[PATCH v3 0/3] PWM backlight interpolation adjustments

[PATCH v3 0/3] PWM backlight interpolation adjustments

[Alexandru Stan]

I was trying to adjust the brightness-levels for the trogdor boards:
https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/2291209
Like on a lot of panels, trogdor's low end needs to be cropped,
and now that we have the interpolation stuff I wanted to make use of it
and bake in even the curve that's customary to have on chromebooks.

I found the current behavior of the pwm_bl driver a little unintuitive
and non-linear. See patch 1 for a suggested fix for this.

A few veyron dts files were relying on this (perhaps weird) behavior.
Those devices also want a minimum brightness like trogdor, so changed
them to use the new way.

Finally, given that trogdor's dts is part of linux-next now, add the
brightness-levels to it, since that's the original reason I was looking at
this.

Changes in v3:
- Reordered patches, since both dts changes will work just fine
  even before the driver change.
- Rewrote a bit of the commit message to describe the new policy,
  as Daniel suggested.
- Removed redundant s64 for something that's always positive

Changes in v2:
- Fixed type promotion in the driver
- Removed "backlight: pwm_bl: Artificially add 0% during interpolation",
  userspace works just fine without it because it already knows how to use
  bl_power for turning off the display.
- Added brightness-levels to trogdor as well, now the dts is upstream.


Alexandru Stan (3):
  ARM: dts: rockchip: veyron: Remove 0 point from brightness-levels
  arm64: dts: qcom: trogdor: Add brightness-levels
  backlight: pwm_bl: Fix interpolation

 arch/arm/boot/dts/rk3288-veyron-jaq.dts      |  2 +-
 arch/arm/boot/dts/rk3288-veyron-minnie.dts   |  2 +-
 arch/arm/boot/dts/rk3288-veyron-tiger.dts    |  2 +-
 arch/arm64/boot/dts/qcom/sc7180-trogdor.dtsi |  9 +++
 drivers/video/backlight/pwm_bl.c             | 70 +++++++++-----------
 5 files changed, 43 insertions(+), 42 deletions(-)

-- 
2.28.0

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[PATCH v3 3/3] backlight: pwm_bl: Fix interpolation

[Alexandru Stan]

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(-)

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;
-- 
2.28.0

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Re: [PATCH v3 3/3] backlight: pwm_bl: Fix interpolation

[Daniel Thompson]

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.
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Re: [PATCH v3 0/3] PWM backlight interpolation adjustments

[Heiko Stuebner]

On Wed, 21 Oct 2020 22:04:42 -0700, Alexandru Stan wrote:
> I was trying to adjust the brightness-levels for the trogdor boards:
> https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/2291209
> Like on a lot of panels, trogdor's low end needs to be cropped,
> and now that we have the interpolation stuff I wanted to make use of it
> and bake in even the curve that's customary to have on chromebooks.
> 
> I found the current behavior of the pwm_bl driver a little unintuitive
> and non-linear. See patch 1 for a suggested fix for this.
> 
> [...]

Applied, thanks!

[1/1] ARM: dts: rockchip: Remove 0 point from brightness-levels on rk3288-veyron
      commit: 225c59b9235a421cdb219be5fbc13126a49714a6

Best regards,
-- 
Heiko Stuebner <heiko@sntech.de>
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Re: [PATCH v3 3/3] backlight: pwm_bl: Fix interpolation

[Alexandru M Stan]

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)
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Re: [PATCH v3 3/3] backlight: pwm_bl: Fix interpolation

[Lee Jones]

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.

-- 
Lee Jones [李琼斯]
Senior Technical Lead - Developer Services
Linaro.org │ Open source software for Arm SoCs
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