[PATCH] backlight: pwm_bl: Fix heuristic to determine number of brightness levels

[Enric Balletbo i Serra]

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 at chromium.org>

Tested on Samsung Chromebook Plus (16-bit pwm)

Tested-by: Enric Balletbo i Serra <enric.balletbo at collabora.com>


> ---
>  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)
>