Hi All,
As discussed in the "[RFC] drm/kms: control display brightness through drm_connector properties" thread, step 1 of the plan is to stop registering multiple /sys/class/backlight devs for a single display.
On x86 there can be multiple firmware + direct-hw-access methods for controlling the backlight and in some cases the kernel registers multiple backlight-devices for a single internal laptop LCD panel, 2 common scenarios where this happens are:
1) i915 and nouveau unconditionally register their "native" backlight dev even on devices where /sys/class/backlight/acpi_video0 must be used to control the backlight, relying on userspace to prefer the "firmware" acpi_video0 device over "native" devices.
2) amdgpu and nouveau rely on the acpi_video driver initializing before them, which currently causes /sys/class/backlight/acpi_video0 to usually show up and then they register their own native backlight driver after which the drivers/acpi/video_detect.c code unregisters the acpi_video0 device. This means that userspace briefly sees 2 devices and the disappearing of acpi_video0 after a brief time confuses the systemd backlight level save/restore code, see e.g.: https://bbs.archlinux.org/viewtopic.php?id=269920
Fixing kms driver unconditionally register their "native" backlight dev =======================================================================
The plan for fixing 1) is to add a "bool native_backlight_available" parameter to acpi_video_get_backlight_type(), which will be set to true when called by e.g. the i915 code to check if it should register its native backlight-device. This way acpi_video_get_backlight_type() will know that a native backlight-device is (will be) available even though it has not been registered yet and then it can return acpi_backlight_native if that is the best option.
And then the i915 code will only actually register its native backlight when acpi_backlight_native gets returned, thus hiding it in scenario 1.
Fixing acpi_video0 getting registered for a brief time ======================================================
ATM the acpi_video code will delay parsing the ACPI video extensions when an i915 opregion is present and will immediately parse these + register an acpi_video backlight device on laptops without Intel integrated graphics. On laptops with i915 gfx the i915 driver calls acpi_video_register() near the end of its probe() function when things are ready for the acpi_video code to run, avoiding scenario 2.
Where as on systems without i915 gfx acpi_video's module_init() immediately calls acpi_video_register() and then later the ACPI video_detect code calls acpi_video_unregister_backlight() to hide the acpi_video# backlight-device on systems where the native backlight-device should be used. The plan to fix this is to add an acpi_video_register_backlight() and to make acpi_video_register() do all the usual ACPI video extension probing, but have it skip the actual registering of the backlight devices and have drivers explicitly call acpi_video_register() after they have setup their own native backlight-device. This way acpi_video_get_backlight_type() already will know that a native backlight-device is available (and preferred) when acpi_video_register_backlight() runs and the registering of the acpi_video# device will then be skipped, removing it briefly showing up and disappearing again.
One problem with this approach is that this relies on the GPU driver to call acpi_video_register_backlight() when it is done. One could argue that this is actually a feature, we have had issues with some desktops where acpi_video falsely registers its backlight (even though there is no internal LCD panel), but this will likely cause issues on some systems (1). So the plan is to queue a delayed work with an 8 second (1) delay from acpi_video_register() and have that register the backlight-device if not done already.
Other issues ============
The above only takes native vs acpi_video backlight issues into account, there are also a couple of other scenarios which may lead to multiple backlight-devices getting registered:
1) Apple laptops using the apple_gmux driver 2) Nvidia laptops using the (new) nvidia-wmi-ec-backlight driver 3) drivers/platform/x86 drivers calling acpi_video_set_dmi_backlight_type() to override the normal acpi_video_get_backlight_type() heuristics after the native/acpi_video drivers have already loaded
The plan for 1) + 2) is to extend the acpi_backlight_type enum with acpi_backlight_gmux and acpi_backlight_nvidia_wmi_ec values and to add detection of these 2 to acpi_video_get_backlight_type().
The plan for 3) is to move the DMI quirks from drivers/platform/x86 drivers which call acpi_video_set_dmi_backlight_type() in to the existing DMI quirk table in drivers/acpi/video_detect.c, so that they will be available during the first/every call of acpi_video_get_backlight_type() and then remove acpi_video_set_dmi_backlight_type()
Regards,
Hans
Footnotes:
1) E.g. systems using the nvidia binary driver, esp. systems using the legacy nvidia binary driver versions. I at least expect the current nvidia binary driver versions to get updated to actually call acpi_video_register_backlight() when necessary.
2) The 8 second delay is the timeout plymouth uses waiting for a kms device to show up when showing the bootsplash. This is based on the amdgpu driver taking 5 seconds to initialize on some systems + margin.