Re: [PATCH 4/5] backlight: pwm_bl: switch to power-of-2 base for fixed-point math

[Daniel Thompson]

On Thu, Sep 19, 2019 at 04:06:19PM +0200, Rasmus Villemoes wrote:
> Using a power-of-2 instead of power-of-10 base makes the computations
> much cheaper. 2^16 is safe; retval never becomes more than 2^48 +
> 2^16/2. On a 32 bit platform, the very expensive 64/32 division at the
> end of cie1931() instead becomes essentially free (a shift by 32 is
> just a register rename).
> 
> Signed-off-by: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>

Reviewed-by: Daniel Thompson <daniel.thompson@xxxxxxxxxx>

> ---
>  drivers/video/backlight/pwm_bl.c | 22 ++++++++++++----------
>  1 file changed, 12 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
> index aee6839e024a..102bc191310f 100644
> --- a/drivers/video/backlight/pwm_bl.c
> +++ b/drivers/video/backlight/pwm_bl.c
> @@ -148,7 +148,8 @@ static const struct backlight_ops pwm_backlight_ops = {
>  };
>  
>  #ifdef CONFIG_OF
> -#define PWM_LUMINANCE_SCALE	10000 /* luminance scale */
> +#define PWM_LUMINANCE_SHIFT	16
> +#define PWM_LUMINANCE_SCALE	(1 << PWM_LUMINANCE_SHIFT) /* luminance scale */
>  
>  /*
>   * CIE lightness to PWM conversion.
> @@ -165,23 +166,25 @@ static const struct backlight_ops pwm_backlight_ops = {
>   * The following function does the fixed point maths needed to implement the
>   * above formula.
>   */
> -static u64 cie1931(unsigned int lightness, unsigned int scale)
> +static u64 cie1931(unsigned int lightness)
>  {
>  	u64 retval;
>  
>  	/*
>  	 * @lightness is given as a number between 0 and 1, expressed
> -	 * as a fixed-point number in scale @scale. Convert to a
> -	 * percentage, still expressed as a fixed-point number, so the
> -	 * above formulas can be applied.
> +	 * as a fixed-point number in scale
> +	 * PWM_LUMINANCE_SCALE. Convert to a percentage, still
> +	 * expressed as a fixed-point number, so the above formulas
> +	 * can be applied.
>  	 */
>  	lightness *= 100;
> -	if (lightness <= (8 * scale)) {
> +	if (lightness <= (8 * PWM_LUMINANCE_SCALE)) {
>  		retval = DIV_ROUND_CLOSEST(lightness * 10, 9033);
>  	} else {
> -		retval = (lightness + (16 * scale)) / 116;
> +		retval = (lightness + (16 * PWM_LUMINANCE_SCALE)) / 116;
>  		retval *= retval * retval;
> -		retval = DIV_ROUND_CLOSEST_ULL(retval, (scale * scale));
> +		retval += PWM_LUMINANCE_SCALE/2;
> +		retval >>= 2*PWM_LUMINANCE_SHIFT;
>  	}
>  
>  	return retval;
> @@ -215,8 +218,7 @@ int pwm_backlight_brightness_default(struct device *dev,
>  	/* Fill the table using the cie1931 algorithm */
>  	for (i = 0; i < data->max_brightness; i++) {
>  		retval = cie1931((i * PWM_LUMINANCE_SCALE) /
> -				 data->max_brightness, PWM_LUMINANCE_SCALE) *
> -				 period;
> +				 data->max_brightness) * period;
>  		retval = DIV_ROUND_CLOSEST_ULL(retval, PWM_LUMINANCE_SCALE);
>  		if (retval > UINT_MAX)
>  			return -EINVAL;
> -- 
> 2.20.1