Re: [PATCH v3 3/4] backlight: pwm_bl: compute brightness of LED linearly to human eye.

[Daniel Thompson]

On Thu, Feb 08, 2018 at 12:30:31PM +0100, Enric Balletbo i Serra wrote:
> When you want to change the brightness using a PWM signal, one thing you
> need to consider is how human perceive the brightness. Human perceive
> the brightness change non-linearly, we have better sensitivity at low
> luminance than high luminance, so to achieve perceived linear dimming,
> the brightness must be matches to the way our eyes behave. The CIE 1931
> lightness formula is what actually describes how we perceive light.
> 
> This patch computes a default table with the brightness levels filled
> with the numbers provided by the CIE 1931 algorithm, the number of the
> brightness levels is calculated based on the PWM resolution.
> 
> The calculation of the table using the CIE 1931 algorithm is enabled by
> default when you do not define the 'brightness-levels' propriety in your
> device tree.
> 
> Signed-off-by: Enric Balletbo i Serra <enric.balletbo@xxxxxxxxxxxxx>

Acked-by: Daniel Thompson <daniel.thompson@xxxxxxxxxx>

> ---
> Changes since v2:
> - None
> Changes since v1:
> - Fix an horrible and unjustifiable mistake a missing { fortunately
>   catched very quick.
> 
>  drivers/video/backlight/pwm_bl.c | 149 +++++++++++++++++++++++++++++++++++----
>  1 file changed, 136 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
> index f0a108ab570a..d047d875f251 100644
> --- a/drivers/video/backlight/pwm_bl.c
> +++ b/drivers/video/backlight/pwm_bl.c
> @@ -143,6 +143,107 @@ static const struct backlight_ops pwm_backlight_ops = {
>  };
>  
>  #ifdef CONFIG_OF
> +#define PWM_LUMINANCE_SCALE	10000 /* luminance scale */
> +
> +/* An integer based power function */
> +static u64 int_pow(u64 base, int exp)
> +{
> +	u64 result = 1;
> +
> +	while (exp) {
> +		if (exp & 1)
> +			result *= base;
> +		exp >>= 1;
> +		base *= base;
> +	}
> +
> +	return result;
> +}
> +
> +/*
> + * CIE lightness to PWM conversion.
> + *
> + * The CIE 1931 lightness formula is what actually describes how we perceive
> + * light:
> + *          Y = (L* / 902.3)           if L* â 0.08856
> + *          Y = ((L* + 16) / 116)^3    if L* > 0.08856
> + *
> + * 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
> + * perceives the screen to be, and is a number between 0 and 100.
> + *
> + * The following function does the fixed point maths needed to implement the
> + * above formula.
> + */
> +static u64 cie1931(unsigned int lightness, unsigned int scale)
> +{
> +	u64 retval;
> +
> +	lightness *= 100;
> +	if (lightness <= (8 * scale)) {
> +		retval = DIV_ROUND_CLOSEST_ULL(lightness * 10, 9023);
> +	} else {
> +		retval = int_pow((lightness + (16 * scale)) / 116, 3);
> +		retval = DIV_ROUND_CLOSEST_ULL(retval, (scale * scale));
> +	}
> +
> +	return retval;
> +}
> +
> +/*
> + * Create a default correction table for PWM values to create linear brightness
> + * for LED based backlights using the CIE1931 algorithm.
> + */
> +static
> +int pwm_backlight_brightness_default(struct device *dev,
> +				     struct platform_pwm_backlight_data *data,
> +				     unsigned int period)
> +{
> +	unsigned int counter = 0;
> +	unsigned int i, n;
> +	u64 retval;
> +
> +	/*
> +	 * Count the number of bits needed to represent the period number. The
> +	 * number of bits is used to calculate the number of levels used for the
> +	 * brightness-levels table, the purpose of this calculation is have a
> +	 * pre-computed table with enough levels to get linear brightness
