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backlight: pwm_bl: Switch to power-of-2 base for fixed-point math 

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 +
2^32/2. 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>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
This commit is contained in:
Rasmus Villemoes 2019-10-08 14:03:27 +02:00 committed by Lee Jones
parent 407feae1ca
commit ca58b37034
1 changed files with 12 additions and 10 deletions
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22
drivers/video/backlight/pwm_bl.c
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@ -149,7 +149,8 @@ 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.
@ -166,23 +167,25 @@ 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 += 1ULL << (2*PWM_LUMINANCE_SHIFT - 1);
retval >>= 2*PWM_LUMINANCE_SHIFT;
}
return retval;
@ -216,8 +219,7 @@ 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;