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>

diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
index ee85055..efb4efc 100644 (file)
--- a/drivers/video/backlight/pwm_bl.c
+++ b/drivers/video/backlight/pwm_bl.c
@@ -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;