Hi, On 9/28/22 12:04, Jani Nikula wrote: > On Fri, 09 Sep 2022, Hans de Goede <hdego...@redhat.com> wrote: >> Hi all, >> >> Here is v2 of my "drm/kms: control display brightness through drm_connector >> properties" RFC: >> >> Changes from version 1: >> - Drop bl_brightness_0_is_min_brightness from list of new connector >> properties. >> - Clearly define that 0 is always min-brightness when setting the brightness >> through the connector properties. >> - Drop bl_brightness_control_method from list of new connector >> properties. >> - Phase 1 of the plan has been completed >> >> As discussed already several times in the past: >> https://www.x.org/wiki/Events/XDC2014/XDC2014GoedeBacklight/ >> >> https://lore.kernel.org/all/4b17ba08-39f3-57dd-5aad-d37d844b0...@linux.intel.com/ >> >> The current userspace API for brightness control offered by >> /sys/class/backlight devices has various issues: >> >> 1. There is no way to map the backlight device to a specific >> display-output / panel (1) >> 2. Controlling the brightness requires root-rights requiring >> desktop-environments to use suid-root helpers for this. >> 3. The meaning of 0 is not clearly defined, it can be either off, >> or minimum brightness at which the display is still readable >> (in a low light environment) >> 4. It's not possible to change both the gamma and the brightness in the >> same KMS atomic commit. You'd want to be able to reduce brightness to >> conserve power, and counter the effects of that by changing gamma to >> reach a visually similar image. And you'd want to have the changes take >> effect at the same time instead of reducing brightness at some frame and >> change gamma at some other frame. This is pretty much impossible to do >> via the sysfs interface. >> >> As already discussed on various conference's hallway tracks >> and as has been proposed on the dri-devel list once before (2), >> it seems that there is consensus that the best way to to solve these >> 2 issues is to add support for controlling a video-output's brightness >> through properties on the drm_connector. >> >> This RFC outlines my plan to try and actually implement this, >> which has 3 phases: >> >> >> Phase 1: 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. >> >> A plan to fix this was posted here: >> https://lore.kernel.org/dri-devel/98519ba0-7f18-201a-ea34-652f50343...@redhat.com/ >> And a pull-req actually implementing this plan has been send out this week: >> https://lore.kernel.org/dri-devel/261afe3d-7790-e945-adf6-a2c96c9b1...@redhat.com/ >> >> >> Phase 2: Add drm_connector properties mirroring the matching backlight device >> ============================================================================= >> >> The plan is to add a drm_connector helper function, which optionally takes >> a pointer to the backlight device for the GPU's native backlight device, >> which will then mirror the backlight settings from the backlight device >> in a set of read/write brightness* properties on the connector. >> >> This function can then be called by GPU drivers for the drm_connector for >> the internal panel and it will then take care of everything. When there >> is no native GPU backlight device, or when it should not be used then >> (on x86) the helper will use the acpi_video_get_backlight_type() to >> determine which backlight-device should be used instead and it will find >> + mirror that one. >> >> >> Phase 3: Deprecate /sys/class/backlight uAPI >> ============================================ >> >> Once most userspace has moved over to using the new drm_connector >> brightness props, a Kconfig option can be added to stop exporting >> the backlight-devices under /sys/class/backlight. The plan is to >> just disable the sysfs interface and keep the existing backlight-device >> internal kernel abstraction as is, since some abstraction for (non GPU >> native) backlight devices will be necessary regardless. >> >> It is unsure if we will ever be able to do this. For example people using >> non fully integrated desktop environments like e.g. sway often use custom >> scripts binded to hotkeys to get functionality like the brightness >> up/down keyboard hotkeys changing the brightness. This typically involves >> e.g. the xbacklight utility. >> >> Even if the xbacklight utility is ported to use kms with the new connector >> object brightness properties then this still will not work because >> changing the properties will require drm-master rights and e.g. sway will >> already hold those. >> >> >> The drm_connector brightness properties >> ======================================= >> >> The new uAPI for this consists of 2 properties: >> >> 1. "display brightness": rw 0-int32_max property controlling the brightness >> setting >> of the connected display. The actual maximum of this will be less then >> int32_max and is given in "display brightness max". > > This could use a few words explaining the choice of range and property > type. (I assume it's because you can't change a range property's max at > runtime. Which is also why you need a separate max property.)
