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With commit 88ba95bedb79 ("backlight: pwm_bl: Compute brightness of
LED linearly to human eye") the number of set bits (aka hweight())
in the PWM period is used in the heuristic to determine the number
of brightness levels, when the brightness table isn't specified in
the DT. The number of set bits doesn't provide a reliable clue about
the length of the period, instead change the heuristic to:
nlevels = period / fls(period)
Also limit the maximum number of brightness levels to 4096 to avoid
excessively large tables.
With this the number of levels increases monotonically with the PWM
period, until the maximum of 4096 levels is reached:
period (ns) # levels
100 16
500 62
1000 111
5000 416
10000 769
50000 3333
100000 4096
Fixes: 88ba95bedb79 ("backlight: pwm_bl: Compute brightness of LED linearly to human eye")
Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Tested-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
73fbfc4994