On Tue, Aug 20, 2019 at 04:49:21PM +0200, Daniel Vetter wrote: > On Mon, Aug 19, 2019 at 11:50 AM Daniel Thompson > <daniel.thompson@xxxxxxxxxx> wrote: > > On Mon, Aug 19, 2019 at 07:46:28AM +0200, Uwe Kleine-König wrote: > > > And the big upside is that in the end (i.e. when all kernel drivers and > > > userspace applications are adapted to provide/consume the "correct" > > > curve) the result is simpler. > > > > My view is that this convergence will eventually be achieved but it will > > happen through the obsolescence of the backlight sysfs interface. The > > sysfs interface has other flaws, in particular no integration with the > > DRM connector API. > > > > Thus I would expect an alternative interface to emerge, most likely as > > part of the DRM connector API. I'd expect such a new API to a > > perceptual scale and to have a fixed max brightness with enough > > steps to support animated backlight effects (IIRC 0..100 has been > > proposed in the past) > > > > In the mean time getting the existing collection of backlight drivers > > marked up as linear/logarithmic/etc will ease the introduction of that > > API because, within the kernel, we might have gathered enough knowledge > > to have some hope of correctly mapping each backlight onto a > > standardized scale. > > In case people wonder why the drm connector based backlight interface > hasn't happened ages ago, some more context: > > - userspace (well libbacklight) selects the right backlight, using > some priority search. Plus blacklists in drivers to make sure they're > not overriding the real backlight driver (e.g. acpi has higher > priority in libbacklight, but on modern system it's not the backlight > driver you want. If we move that into the kernel it's going to be > somewhat a mess, since defacto you never know when loading is complete > and you actually have the right backlight driver. > > This isn't a problem on DT platforms, but really just for x86/acpi > platforms. But if we don't fix them, then userspace adoption of these > new interfaces will likely be too low to matter. > > - second issue is that right now the kms client is supposed to handle > backlight around modeset, like fbdev does through the fb notifier. > Except for drivers which do handle the backlight across modesets, but > maybe not the right backlight. If we move the backlight interface to > drm connectors then the right thing would be for the drm driver to > handle backlight enable/disable across modesets. But to make that > work, userspace needs to stop touching it (otherwise userspace first > disables, then the kernel and then on restore the two fight and > usually black screen wins), and that's a bit a tricky uapi problem of > not breaking existing userspace. > > - finally there's some userspace which assumes the lowest backlight > setting is actually off, and uses that to do fast modesets. This > doesn't work on most ACPI backlights, so I think that problem isn't > widespread. > > Anyway from watching from afar, I think this clarification on what the > backlight scale means internally should at least help us somewhat in > the long term. But the long term solution itself needs someone with > way too much time I fear, so lets not hold up anything on that. Thanks for sharing your views on this. Daniel. _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
