| Message ID | 20190919140620.32407-4-linux@rasmusvillemoes.dk |
|---|---|
| State | Not Applicable |
| Headers | show |
| Series | [1/5] backlight: pwm_bl: fix cie1913 comments and constant | expand |
On Thu, Sep 19, 2019 at 04:06:19PM +0200, 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 + > 2^16/2. 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@rasmusvillemoes.dk> Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org> > --- > 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
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;
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@rasmusvillemoes.dk> --- drivers/video/backlight/pwm_bl.c | 22 ++++++++++++---------- 1 file changed, 12 insertions(+), 10 deletions(-)