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

On Mon, Jul 20, 2020 at 09:25:20PM -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.

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.

Reviewed-by: Douglas Anderson dianders@chromium.org Reviewed-by: Matthias Kaehlcke mka@chromium.org Signed-off-by: Alexandru Stan amstan@chromium.org

Apologies for the delay. Patch 2/3 meant I had some thinking to do... and then the holiday's took their toll.

Overall this looks good, just some quibbles about broken 64-bit maths.

---

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 82b8d7594701..5193a72305a2 100644 --- a/drivers/video/backlight/pwm_bl.c +++ b/drivers/video/backlight/pwm_bl.c @@ -235,8 +235,7 @@ 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;

@@ -265,12 +264,11 @@ 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)

@@ -278,7 +276,7 @@ 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;

@@ -303,7 +301,13 @@ 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;
  

• 	u32 y1, y2;
  

• 	s64 dx, dy;
  

dx should be 32-bit. It will be truncated to 32-bit when passed to div_s64() so this type is actively misleading about how the maths works.

• 	if (num_input_levels < 2) {
  	dev_err(dev, "can't interpolate\n");
  	return -EINVAL;
  }
  

@@ -313,14 +317,7 @@ 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);
  
  

@@ -332,24 +329,25 @@ 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 = y2 - y1;
  

This is an u32 expression being assigned to a s64. I could be rusty on my fixed point maths but won't this promote too late for the 64-bitness of dy to be useful?

Daniel.

• 		for (x = x1; x < x2; x++) {
  

• 			table[x] = y1 +
  

• 				div_s64(dy * (x - x1), dx);
  	}
  }
  

• 	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
  

@@ -358,15 +356,9 @@ 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;

-- 2.27.0