Hi Matthias,
On 12/6/19 20:00, Matthias Kaehlcke wrote:
> 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@xxxxxxxxxxxx>
Tested on Samsung Chromebook Plus (16-bit pwm)
Tested-by: Enric Balletbo i Serra <enric.balletbo@xxxxxxxxxxxxx>
> ---
> drivers/video/backlight/pwm_bl.c | 24 ++++++------------------
> 1 file changed, 6 insertions(+), 18 deletions(-)
>
> diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
> index fb45f866b923..0b7152fa24f7 100644
> --- a/drivers/video/backlight/pwm_bl.c
> +++ b/drivers/video/backlight/pwm_bl.c
> @@ -194,29 +194,17 @@ int pwm_backlight_brightness_default(struct device *dev,
> struct platform_pwm_backlight_data *data,
> unsigned int period)
> {
> - unsigned int counter = 0;
> - unsigned int i, n;
> + unsigned int i;
> u64 retval;
>
> /*
> - * Count the number of bits needed to represent the period number. The
> - * number of bits is used to calculate the number of levels used for the
> - * brightness-levels table, the purpose of this calculation is have a
> - * pre-computed table with enough levels to get linear brightness
> - * perception. The period is divided by the number of bits so for a
> - * 8-bit PWM we have 255 / 8 = 32 brightness levels or for a 16-bit PWM
> - * we have 65535 / 16 = 4096 brightness levels.
> - *
> - * Note that this method is based on empirical testing on different
> - * devices with PWM of 8 and 16 bits of resolution.
> + * Once we have 4096 levels there's little point going much higher...
> + * neither interactive sliders nor animation benefits from having
> + * more values in the table.
> */
> - n = period;
> - while (n) {
> - counter += n % 2;
> - n >>= 1;
> - }
> + data->max_brightness =
> + min((int)DIV_ROUND_UP(period, fls(period)), 4096);
>
> - data->max_brightness = DIV_ROUND_UP(period, counter);
> data->levels = devm_kcalloc(dev, data->max_brightness,
> sizeof(*data->levels), GFP_KERNEL);
> if (!data->levels)
>
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