[PATCH v2 0/4] backlight: pwm_bl: optimizations and small fix for cie1913

[PATCH v2 0/4] backlight: pwm_bl: optimizations and small fix for cie1913

[Rasmus Villemoes]

These patches optimize the cie1913() implementation by using fewer 64
bit divisions and multiplications. It also contains a minor fix for
the linear constant used.

v2:
- Drop patch 5.
- Fix thinko in patch 4, otherwise no code change.
- Better changelog in patch 3.
- Add Daniel's Reviewed-By to the four patches.

Daniel, I took the liberty of adding your R-B to patch 4 despite
changing it a little to fix a thinko - I should add 1<<31 and not
1<<15. Please tell me if that was inappropriate.

Rasmus Villemoes (4):
  backlight: pwm_bl: fix cie1913 comments and constant
  backlight: pwm_bl: eliminate a 64/32 division
  backlight: pwm_bl: drop use of int_pow()
  backlight: pwm_bl: switch to power-of-2 base for fixed-point math

 drivers/video/backlight/pwm_bl.c | 29 +++++++++++++++++++----------
 1 file changed, 19 insertions(+), 10 deletions(-)

-- 
2.20.1

[PATCH v2 1/4] backlight: pwm_bl: fix cie1913 comments and constant

[Rasmus Villemoes]

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(-)

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));
-- 
2.20.1

Re: [PATCH v2 1/4] backlight: pwm_bl: fix cie1913 comments and constant

[Lee Jones]

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.

-- 
Lee Jones [李琼斯]
Linaro Services Technical Lead
Linaro.org │ Open source software for ARM SoCs
Follow Linaro: Facebook | Twitter | Blog

Re: [PATCH v2 1/4] backlight: pwm_bl: fix cie1913 comments and constant

[Rasmus Villemoes]

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 v2 2/4] backlight: pwm_bl: eliminate a 64/32 division

[Rasmus Villemoes]

lightness*1000 is nowhere near overflowing 32 bits, so we can just use
an ordinary 32/32 division, which is much cheaper than the 64/32 done
via do_div().

Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
---
 drivers/video/backlight/pwm_bl.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
index cc44a02e95c7..672c5e7cfcd1 100644
--- a/drivers/video/backlight/pwm_bl.c
+++ b/drivers/video/backlight/pwm_bl.c
@@ -177,7 +177,7 @@ static u64 cie1931(unsigned int lightness, unsigned int scale)
 	 */
 	lightness *= 100;
 	if (lightness <= (8 * scale)) {
-		retval = DIV_ROUND_CLOSEST_ULL(lightness * 10, 9033);
+		retval = DIV_ROUND_CLOSEST(lightness * 10, 9033);
 	} else {
 		retval = int_pow((lightness + (16 * scale)) / 116, 3);
 		retval = DIV_ROUND_CLOSEST_ULL(retval, (scale * scale));
-- 
2.20.1

Re: [PATCH v2 2/4] backlight: pwm_bl: eliminate a 64/32 division

[Lee Jones]

On Tue, 08 Oct 2019, Rasmus Villemoes wrote:

> lightness*1000 is nowhere near overflowing 32 bits, so we can just use
> an ordinary 32/32 division, which is much cheaper than the 64/32 done
> via do_div().
> 
> Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
> Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> ---
>  drivers/video/backlight/pwm_bl.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Applied, thanks.

-- 
Lee Jones [李琼斯]
Linaro Services Technical Lead
Linaro.org │ Open source software for ARM SoCs
Follow Linaro: Facebook | Twitter | Blog

[PATCH v2 3/4] backlight: pwm_bl: drop use of int_pow()

[Rasmus Villemoes]

For a fixed small exponent of 3, it is more efficient to simply use
two explicit multiplications rather than calling the int_pow() library
function: Aside from the function call overhead, its implementation
using repeated squaring means it ends up doing four 64x64
multiplications.

Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
---
 drivers/video/backlight/pwm_bl.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
index 672c5e7cfcd1..273d3fb628a0 100644
--- a/drivers/video/backlight/pwm_bl.c
+++ b/drivers/video/backlight/pwm_bl.c
@@ -179,7 +179,8 @@ static u64 cie1931(unsigned int lightness, unsigned int scale)
 	if (lightness <= (8 * scale)) {
 		retval = DIV_ROUND_CLOSEST(lightness * 10, 9033);
 	} else {
-		retval = int_pow((lightness + (16 * scale)) / 116, 3);
+		retval = (lightness + (16 * scale)) / 116;
+		retval *= retval * retval;
 		retval = DIV_ROUND_CLOSEST_ULL(retval, (scale * scale));
 	}
 
-- 
2.20.1

Re: [PATCH v2 3/4] backlight: pwm_bl: drop use of int_pow()

[Lee Jones]

On Tue, 08 Oct 2019, Rasmus Villemoes wrote:

> For a fixed small exponent of 3, it is more efficient to simply use
> two explicit multiplications rather than calling the int_pow() library
> function: Aside from the function call overhead, its implementation
> using repeated squaring means it ends up doing four 64x64
> multiplications.
> 
> Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
> Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> ---
>  drivers/video/backlight/pwm_bl.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)

Applied, thanks.

-- 
Lee Jones [李琼斯]
Linaro Services Technical Lead
Linaro.org │ Open source software for ARM SoCs
Follow Linaro: Facebook | Twitter | Blog

[PATCH v2 4/4] backlight: pwm_bl: switch to power-of-2 base for fixed-point math

[Rasmus Villemoes]

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).

Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Rasmus Villemoes <linux@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 273d3fb628a0..a99c2210c935 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 += 1ULL << (2*PWM_LUMINANCE_SHIFT - 1);
+		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

Re: [PATCH v2 4/4] backlight: pwm_bl: switch to power-of-2 base for fixed-point math

[Lee Jones]

On Tue, 08 Oct 2019, Rasmus Villemoes wrote:

> 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).
> 
> Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
> Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> ---
>  drivers/video/backlight/pwm_bl.c | 22 ++++++++++++----------
>  1 file changed, 12 insertions(+), 10 deletions(-)

Applied, thanks.

-- 
Lee Jones [李琼斯]
Linaro Services Technical Lead
Linaro.org │ Open source software for ARM SoCs
Follow Linaro: Facebook | Twitter | Blog