Reviewed-by: Lyude Paul <lyude@xxxxxxxxxx> On Wed, 2022-08-24 at 14:15 +0200, Hans de Goede wrote: > Add an entry summarizing the discussion about dealing with brightness > control on devices with more then 1 internal panel. > > The original discussion can be found here: > https://lore.kernel.org/dri-devel/20220517152331.16217-1-hdegoede@xxxxxxxxxx/ > > Signed-off-by: Hans de Goede <hdegoede@xxxxxxxxxx> > --- > Documentation/gpu/todo.rst | 68 ++++++++++++++++++++++++++++++++++++++ > 1 file changed, 68 insertions(+) > > diff --git a/Documentation/gpu/todo.rst b/Documentation/gpu/todo.rst > index 7634c27ac562..393d218e4a0c 100644 > --- a/Documentation/gpu/todo.rst > +++ b/Documentation/gpu/todo.rst > @@ -679,6 +679,74 @@ Contact: Sam Ravnborg > > Level: Advanced > > +Brightness handling on devices with multiple internal panels > +============================================================ > + > +On x86/ACPI devices there can be multiple backlight firmware interfaces: > +(ACPI) video, vendor specific and others. As well as direct/native (PWM) > +register programming by the KMS driver. > + > +To deal with this backlight drivers used on x86/ACPI call > +acpi_video_get_backlight_type() which has heuristics (+quirks) to select > +which backlight interface to use; and backlight drivers which do not match > +the returned type will not register themselves, so that only one backlight > +device gets registered (in a single GPU setup, see below). > + > +At the moment this more or less assumes that there will only > +be 1 (internal) panel on a system. > + > +On systems with 2 panels this may be a problem, depending on > +what interface acpi_video_get_backlight_type() selects: > + > +1. native: in this case the KMS driver is expected to know which backlight > + device belongs to which output so everything should just work. > +2. video: this does support controlling multiple backlights, but some work > + will need to be done to get the output <-> backlight device mapping > + > +The above assumes both panels will require the same backlight interface type. > +Things will break on systems with multiple panels where the 2 panels need > +a different type of control. E.g. one panel needs ACPI video backlight control, > +where as the other is using native backlight control. Currently in this case > +only one of the 2 required backlight devices will get registered, based on > +the acpi_video_get_backlight_type() return value. > + > +If this (theoretical) case ever shows up, then supporting this will need some > +work. A possible solution here would be to pass a device and connector-name > +to acpi_video_get_backlight_type() so that it can deal with this. > + > +Note in a way we already have a case where userspace sees 2 panels, > +in dual GPU laptop setups with a mux. On those systems we may see > +either 2 native backlight devices; or 2 native backlight devices. > + > +Userspace already has code to deal with this by detecting if the related > +panel is active (iow which way the mux between the GPU and the panels > +points) and then uses that backlight device. Userspace here very much > +assumes a single panel though. It picks only 1 of the 2 backlight devices > +and then only uses that one. > + > +Note that all userspace code (that I know off) is currently hardcoded > +to assume a single panel. > + > +Before the recent changes to not register multiple (e.g. video + native) > +/sys/class/backlight devices for a single panel (on a single GPU laptop), > +userspace would see multiple backlight devices all controlling the same > +backlight. > + > +To deal with this userspace had to always picks one preferred device under > +/sys/class/backlight and will ignore the others. So to support brightness > +control on multiple panels userspace will need to be updated too. > + > +There are plans to allow brightness control through the KMS API by adding > +a "display brightness" property to drm_connector objects for panels. This > +solves a number of issues with the /sys/class/backlight API, including not > +being able to map a sysfs backlight device to a specific connector. Any > +userspace changes to add support for brightness control on devices with > +multiple panels really should build on top of this new KMS property. > + > +Contact: Hans de Goede > + > +Level: Advanced > + > Outside DRM > =========== > -- Cheers, Lyude Paul (she/her) Software Engineer at Red Hat
