[v2,1/4] backlight: pwm_bl: fix cie1913 comments and constant
diff mbox series

Message ID 20191008120327.24208-2-linux@rasmusvillemoes.dk
State New
Headers show
Series
  • backlight: pwm_bl: optimizations and small fix for cie1913
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Commit Message

Rasmus Villemoes Oct. 8, 2019, 12:03 p.m. UTC
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.

Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
---
 drivers/video/backlight/pwm_bl.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

Comments

Lee Jones Oct. 14, 2019, 7:27 a.m. UTC | #1
On Tue, 08 Oct 2019, Rasmus Villemoes wrote:

> 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.
> 
> Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
> Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> ---
>  drivers/video/backlight/pwm_bl.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)

Applied, thanks.
Rasmus Villemoes Oct. 23, 2019, 8:21 p.m. UTC | #2
On 14/10/2019 09.27, Lee Jones wrote:

> Applied, thanks.

I'm not seeing the series in next-20191023, should it be there?

Rasmus

Patch
diff mbox series

diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
index 746eebc411df..cc44a02e95c7 100644
--- a/drivers/video/backlight/pwm_bl.c
+++ b/drivers/video/backlight/pwm_bl.c
@@ -155,8 +155,8 @@  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
@@ -169,9 +169,15 @@  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));