On Wed, 18 May 2022, Hans de Goede <hdegoede@xxxxxxxxxx> wrote: > Hi, > > On 5/18/22 10:44, Jani Nikula wrote: >> On Tue, 17 May 2022, Hans de Goede <hdegoede@xxxxxxxxxx> wrote: >>> Hi All, >>> >>> As mentioned in my RFC titled "drm/kms: control display brightness through >>> drm_connector properties": >>> https://lore.kernel.org/dri-devel/0d188965-d809-81b5-74ce-7d30c49fee2d@xxxxxxxxxx/ >>> >>> The first step towards this is to deal with some existing technical debt >>> in backlight handling on x86/ACPI boards, specifically we need to stop >>> registering multiple /sys/class/backlight devs for a single display. >> >> I guess my question here is, how do you know it's for a *single* >> display? >> >> There are already designs out there with two flat panels, with >> independent brightness controls. They're still rare and I don't think we >> handle them very well. But we've got code to register multiple native >> backlight interfaces, see e.g. commit 20f85ef89d94 ("drm/i915/backlight: >> use unique backlight device names"). >> >> So imagine a design where one of the panels needs backlight control via >> ACPI and the other via native backlight control. Granted, I don't know >> if one exists, but I think it's very much in the realm of possible >> things the OEMs might do. For example, have an EC PWM for primary panel >> backlight, and use GPU PWM for secondary. How do you know you actually >> do need to register two interfaces? > > On x86/ACPI devices this is all driven by acpi_video_get_backlight_type() / > by the drivers/acpi/video_detect.c code. That code already will break on > systems where there are 2 backlight controls, with one being ACPI based > and the other a native GPU PWM backlight device. > > In this scenario as soon as the native GPU PWM backlight device shows up > then, if acpi_video_get_backlight_type()==native, video_detect.c will > currently unregister the acpi_video# backlight device(s) since userspace > prefers the firmware type over the native type, so for userspace to > actually honor the acpi_video_get_backlight_type()==native we need to get > the acpi_video# backlight device "out of the way" which is currently > handled by unregistering it. > > Note in a way we already have a case where userspace sees 2 panels, > in hybrid laptop setups with a mux and on those systems we may see > either 2 native backlight devices; or 2 native backlight devices + > 2 acpi_video backlight devices with userspace preferring the ACPI > ones. > > Also note that userspace already has code to detect 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. > >> I'm fine with dealing with such cases as they arise to avoid >> over-engineering up front, but I also don't want us to completely paint >> ourselves in a corner either. > > Right. Note that the current code (both with and without this patchset) > already will work fine from a kernel pov as long as both panels > are either using native GPU PWM or are both using ACPI. But if we > ever get a mix then this will need special handling. > > Note that all userspace code I know is currently hardcoded > to assume a single panel. Userspace already picks one preferred > device under /sys/class/backlight and ignores the rest. Actually > atm userspace must behave this way, because on x86/ACPI boards we > do often register multiple backlight devices for a single panel. > > So in a way moving to registering only a single backlight device > prepares for actually having multiple panels work. > > Also keep in mind that this is preparation work for making the > panel brightness a drm_connector property. When the panel uses > a backlight device other then the native GPU PWM to control the > brightness then the helper code for this needs to have a way to > select which backlight_device to use then and the non native > types are not "linked" to a specific connector so in this case > we really need there to be only 1 backlight device registered > so that the code looking up the (non native) backlight device > for the connector gets the right one and not merely the one > which