> +	 * perception. The period is divided by the number of bits so for a
> +	 * 8-bit PWM we have 255 / 8 = 32 brightness levels or for a 16-bit PWM
> +	 * we have 65535 / 16 = 4096 brightness levels.
> +	 *
> +	 * Note that this method is based on empirical testing on different
> +	 * devices with PWM of 8 and 16 bits of resolution.
> +	 */
> +	n = period;
> +	while (n) {
> +		counter += n % 2;
> +		n >>= 1;
> +	}
> +
> +	data->max_brightness = DIV_ROUND_UP(period, counter);
> +	data->levels = devm_kcalloc(dev, data->max_brightness,
> +				    sizeof(*data->levels), GFP_KERNEL);
> +	if (!data->levels)
> +		return -ENOMEM;
> +
> +	/* 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;
> +		retval = DIV_ROUND_CLOSEST_ULL(retval, PWM_LUMINANCE_SCALE);
> +		if (retval > UINT_MAX)
> +			return -EINVAL;
> +		data->levels[i] = (unsigned int)retval;
> +	}
> +
> +	data->dft_brightness = data->max_brightness / 2;
> +	data->max_brightness--;
> +
> +	return 0;
> +}
> +
>  static int pwm_backlight_parse_dt(struct device *dev,
>  				  struct platform_pwm_backlight_data *data)
>  {
> @@ -161,10 +262,13 @@ static int pwm_backlight_parse_dt(struct device *dev,
>  
>  	memset(data, 0, sizeof(*data));
>  
> -	/* determine the number of brightness levels */
> +	/*
> +	 * Determine the number of brightness levels, if this property is not
> +	 * set a default table of brightness levels will be used.
> +	 */
>  	prop = of_find_property(node, "brightness-levels", &length);
>  	if (!prop)
> -		return -EINVAL;
> +		return 0;
>  
>  	data->max_brightness = length / sizeof(u32);
>  
> @@ -294,6 +398,14 @@ static int pwm_backlight_parse_dt(struct device *dev,
>  {
>  	return -ENODEV;
>  }
> +
> +static
> +int pwm_backlight_brightness_default(struct device *dev,
> +				     struct platform_pwm_backlight_data *data,
> +				     unsigned int period)
> +{
> +	return -ENODEV;
> +}
>  #endif
>  
>  static int pwm_backlight_initial_power_state(const struct pwm_bl_data *pb)
> @@ -334,7 +446,9 @@ static int pwm_backlight_probe(struct platform_device *pdev)
>  	struct backlight_device *bl;
>  	struct device_node *node = pdev->dev.of_node;
>  	struct pwm_bl_data *pb;
> +	struct pwm_state state;
>  	struct pwm_args pargs;
> +	unsigned int i;
>  	int ret;
>  
>  	if (!data) {
> @@ -359,17 +473,6 @@ static int pwm_backlight_probe(struct platform_device *pdev)
>  		goto err_alloc;
>  	}
>  
> -	if (data->levels) {
> -		unsigned int i;
> -
> -		for (i = 0; i <= data->max_brightness; i++)
> -			if (data->levels[i] > pb->scale)
> -				pb->scale = data->levels[i];
> -
> -		pb->levels = data->levels;
> -	} else
> -		pb->scale = data->max_brightness;
> -
>  	pb->notify = data->notify;
>  	pb->notify_after = data->notify_after;
>  	pb->check_fb = data->check_fb;
> @@ -436,6 +539,26 @@ static int pwm_backlight_probe(struct platform_device *pdev)
>  
>  	dev_dbg(&pdev->dev, "got pwm for backlight\n");
>  
> +	if (!data->levels) {
> +		/* Get the PWM period (in nanoseconds) */
> +		pwm_get_state(pb->pwm, &state);
> +
> +		ret = pwm_backlight_brightness_default(&pdev->dev, data,
> +						       state.period);
> +		if (ret < 0) {
> +			dev_err(&pdev->dev,
> +				"failed to setup default brightness table\n");
> +			goto err_alloc;
> +		}
> +	}
> +
> +	for (i = 0; i <= data->max_brightness; i++) {
> +		if (data->levels[i] > pb->scale)
> +			pb->scale = data->levels[i];
> +
> +		pb->levels = data->levels;
> +	}
> +
>  	/*
>  	 * FIXME: pwm_apply_args() should be removed when switching to
>  	 * the atomic PWM API.
> -- 
> 2.15.1
>