[PATCH v3 2/4] backlight: Expose brightness curve type through sysfs

[Daniel Thompson]

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 at linaro.org> 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.