Messages in this thread | | | Date | Tue, 11 Jun 2019 14:58:59 -0700 | From | Matthias Kaehlcke <> | Subject | Re: [PATCH 2/2] backlight: pwm_bl: Get number of brightness levels for CIE 1931 from the device tree |
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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.
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