Right. One of the things some users are asking for is brightness control for external monitors through e.g. DDC/DI which means that the control only becomes available when the monitor is plugged in, which with laptops may be much later then boot. > >> Unlike the /sys/class/backlight/foo/brightness this brightness property >> has a clear definition for the value 0. The kernel must ensure that 0 >> means minimum brightness (so 0 should _never_ turn the backlight off). >> If necessary the kernel must enforce a minimum value by adding >> an offset to the value seen in the property to ensure this behavior. >> >> For example if necessary the driver must clamp 0-255 to 10-255, which then >> becomes 0-245 on the brightness property, adding 10 internally to writes >> done to the brightness property. This adding of an extra offset when >> necessary must only be done on the brightness property, >> the /sys/class/backlight interface should be left unchanged to not break >> userspace which may rely on 0 = off on some systems. >> >> Note amdgpu already does something like this even for /sys/class/backlight, >> see the use of AMDGPU_DM_DEFAULT_MIN_BACKLIGHT in amdgpu. >> >> Also whenever possible the kernel must ensure that the brightness range >> is in perceived brightness, but this cannot always be guaranteed. > > Do you mean every step should be a visible change? No that would mean that more or less raw 16 bit pwm controls would need to loose a lot of range. This is more about what Ville mentions further in the thread (and which you mention below) when the raw PWM directly controls a LED backlight, or when it linearly controls a current-source for a LED backlight then when going from 10% dutycycle (which would be 0) to 20% would double the brightness where as going from 90% to 100& only adds 11%. Some brightness control methods use a curve to make things feel more linear giving a similar % brightness increase on all parts of the curve. What I'm trying to say here is that ideally the control should always follow such a curve (but only if available, the kernel should not make up its own curves). > >> 2. "display brightness max": ro 0-int32_max property giving the actual >> maximum >> of the display's brightness setting. This will report 0 when brightness >> control is not available. >> >> The value of "display brightness max" may change at runtime, either by >> a legacy drivers/platform/x86 backlight driver loading after the drm >> driver has loaded; or when plugging in a monitor which allows brightness >> control over DDC/CI. In both these cases the max value will change from 0 >> to the actual max value, indicating backlight control has become available >> on this connector. > > I think this could be a bit more restrictive in stating the allowed > runtime changes. Is it only 0 -> actual max for non-hotpluggable > displays, nothing else, and additionally actual max -> 0 when unplugging > a display? Right, only 0 -> actual max for non-hotpluggable displays, nothing else and additionally actual max -> 0 when unplugging a display? I'll adjust the wording here a bit, but I don't plan to make the non-hotpluggable vs hotpluggable difference though. I'm not sure that is helpful. With LCD panel muxes used in some hybrid GPU designs even a laptop panel can be hotplugged (from the GPU pov on a switch its unplugged/plugged-in). > >> >> >> Future extensions >> ================= >> >> Some hardware do adaptive brightness in hardware, rather then providing >> an ALS sensor and letting userspace handle this. >> >> One example of this is the Steam deck, which currently uses some custom >> sysfs attributes to allow tweaking (and enable/disable?) the adaptive >> brightness. Adding standardized uAPI for this through new >> "display brightness *" properties seems like a natural extension of this >> proposal. > > Another example is adjusting for non-linear luminance curve. It would be > nicer for equal steps in brightness value to have similar luminance > changes. Actually that is sorta what I meant above. So I guess what you are suggesting is allowing userspace to load some adjustment curve into the kernel ? That would definitely fall under the "Future Extensions" bit, but yeah that would be interesting. In some cases we may even be able to pre-populate such a curve with firmware provided data. > I think sometimes we lose precision with the limited number of > steps in UIs. But this is thinking pretty far ahead. :) > > Other than the minor clarifications, the plans sounds good to me. Thank you for the review. > Thanks again for doing all this, much appreciated. You're welcome. Regards, Hans > > > BR, > Jani. > >> >> Regards, >> >> Hans >> >> >> 1) The need to be able to map the backlight device to a specific display >> has become clear once more with the recent proposal to add DDCDI support: >> https://lore.kernel.org/lkml/20220403230850.2986-1-yusisameri...@gmail.com/ >> >> 2) >> https://lore.kernel.org/all/4b17ba08-39f3-57dd-5aad-d37d844b0...@linux.intel.com/ >> Note this proposal included a method for userspace to be able to tell the >> kernel if the native/acpi_video/vendor backlight device should be used, >> but this has been solved in the kernel for years now: >> https://www.spinics.net/lists/linux-acpi/msg50526.html >> An initial implementation of this proposal is available here: >> https://cgit.freedesktop.org/~mperes/linux/log/?h=backlight >> >> >