happened to get registered first. > > And I believe that having the panel brightness be a drm_connector > property is the way to make it possible for userspace to deal > with the multiple panels which each have a separate brightness > control case. Agreed. Thanks for the explanations and recording them here. BR, Jani. > > Regards, > > Hans > > > > > >> >> BR, >> Jani. >> >> >>> >>> This series implements my RFC describing my plan for these cleanups: >>> https://lore.kernel.org/dri-devel/98519ba0-7f18-201a-ea34-652f50343158@xxxxxxxxxx/ >>> >>> Specifically patches 1-6 implement the "Fixing kms driver unconditionally >>> register their "native" backlight dev" part. >>> >>> And patches 7-14 implement the "Fixing acpi_video0 getting registered for >>> a brief time" time. >>> >>> Note this series does not deal yet with the "Other issues" part, I plan >>> to do a follow up series for that. >>> >>> The changes in this series are good to have regardless of the further >>> "drm/kms: control display brightness through drm_connector properties" >>> plans. So I plan to push these upstream once they are ready (once >>> reviewed). Since this crosses various subsystems / touches multiple >>> kms drivers my plan is to provide an immutable branch based on say >>> 5.19-rc1 and then have that get merged into all the relevant trees. >>> >>> Please review. >>> >>> Regards, >>> >>> Hans >>> >>> >>> Hans de Goede (14): >>> ACPI: video: Add a native function parameter to >>> acpi_video_get_backlight_type() >>> drm/i915: Don't register backlight when another backlight should be >>> used >>> drm/amdgpu: Don't register backlight when another backlight should be >>> used >>> drm/radeon: Don't register backlight when another backlight should be >>> used >>> drm/nouveau: Don't register backlight when another backlight should be >>> used >>> ACPI: video: Drop backlight_device_get_by_type() call from >>> acpi_video_get_backlight_type() >>> ACPI: video: Remove acpi_video_bus from list before tearing it down >>> ACPI: video: Simplify acpi_video_unregister_backlight() >>> ACPI: video: Make backlight class device registration a separate step >>> ACPI: video: Remove code to unregister acpi_video backlight when a >>> native backlight registers >>> drm/i915: Call acpi_video_register_backlight() >>> drm/nouveau: Register ACPI video backlight when nv_backlight >>> registration fails >>> drm/amdgpu: Register ACPI video backlight when skipping amdgpu >>> backlight registration >>> drm/radeon: Register ACPI video backlight when skipping radeon >>> backlight registration >>> >>> drivers/acpi/acpi_video.c | 69 ++++++++++++++----- >>> drivers/acpi/video_detect.c | 53 +++----------- >>> drivers/gpu/drm/Kconfig | 2 + >>> .../gpu/drm/amd/amdgpu/atombios_encoders.c | 14 +++- >>> .../gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 9 +++ >>> .../gpu/drm/i915/display/intel_backlight.c | 7 ++ >>> drivers/gpu/drm/i915/display/intel_display.c | 1 + >>> drivers/gpu/drm/i915/display/intel_opregion.c | 2 +- >>> drivers/gpu/drm/nouveau/nouveau_backlight.c | 14 ++++ >>> drivers/gpu/drm/radeon/atombios_encoders.c | 7 ++ >>> drivers/gpu/drm/radeon/radeon_encoders.c | 11 ++- >>> .../gpu/drm/radeon/radeon_legacy_encoders.c | 7 ++ >>> drivers/platform/x86/acer-wmi.c | 2 +- >>> drivers/platform/x86/asus-laptop.c | 2 +- >>> drivers/platform/x86/asus-wmi.c | 4 +- >>> drivers/platform/x86/compal-laptop.c | 2 +- >>> drivers/platform/x86/dell/dell-laptop.c | 2 +- >>> drivers/platform/x86/eeepc-laptop.c | 2 +- >>> drivers/platform/x86/fujitsu-laptop.c | 4 +- >>> drivers/platform/x86/ideapad-laptop.c | 2 +- >>> drivers/platform/x86/intel/oaktrail.c | 2 +- >>> drivers/platform/x86/msi-laptop.c | 2 +- >>> drivers/platform/x86/msi-wmi.c | 2 +- >>> drivers/platform/x86/samsung-laptop.c | 2 +- >>> drivers/platform/x86/sony-laptop.c | 2 +- >>> drivers/platform/x86/thinkpad_acpi.c | 4 +- >>> drivers/platform/x86/toshiba_acpi.c | 2 +- >>> include/acpi/video.h | 8 ++- >>> 28 files changed, 156 insertions(+), 84 deletions(-) >> > -- Jani Nikula, Intel Open Source Graphics Center
