[PATCH 4/5] backlight: pwm_bl: switch to power-of-2 base for fixed-point math
Rasmus Villemoes
linux at rasmusvillemoes.dk
Thu Sep 19 14:06:19 UTC 2019
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>
---
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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