Re: [PATCH 2/2] backlight: pwm_bl: Get number of brightness levels for CIE 1931 from the device tree

[Matthias Kaehlcke <mka@chromium.org>]

Hi Pavel,

On Tue, Jun 11, 2019 at 12:18:43PM +0200, Pavel Machek wrote:
> On Mon 2019-06-10 16:37:39, Matthias Kaehlcke wrote:
> > Commit 88ba95bedb79 ("backlight: pwm_bl: Compute brightness of LED
> > linearly to human eye") uses pwm_period / hweight32(pwm_period) as
> > as heuristic to determine the number of brightness levels when the DT
> > doesn't provide a brightness level table. This heuristic is broken
> > and can result in excessively large brightness tables.
> > 
> > Instead of using the heuristic try to retrieve the number of
> > brightness levels from the device tree (property 'max-brightness'
> > + 1). If the value is not specified use a default of 256 levels.
> > 
> > Fixes: 88ba95bedb79 ("backlight: pwm_bl: Compute brightness of LED linearly to human eye")
> 
> I don't think this one is suitable for stable. I'm pretty sure the
> heuristics works well for many boards, and you just replaced it with
> another heuristics ("256").

whether the patch is suitable for stable/upstream is certainly
debatable, in any case I'd argue the current heuristic is bogus and
works by accident or at a cost:

nlevels = period / hweight(period)

w/ period = 131071 ns  (0x1FFFF)

  nlevels = 131071 / 17 = 7710

w/ period = 131072 ns (0x20000)

  nlevels = 131072 / 1 = 131072

and some PWMs use significantly higher periods like 1 ms or 10 ms.