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

[Daniel Thompson]

On 12/06/2019 19:00, 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>

As I understand it we can't determine the PWM quantization without 
actually programming it so the table could still be oversized after this 
patch (e.g. multiple entries end up with same physical brightness) but 
since it should always be monotonic and the table size will cap out at a 
sane value then:

Acked-by: Daniel Thompson <daniel.thompson at linaro.org>


> ---
>   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)
>