Re: [PATCH v2 1/3] backlight: pwm_bl: Fix interpolation

[Daniel Thompson <daniel.thompson@linaro.org>]

On Tue, Oct 13, 2020 at 01:01:01AM -0700, Alexandru Stan wrote:
> Whenever num-interpolated-steps was larger than the distance
> between 2 consecutive brightness levels the table would get really
> discontinuous. The slope of the interpolation would stick with
> integers only and if it was 0 the whole line segment would get skipped.
> 
> Example settings:
> 	brightness-levels = <0 1 2 4 8 16 32 64 128 256>;
> 	num-interpolated-steps = <16>;
> 
> The distances between 1 2 4 and 8 would be 1, and only starting with 16
> it would start to interpolate properly.

Both comments a perilously close to nitpicking but enough that I wanted
to reply...

I'd suggest that the current behaviour as having two properties.

1. It was designed to generate strictly increasing tables (no repeated
   values).

2. It's implementation contains quantization errors when calculating the
   step size. This results in both the discards of some interpolated
   steps you mentioned (it is possible to insert extra steps between 4
   and 8 whilst retaining a strictly increasing table). It also
   results in a potentially large undershoot when multiplying a step
   size (64 interpolated steps and a gap of 127 is likely to get a visual
   jump as we hop through 63 physical steps in one go).

#1 can is a policy that can be changed. #2 is a bug that could be fixed.

To be clear I don't object to generating a monotonically increasing
table but I'd prefer the policy change to be explicitly described in
the description.


> Let's change it so there's always interpolation happening, even if
> there's no enough points available (read: values in the table would
> appear more than once). This should match the expected behavior much
> more closely.
> 
> Signed-off-by: Alexandru Stan <amstan@chromium.org>
> ---
> 
>  drivers/video/backlight/pwm_bl.c | 70 ++++++++++++++------------------
>  1 file changed, 31 insertions(+), 39 deletions(-)
> 
> diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
> index dfc760830eb9..3e77f6b73fd9 100644
> --- a/drivers/video/backlight/pwm_bl.c
> +++ b/drivers/video/backlight/pwm_bl.c
> @@ -230,8 +230,7 @@ static int pwm_backlight_parse_dt(struct device *dev,
>  				  struct platform_pwm_backlight_data *data)
>  {
>  	struct device_node *node = dev->of_node;
> -	unsigned int num_levels = 0;
> -	unsigned int levels_count;
> +	unsigned int num_levels;
>  	unsigned int num_steps = 0;
>  	struct property *prop;
>  	unsigned int *table;
> @@ -260,12 +259,11 @@ static int pwm_backlight_parse_dt(struct device *dev,
>  	if (!prop)
>  		return 0;
>  
> -	data->max_brightness = length / sizeof(u32);
> +	num_levels = length / sizeof(u32);
>  
>  	/* read brightness levels from DT property */
> -	if (data->max_brightness > 0) {
> -		size_t size = sizeof(*data->levels) * data->max_brightness;
> -		unsigned int i, j, n = 0;
> +	if (num_levels > 0) {
> +		size_t size = sizeof(*data->levels) * num_levels;
>  
>  		data->levels = devm_kzalloc(dev, size, GFP_KERNEL);
>  		if (!data->levels)
> @@ -273,7 +271,7 @@ static int pwm_backlight_parse_dt(struct device *dev,
>  
>  		ret = of_property_read_u32_array(node, "brightness-levels",
>  						 data->levels,
> -						 data->max_brightness);
> +						 num_levels);
>  		if (ret < 0)
>  			return ret;
>  
> @@ -298,7 +296,13 @@ static int pwm_backlight_parse_dt(struct device *dev,
>  		 * between two points.
>  		 */
>  		if (num_steps) {
> -			if (data->max_brightness < 2) {
> +			unsigned int num_input_levels = num_levels;
> +			unsigned int i;
> +			u32 x1, x2, x, dx;
> +			u32 y1, y2;
> +			s64 dy;
> +
> +			if (num_input_levels < 2) {
>  				dev_err(dev, "can't interpolate\n");
>  				return -EINVAL;
>  			}
> @@ -308,14 +312,7 @@ static int pwm_backlight_parse_dt(struct device *dev,
>  			 * taking in consideration the number of interpolated
>  			 * steps between two levels.
>  			 */
> -			for (i = 0; i < data->max_brightness - 1; i++) {
> -				if ((data->levels[i + 1] - data->levels[i]) /
> -				   num_steps)
> -					num_levels += num_steps;
> -				else
> -					num_levels++;
> -			}
> -			num_levels++;
> +			num_levels = (num_input_levels - 1) * num_steps + 1;
>  			dev_dbg(dev, "new number of brightness levels: %d\n",
>  				num_levels);
>  
> @@ -327,24 +324,25 @@ static int pwm_backlight_parse_dt(struct device *dev,
>  			table = devm_kzalloc(dev, size, GFP_KERNEL);
>  			if (!table)
>  				return -ENOMEM;
> -
> -			/* Fill the interpolated table. */
> -			levels_count = 0;
> -			for (i = 0; i < data->max_brightness - 1; i++) {
> -				value = data->levels[i];
> -				n = (data->levels[i + 1] - value) / num_steps;
> -				if (n > 0) {
> -					for (j = 0; j < num_steps; j++) {
> -						table[levels_count] = value;
> -						value += n;
> -						levels_count++;
> -					}
> -				} else {
> -					table[levels_count] = data->levels[i];
> -					levels_count++;
> +			/*
> +			 * Fill the interpolated table[x] = y
> +			 * by draw lines between each (x1, y1) to (x2, y2).
> +			 */
> +			dx = num_steps;
> +			for (i = 0; i < num_input_levels - 1; i++) {
> +				x1 = i * dx;
> +				x2 = x1 + dx;
> +				y1 = data->levels[i];
> +				y2 = data->levels[i + 1];
> +				dy = (s64)y2 - y1;
> +
> +				for (x = x1; x < x2; x++) {
> +					table[x] = y1 +
> +						div_s64(dy * ((s64)x - x1), dx);

I don't think it is possible for x - x1 to be negative (e.g. what is the
s64 for). Obviously it makes little functional difference whether the
cast is there or not but I don't like fixed point code that has been
written with "just in case" casts.


Daniel.


>  				}
>  			}
> -			table[levels_count] = data->levels[i];
> +			/* Fill in the last point, since no line starts here. */
> +			table[x2] = y2;
>  
>  			/*
>  			 * As we use interpolation lets remove current
> @@ -353,15 +351,9 @@ static int pwm_backlight_parse_dt(struct device *dev,
>  			 */
>  			devm_kfree(dev, data->levels);
>  			data->levels = table;
> -
> -			/*
> -			 * Reassign max_brightness value to the new total number
> -			 * of brightness levels.
> -			 */
> -			data->max_brightness = num_levels;
>  		}
>  
> -		data->max_brightness--;
> +		data->max_brightness = num_levels - 1;
>  	}
>  
>  	return 0;
> -- 
> 2.28.0
>