# [v3,3/3] backlight: pwm_bl: Fix interpolation

Message ID 20201021220404.v3.3.I4dcea1c90e9da3902d466033aa73351e19e49c49@changeid (mailing list archive) New, archived show PWM backlight interpolation adjustments | expand

## Commit Message

Alexandru M Stan Oct. 22, 2020, 5:04 a.m. UTC
```The previous behavior was a little unexpected, its properties/problems:
1. It was designed to generate strictly increasing values (no repeats)
2. It had quantization errors when calculating step size. Resulting in
unexpected jumps near the end of some segments.

Example settings:
brightness-levels = <0 1 2 4 8 16 32 64 128 256>;
num-interpolated-steps = <16>;

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.

The distances between 1 2 4 and 8 would be 1 (property #1 fighting us),
and only starting with 16 it would start to interpolate properly.

Property #1 is not enough. The goal here is more than just monotonically
increasing. We should still care about the shape of the curve. Repeated
points might be desired if we're in the part of the curve where we want
to go slow (aka slope near 0).

Problem #2 is plainly a bug. Imagine if the 64 entry was 63 instead,
the calculated slope on the 32-63 segment will be almost half as it
should be.

The most expected and simplest algorithm for interpolation is linear
interpolation, which would handle both problems.
Let's just implement that!

Take pairs of points from the brightness-levels array and linearly
interpolate between them. On the X axis (what userspace sees) we'll
now have equally sized intervals (num-interpolated-steps sized,
as opposed to before where we were at the mercy of quantization).

END

Signed-off-by: Alexandru Stan <amstan@chromium.org>
---

drivers/video/backlight/pwm_bl.c | 70 ++++++++++++++------------------
1 file changed, 31 insertions(+), 39 deletions(-)
```

