Linux-PWM Archive on lore.kernel.org
 help / color / Atom feed
From: Alexandru Stan <amstan@chromium.org>
To: "Thierry Reding" <thierry.reding@gmail.com>,
	"Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>,
	"Lee Jones" <lee.jones@linaro.org>,
	"Daniel Thompson" <daniel.thompson@linaro.org>,
	"Jingoo Han" <jingoohan1@gmail.com>,
	"Bartlomiej Zolnierkiewicz" <b.zolnierkie@samsung.com>,
	"Heiko Stuebner" <heiko@sntech.de>,
	"Rob Herring" <robh+dt@kernel.org>
Cc: Matthias Kaehlcke <mka@chromium.org>,
	Douglas Anderson <dianders@chromium.org>,
	Enric Balletbo i Serra <enric.balletbo@collabora.com>,
	Alexandru Stan <amstan@chromium.org>,
	dri-devel@lists.freedesktop.org, linux-fbdev@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-pwm@vger.kernel.org
Subject: [PATCH 1/3] backlight: pwm_bl: Fix interpolation
Date: Mon, 20 Jul 2020 21:25:20 -0700
Message-ID: <20200720212502.1.I4dcea1c90e9da3902d466033aa73351e19e49c49@changeid> (raw)
In-Reply-To: <20200721042522.2403410-1-amstan@chromium.org>

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>
---

 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;
+
+			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;
+
+				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

  reply index

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-21  4:25 [PATCH 0/3] PWM backlight interpolation adjustments Alexandru Stan
2020-07-21  4:25 ` Alexandru Stan [this message]
2020-09-04 11:27   ` [PATCH 1/3] backlight: pwm_bl: Fix interpolation Daniel Thompson
2020-07-21  4:25 ` [PATCH 2/3] backlight: pwm_bl: Artificially add 0% during interpolation Alexandru Stan
2020-08-07  8:21   ` daniel
2020-08-13 13:45     ` Daniel Thompson
2020-09-04 11:38   ` Daniel Thompson
2020-09-07  7:50     ` Daniel Vetter
2020-09-09 14:45       ` Daniel Thompson
2020-09-09 15:03         ` Daniel Vetter
2020-09-10  7:47           ` Daniel Vetter
2020-09-09 18:42     ` Alexandru M Stan
2020-08-05 21:04 ` [PATCH 0/3] PWM backlight interpolation adjustments Alexandru M Stan

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20200720212502.1.I4dcea1c90e9da3902d466033aa73351e19e49c49@changeid \
    --to=amstan@chromium.org \
    --cc=b.zolnierkie@samsung.com \
    --cc=daniel.thompson@linaro.org \
    --cc=dianders@chromium.org \
    --cc=dri-devel@lists.freedesktop.org \
    --cc=enric.balletbo@collabora.com \
    --cc=heiko@sntech.de \
    --cc=jingoohan1@gmail.com \
    --cc=lee.jones@linaro.org \
    --cc=linux-fbdev@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-pwm@vger.kernel.org \
    --cc=mka@chromium.org \
    --cc=robh+dt@kernel.org \
    --cc=thierry.reding@gmail.com \
    --cc=u.kleine-koenig@pengutronix.de \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Linux-PWM Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/linux-pwm/0 linux-pwm/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linux-pwm linux-pwm/ https://lore.kernel.org/linux-pwm \
		linux-pwm@vger.kernel.org
	public-inbox-index linux-pwm

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-pwm


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git