Commit ca58b370 authored by Rasmus Villemoes's avatar Rasmus Villemoes Committed by Lee Jones
Browse files

backlight: pwm_bl: Switch to power-of-2 base for fixed-point math 



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).

Signed-off-by: default avatarRasmus Villemoes <linux@rasmusvillemoes.dk>
Reviewed-by: default avatarDaniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: default avatarLee Jones <lee.jones@linaro.org>
parent 407feae1

drivers/video/backlight/pwm_bl.c

+12 −10
Original line number Diff line number Diff line
@@ -149,7 +149,8 @@ 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.
@@ -166,23 +167,25 @@ 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;
@@ -216,8 +219,7 @@ 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;

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