Daniel Thompson Oct. 28, 2020, 3:12 p.m. UTC | #1
```On Wed, Oct 21, 2020 at 10:04:45PM -0700, Alexandru Stan wrote:
> The previous behavior was a little unexpected, its properties/problems:
> 1. It was designed to generate strictly increasing values (no repeats)
> 2. It had quantization errors when calculating step size. Resulting in
> unexpected jumps near the end of some segments.
>
> Example settings:
> 	brightness-levels = <0 1 2 4 8 16 32 64 128 256>;
> 	num-interpolated-steps = <16>;
>
> 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.
>
> The distances between 1 2 4 and 8 would be 1 (property #1 fighting us),
> and only starting with 16 it would start to interpolate properly.
>
> Property #1 is not enough. The goal here is more than just monotonically
> increasing. We should still care about the shape of the curve. Repeated
> points might be desired if we're in the part of the curve where we want
> to go slow (aka slope near 0).
>
> Problem #2 is plainly a bug. Imagine if the 64 entry was 63 instead,
> the calculated slope on the 32-63 segment will be almost half as it
> should be.
>
> The most expected and simplest algorithm for interpolation is linear
> interpolation, which would handle both problems.
> Let's just implement that!
>
> Take pairs of points from the brightness-levels array and linearly
> interpolate between them. On the X axis (what userspace sees) we'll
> now have equally sized intervals (num-interpolated-steps sized,
> as opposed to before where we were at the mercy of quantization).
>
> END

INTERESTING.

I guess this a copy 'n paste error from some internal log book?
Better removed... but I won't lose sleep over it.

> Signed-off-by: Alexandru Stan <amstan@chromium.org>

I've waited a bit to see how strong the feelings were w.r.t. getting rid
of the division from the table initialization. It was something I was
aware of during an earlier review but it was below my personal nitpicking
threshold (which could be badly calibrated... hence waiting). However
it's all been quiet so:

Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>

Daniel.
```
Alexandru M Stan Nov. 2, 2020, 6:52 p.m. UTC | #2
```On Wed, Oct 28, 2020 at 8:12 AM Daniel Thompson
<daniel.thompson@linaro.org> wrote:
>
> On Wed, Oct 21, 2020 at 10:04:45PM -0700, Alexandru Stan wrote:
> > The previous behavior was a little unexpected, its properties/problems:
> > 1. It was designed to generate strictly increasing values (no repeats)
> > 2. It had quantization errors when calculating step size. Resulting in
> > unexpected jumps near the end of some segments.
> >
> > Example settings:
> >       brightness-levels = <0 1 2 4 8 16 32 64 128 256>;
> >       num-interpolated-steps = <16>;
> >
> > 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.
> >
> > The distances between 1 2 4 and 8 would be 1 (property #1 fighting us),
> > and only starting with 16 it would start to interpolate properly.
> >
> > Property #1 is not enough. The goal here is more than just monotonically
> > increasing. We should still care about the shape of the curve. Repeated
> > points might be desired if we're in the part of the curve where we want
> > to go slow (aka slope near 0).
> >
> > Problem #2 is plainly a bug. Imagine if the 64 entry was 63 instead,
> > the calculated slope on the 32-63 segment will be almost half as it
> > should be.
> >
> > The most expected and simplest algorithm for interpolation is linear
> > interpolation, which would handle both problems.
> > Let's just implement that!
> >
> > Take pairs of points from the brightness-levels array and linearly
> > interpolate between them. On the X axis (what userspace sees) we'll
> > now have equally sized intervals (num-interpolated-steps sized,
> > as opposed to before where we were at the mercy of quantization).
> >
> > END
>
> INTERESTING.
>
> I guess this a copy 'n paste error from some internal log book?
> Better removed... but I won't lose sleep over it.

Sorry! Yeah, I mistakenly duplicated the "END" line in patman.

>
>
> > Signed-off-by: Alexandru Stan <amstan@chromium.org>
>
> I've waited a bit to see how strong the feelings were w.r.t. getting rid
> of the division from the table initialization. It was something I was
> aware of during an earlier review but it was below my personal nitpicking
> threshold (which could be badly calibrated... hence waiting). However
> it's all been quiet so:
>
> Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
>
>
> Daniel.

Alexandru Stan (amstan)
```
Lee Jones Nov. 4, 2020, 3:54 p.m. UTC | #3
```On Wed, 21 Oct 2020, Alexandru Stan wrote:

> The previous behavior was a little unexpected, its properties/problems:
> 1. It was designed to generate strictly increasing values (no repeats)
> 2. It had quantization errors when calculating step size. Resulting in
> unexpected jumps near the end of some segments.
>
> Example settings:
> 	brightness-levels = <0 1 2 4 8 16 32 64 128 256>;
> 	num-interpolated-steps = <16>;
>
> 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.
>
> The distances between 1 2 4 and 8 would be 1 (property #1 fighting us),
> and only starting with 16 it would start to interpolate properly.
>
> Property #1 is not enough. The goal here is more than just monotonically
> increasing. We should still care about the shape of the curve. Repeated
> points might be desired if we're in the part of the curve where we want
> to go slow (aka slope near 0).
>
> Problem #2 is plainly a bug. Imagine if the 64 entry was 63 instead,
> the calculated slope on the 32-63 segment will be almost half as it
> should be.
>
> The most expected and simplest algorithm for interpolation is linear
> interpolation, which would handle both problems.
> Let's just implement that!
>
> Take pairs of points from the brightness-levels array and linearly
> interpolate between them. On the X axis (what userspace sees) we'll
> now have equally sized intervals (num-interpolated-steps sized,
> as opposed to before where we were at the mercy of quantization).
>
> END

I removed this.

> Signed-off-by: Alexandru Stan <amstan@chromium.org>
> ---
>
>  drivers/video/backlight/pwm_bl.c | 70 ++++++++++++++------------------
>  1 file changed, 31 insertions(+), 39 deletions(-)

Applied, thanks.
```

## Patch

```diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
index dfc760830eb9..e48fded3e414 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 * (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
@@ -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;

```