Re: [PATCH 4/5] backlight: pwm_bl: switch to power-of-2 base for fixed-point math
From: Daniel Thompson <hidden>
Date: 2019-10-07 15:32:15
Also in:
dri-devel, linux-pwm, lkml
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 <redacted>
quoted hunk ↗ jump to hunk
--- 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