On Wed, 18 May 2022, Hans de Goede <hdegoede@xxxxxxxxxx> wrote: > Hi, > > On 5/18/22 10:55, Jani Nikula wrote: >> On Tue, 17 May 2022, Hans de Goede <hdegoede@xxxxxxxxxx> 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. Regarding the question below, this is the part that throws me off. >>> >>> 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. > > Can you elaborate the "slightly different explanations in the > commit message and comment" part a bit (so that I can fix this) ? > >> 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. > > Code wise I think this would mean renaming the original and > then adding 2 wrappers, but that is fine with me. I've no real > preference either way and I'm happy with adding a new variant of > acpi_video_get_backlight_type() for the native backlight drivers > any suggestion for a name ? Alternatively, do the native backlight drivers have any need for the actual backlight type information from acpi? They only need to be able to ask if they should register themselves, right? I understand this sounds like bikeshedding, but I'm trying to avoid duplicating the conditions in the drivers where a single predicate function call could be sufficient, and the complexity could be hidden in acpi. if (!acpi_video_backlight_use_native()) return; Perhaps all the drivers/platform/x86/* backlight drivers could use: if (acpi_video_backlight_use_vendor()) ... You can still use the native parameter etc. internally, but just hide the details from everyone else, and, hopefully, make it harder for them to do silly things? BR, Jani. > > Regards, > > Hans > -- Jani Nikula, Intel Open Source Graphics Center
