Re: [PATCH v3 3/4] backlight: pwm_bl: compute brightness of LED linearly to human eye.

[Matthias Kaehlcke <mka@chromium.org>]

Hi Daniel,

On Wed, Jun 12, 2019 at 12:03:25PM +0100, Daniel Thompson wrote:
> On Tue, Jun 11, 2019 at 03:30:19PM -0700, Matthias Kaehlcke wrote:
> > On Tue, Jun 11, 2019 at 09:55:30AM -0700, Brian Norris wrote:
> > > On Tue, Jun 11, 2019 at 3:49 AM Daniel Thompson
> > > <daniel.thompson@linaro.org> wrote:
> > > > This is a long standing flaw in the backlight interfaces. AFAIK generic
> > > > userspaces end up with a (flawed) heuristic.
> > > 
> > > Bingo! Would be nice if we could start to fix this long-standing flaw.
> > 
> > Agreed!
> > 
> > How could a fix look like, a sysfs attribute? Would a boolean value
> > like 'logarithmic_scale' or 'linear_scale' be enough or could more
> > granularity be needed?
> 
> Certainly "linear" (this device will work more or less correctly if the
> userspace applies perceptual curves). Not sure about logarithmic since
> what is actually useful is something that is "perceptually linear"
> (logarithmic is merely a way to approximate that).
> 
> I do wonder about a compatible string like most-detailed to
> least-detailed description. This for a PWM with the auto-generated
> tables we'd see something like:
> 
> cie-1991,perceptual,non-linear
> 
> For something that is non-linear but we are not sure what its tables are
> we can offer just "non-linear".

Thanks for the feedback!

It seems clear that we want a string for the added flexibility. I can
work on a patch with the compatible string like description you
suggested and we can discuss in the review if we want to go with that
or prefer something else.

> > The new attribute could be optional (it only exists if explicitly
> > specified by the driver) or be set to a default based on a heuristic
> > if not specified and be 'fixed' on a case by case basis. The latter
> > might violate "don't break userspace" though, so I'm not sure it's a
> > good idea.
> 
> I think we should avoid any heuristic! There are several drivers and we
> may not be able to work through all of them and make the correct
> decision.

Agreed

> Instead one valid value for the sysfs should be "unknown" and this be
> the default for drivers we have not analysed (this also makes it easy to
> introduce change here).

An "unknown" value sounds good, it allows userspace to just do what it
did/would hace done before this attribute existed.

> We should only set the property to something else for drivers that have
> been reviewed.
> 
> There could be a special case for pwm_bl.c in that I'm prepared to
> assume that the hardware components downstream of the PWM have a
> roughly linear response and that if the user provided tables that their
> function is to provide a perceptually comfortable response.

Unfortunately this isn't universally true :(

At least several Chrome OS devices use a linear brightness scale and
userspace does the transformation in the animated slider. A quick
'git grep -A10 brightness-levels arch' suggests that there are
multiple other devices/platforms using a linear scale.

We could treat devices with a predefined brightness table as
"unknown", unless there is a (new optional) DT property that indicates
the type of the scale.

Cheers

Matthias