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authorMaxime Ripard <maxime@cerno.tech>2022-09-14 14:27:09 +0300
committerMaxime Ripard <maxime@cerno.tech>2022-09-14 14:27:10 +0300
commit3f1a3a28e9e00a84705b62f0fdc5e31d0f935615 (patch)
tree1b579fab836e43ccb94dfb0d48e6651d86512be9 /Documentation/gpu/todo.rst
parenta108772d03d8bdb43258218b00bfe43bbe1e8800 (diff)
parent4f96b1bc156e7076f6efedc2a76a8c7e897c7977 (diff)
downloadlinux-3f1a3a28e9e00a84705b62f0fdc5e31d0f935615.tar.xz
Merge tag 'backlight-detect-refactor-1' of git://git.kernel.org/pub/scm/linux/kernel/git/pdx86/platform-drivers-x86 into drm-misc-next
Immutable backlight-detect-refactor branch between acpi, drm-* and pdx86 Tag (immutable branch) with v6.0-rc1 + the (acpi/x86) backlight detect refactor work. For merging into the acpi, drm-* and pdx86 subsystems. Signed-off-by: Maxime Ripard <maxime@cerno.tech> # -----BEGIN PGP SIGNATURE----- # # iQFIBAABCAAyFiEEuvA7XScYQRpenhd+kuxHeUQDJ9wFAmMVsogUHGhkZWdvZWRl # QHJlZGhhdC5jb20ACgkQkuxHeUQDJ9yy6wgAlig+7hkq940L62lTpj0g2gNQv8zc # HCsMpnU7dnJcZYaEvIjouZhf33ZbN52c0fQq2JWjt7fFX04LLyIiyrJ26Lc293JR # ++yXpJcVoewRGqApy/P3Z05TKUCLll5bexvK4t8isnhOtEXD/nDPWKTLIV2Kd1DK # nLY4KgRznXZ85RhYheUEdidZ7Lwlzt1JVBMq7tpnzu3nVdDExyZmqlqCUITcLynu # ysuASQGr0D2i+1vb9eifHIA3xsQO0S37Bv62aBMBKxB6B8Fz1DYr8VA2YvoT82Hv # IFT0hzCCZ/63Ljga05O78TwraxAQX0RvZWqjqGgnZg6fIBh2hxUiqeQY6g== # =SA1R # -----END PGP SIGNATURE----- # gpg: Signature made Mon 05 Sep 2022 09:25:44 AM IST # gpg: using RSA key BAF03B5D2718411A5E9E177E92EC4779440327DC # gpg: issuer "hdegoede@redhat.com" # gpg: Can't check signature: No public key From: Hans de Goede <hdegoede@redhat.com> Link: https://patchwork.freedesktop.org/patch/msgid/261afe3d-7790-e945-adf6-a2c96c9b1eff@redhat.com
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@@ -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
===========