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:

quoted

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.