backlight: pwm_bl: Fix cie1913 comments and constant

The "break-even" point for the two formulas is L==8, which is also
what the code actually implements. [Incidentally, at that point one
has Y=0.008856, not 0.08856].

Moreover, all the sources I can find say the linear factor is 903.3
rather than 902.3, which makes sense since then the formulas agree at
L==8, both yielding the 0.008856 figure to four significant digits.

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 9c4216f08c5a1bc6b50d48d67ec3385f2934f11b..3525e04791ced61a392f0dfb2dbcfaec6ef6e770 100644 (file)
--- a/drivers/video/backlight/pwm_bl.c
+++ b/drivers/video/backlight/pwm_bl.c
@@ -156,8 +156,8 @@ static const struct backlight_ops pwm_backlight_ops = {
  *
  * The CIE 1931 lightness formula is what actually describes how we perceive
  * light:
- *          Y = (L* / 902.3)           if L* ≤ 0.08856
- *          Y = ((L* + 16) / 116)^3    if L* > 0.08856
+ *          Y = (L* / 903.3)           if L* ≤ 8
+ *          Y = ((L* + 16) / 116)^3    if L* > 8
  *
  * Where Y is the luminance, the amount of light coming out of the screen, and
  * is a number between 0.0 and 1.0; and L* is the lightness, how bright a human
@@ -170,9 +170,15 @@ static u64 cie1931(unsigned int lightness, unsigned int scale)
 {
        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.
+        */
        lightness *= 100;
        if (lightness <= (8 * scale)) {
-               retval = DIV_ROUND_CLOSEST_ULL(lightness * 10, 9023);
+               retval = DIV_ROUND_CLOSEST_ULL(lightness * 10, 9033);
        } else {
                retval = int_pow((lightness + (16 * scale)) / 116, 3);
                retval = DIV_ROUND_CLOSEST_ULL(retval, (scale * scale));