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

["Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>]

Hello Daniel,

On Mon, Aug 19, 2019 at 10:50:37AM +0100, Daniel Thompson wrote:
> On Mon, Aug 19, 2019 at 07:46:28AM +0200, Uwe Kleine-König wrote:
> > Hello Matthias,
> > 
> > On Fri, Aug 16, 2019 at 02:10:51PM -0700, Matthias Kaehlcke wrote:
> > > On Fri, Aug 16, 2019 at 09:47:54PM +0200, Uwe Kleine-König wrote:
> > > > On Fri, Aug 16, 2019 at 10:51:57AM -0700, Matthias Kaehlcke wrote:
> > > > > Hi Uwe,
> > > > > 
> > > > > On Fri, Aug 16, 2019 at 06:51:48PM +0200, Uwe Kleine-König wrote:
> > > > > > On Tue, Jul 09, 2019 at 12:00:05PM -0700, Matthias Kaehlcke wrote:
> > > > > > > Backlight brightness curves can have different shapes. The two main
> > > > > > > types are linear and non-linear curves. The human eye doesn't
> > > > > > > perceive linearly increasing/decreasing brightness as linear (see
> > > > > > > also 88ba95bedb79 "backlight: pwm_bl: Compute brightness of LED
> > > > > > > linearly to human eye"), hence many backlights use non-linear (often
> > > > > > > logarithmic) brightness curves. The type of curve currently is opaque
> > > > > > > to userspace, so userspace often uses more or less reliable heuristics
> > > > > > > (like the number of brightness levels) to decide whether to treat a
> > > > > > > backlight device as linear or non-linear.
> > > > > > > 
> > > > > > > Export the type of the brightness curve via the new sysfs attribute
> > > > > > > 'scale'. The value of the attribute can be 'linear', 'non-linear' or
> > > > > > > 'unknown'. For devices that don't provide information about the scale
> > > > > > > of their brightness curve the value of the 'scale' attribute is 'unknown'.
> > > > > > > 
> > > > > > > Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
> > > > > > 
> > > > > > I wonder what kind of problem you are solving here. Can you describe
> > > > > > that in a few words?
> > > > > 
> > > > > The human eye perceives brightness in a logarithmic manner. For
> > > > > backlights with a linear brightness curve brightness controls like
> > > > > sliders need to use a mapping to achieve a behavior that is perceived
> > > > > as linear-ish (more details: http://www.pathwaylighting.com/products/downloads/brochure/technical_materials_1466797044_Linear+vs+Logarithmic+Dimming+White+Paper.pdf)
> > > > > 
> > > > > As of now userspace doesn't have information about the type of the
> > > > > brightness curve, and often uses heuristics to make a guess, which may
> > > > > be right most of the time, but not always. The new attribute eliminates
> > > > > the need to guess.
> > > > 
> > > > This is about backlights right? So the kernel provides to userspace an
> > > > interval [0, x] for some x and depending on the physics of the the
> > > > backlight configuring x/2 (probably?) either means 50% measured light or
> > > > 50% perceived light, right?
> > > 
> > > correct
> > > 
> > > > I wonder if it would be possible instead of giving different backlight
> > > > implementations the freedom to use either linear or logarithmic (or
> > > > quadratic?) scaling and tell userspace which of the options were picked
> > > > require the drivers to provide a (say) linear scaling and then userspace
> > > > wouldn't need to care about the exact physics.
> > > 
> > > In an ideal world the backlight interface would be consistent as you
> > > suggest, however there are plenty of existing devices which use the
> > > 'other' scaling (regardless of which is chosen as the 'correct'
> > > one). Userspace still has to deal with these. And changing previously
> > > 'logarithmic' drivers to linear (or viceversa) may 'break' userspace,
> > > when it keeps using its 'old' scaling, which now isn't correct anymore.
> > 
> > It might be subjective, or maybe I'm just too optimistic, but I think if
> > there was no policy before about the meaning of
> > 
> > 	echo 17 > brightness
> > 
> > other than "brighter than lower values and darker than higher ones"
> > introducing (say) the scale is intended to represent a linear brightness
> > curve is ok.
> > 
> > Unless userspace jumps through hoops and tries to identify the actual
> > device it is running on it is wrong on some machines anyhow and we're
> > only shifting the set of affected machines with a tighter policy (until
> > that userspace application is fixed).
> 
> I believe that there are two common approaches by userspace at present:
> 
> 1. Assume the scale is perceptual and we can directly map a slider
>    to the backlight value. This is common simply because most ACPI
>    backlights are perceptual and therefore when tested in a laptop
>    it works OK.
> 
> 2. Assume that is max brightness is small (e.g. ACPI) then the
>    scale is perceptual and if the max brightness is large (e.g.
>    a PWM) then the scale is linear and apply a correction
>    function between the slider and the control.
> 
> That historic baggage makes is diffcult to "just define a standardized
> scale"... especially given that if we selected a standardized scale we
> would probably want a perceptual scale with lots of steps (e.g. break
> the heuristic).

With "perceptual" you mean that logarithmic stuff, right?

I would tend to go for linear because this is easily measureable and
also is straight forward to implement in the usual cases (attention: I
assume that "usual" means something like PWM and I don't know much about
the physics of backlights but just assume that a PWM will create a
linear mapping).

> > 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) 

Then work on the new stuff instead of making the old stuff (that is
intended to die) harder to use correctly?

> 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.

It would be enough to do this in a code comment then. That would come
without the need to adapt the old userspace API. Also when the old
solution works at 95% instead of 90% before, it will resist harder to
dying.

Best regards
Uwe

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Pengutronix e.K.                           | Uwe Kleine-König            |
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