering 2 backlight devices for a single display really is undesirable.
>
> On x86 laptops where the native GPU backlight device should be used,
> the registering of other backlight devices is avoided by their drivers
> using acpi_video_get_backlight_type() and only registering their backlight
> if the return value matches their type.
>
> acpi_video_get_backlight_type() uses
> backlight_device_get_by_type(BACKLIGHT_RAW) to determine if a native
> driver is available and will never return native if this returns
> false. This means that the GPU's native backlight registering code
> cannot just call acpi_video_get_backlight_type() to determine if it
> should register its backlight, since acpi_video_get_backlight_type() will
> never return native until the native backlight has already registered.
>
> To fix this add a native function parameter to
> acpi_video_get_backlight_type(), which when set to true will make
> acpi_video_get_backlight_type() behave as if a native backlight has
> already been registered.
>
> Note that all current callers are updated to pass false for the new
> parameter, so this change in itself causes no functional changes.


> diff --git a/drivers/acpi/video_detect.c b/drivers/acpi/video_detect.c
> index becc198e4c22..0a06f0edd298 100644
> --- a/drivers/acpi/video_detect.c
> +++ b/drivers/acpi/video_detect.c
> @@ -17,12 +17,14 @@
>   * Otherwise vendor specific drivers like thinkpad_acpi, asus-laptop,
>   * sony_acpi,... can take care about backlight brightness.
>   *
> - * Backlight drivers can use acpi_video_get_backlight_type() to determine
> - * which driver should handle the backlight.
> + * Backlight drivers can use acpi_video_get_backlight_type() to determine which
> + * driver should handle the backlight. RAW/GPU-driver backlight drivers must
> + * pass true for the native function argument, other drivers must pass false.
>   *
>   * If CONFIG_ACPI_VIDEO is neither set as "compiled in" (y) nor as a module (m)
>   * this file will not be compiled and acpi_video_get_backlight_type() will
> - * always return acpi_backlight_vendor.
> + * return acpi_backlight_native when its native argument is true and
> + * acpi_backlight_vendor when it is false.
>   */

Frankly, I think the boolean native parameter here, and at the call
sites, is confusing, and the slightly different explanations in the
commit message and comment here aren't helping.

I suggest adding a separate function that the native backlight drivers
should use. I think it's more obvious all around, and easier to document
too.


BR,
Jani.

-- 
Jani Nikula, Intel Open Source Graphics Center