On Wed, Sep 28, 2022 at 01:04:01PM +0300, Jani Nikula wrote: > On Fri, 09 Sep 2022, Hans de Goede <hdegoede@xxxxxxxxxx> 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-d37d844b02c6@xxxxxxxxxxxxxxx/ > > > > 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-ea34-652f50343158@xxxxxxxxxx/ > > And a pull-req actually implementing this plan has been send out this week: > > https://lore.kernel.org/dri-devel/261afe3d-7790-e945-adf6-a2c96c9b1eff@xxxxxxxxxx/ > > > > > > 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.) Why don't we just normalize the range to something sensible? > > > 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? Hmm. I guess due to this. I'd prefer the opposite tbh so I could just put in my opregion BCLM patch. It's annoying to have to carry it locally just to have reasonable backlight behaviour. -- Ville Syrjälä Intel
