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@rasmusvillemoes.dk
Reviewed-by: Daniel Thompson daniel.thompson@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.
- 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 = 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;-- 2.20.1