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.