[PATCH 4/5] backlight: pwm_bl: switch to power-of-2 base for fixed-point math
Daniel Thompson
daniel.thompson at linaro.org
Mon Oct 7 15:32:07 UTC 2019
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 at rasmusvillemoes.dk>
Reviewed-by: Daniel Thompson <daniel.thompson at linaro.org>
> ---
> 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
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