[PATCH] pwm-backlight: allow for non-increasing brightness levels

Currently the driver assumes that the values specified in the brightness-levels
device tree property increase as they are parsed from left to right.  But boards
that invert the signal between the PWM output and the backlight will need to
specify decreasing brightness-levels.  This patch removes the assumption that
the last element of the array is the max value, and instead searches the array
for the max value and uses that as the normalizing value when determining the
duty cycle.

Signed-off-by: Mike Dunn <redacted>
---
 drivers/video/backlight/pwm_bl.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
index 1fea627..d66aaa0 100644
--- a/drivers/video/backlight/pwm_bl.c
+++ b/drivers/video/backlight/pwm_bl.c

@@ -27,6 +27,7 @@ struct pwm_bl_data {
 	unsigned int		period;
 	unsigned int		lth_brightness;
 	unsigned int		*levels;
+	unsigned int		max_level;
 	int			(*notify)(struct device *,
 					  int brightness);
 	void			(*notify_after)(struct device *,

@@ -57,7 +58,7 @@ static int pwm_backlight_update_status(struct backlight_device *bl)
 
 		if (pb->levels) {
 			duty_cycle = pb->levels[brightness];
-			max = pb->levels[max];
+			max = pb->levels[pb->max_level];
 		} else {
 			duty_cycle = brightness;
 		}

@@ -195,7 +196,15 @@ static int pwm_backlight_probe(struct platform_device *pdev)
 	}
 
 	if (data->levels) {
-		max = data->levels[data->max_brightness];
+		int i, max_value = 0, max_idx = 0;
+		for (i = 0; i <= data->max_brightness; i++) {
+			if (data->levels[i] > max_value) {
+				max_value = data->levels[i];
+				max_idx = i;
+			}
+		}
+		pb->max_level = max_idx;
+		max = data->levels[max_idx];
 		pb->levels = data->levels;
 	} else
 		max = data->max_brightness;

-- 
1.8.1.5

Hi Mike,

On Thu, Sep 12, 2013 at 11:35:52AM -0700, Mike Dunn wrote:

Currently the driver assumes that the values specified in the brightness-levels
device tree property increase as they are parsed from left to right.  But boards
that invert the signal between the PWM output and the backlight will need to
specify decreasing brightness-levels.  This patch removes the assumption that
the last element of the array is the max value, and instead searches the array
for the max value and uses that as the normalizing value when determining the
duty cycle.

Note there's also support for inverted PWMs in the PWM framework
provided your hardware supports this.

Sascha

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On 09/17/2013 02:36 AM, Sascha Hauer wrote:

Hi Mike,

On Thu, Sep 12, 2013 at 11:35:52AM -0700, Mike Dunn wrote:

quoted

Currently the driver assumes that the values specified in the brightness-levels
device tree property increase as they are parsed from left to right.  But boards
that invert the signal between the PWM output and the backlight will need to
specify decreasing brightness-levels.  This patch removes the assumption that
the last element of the array is the max value, and instead searches the array
for the max value and uses that as the normalizing value when determining the
duty cycle.

Note there's also support for inverted PWMs in the PWM framework
provided your hardware supports this.

Yes, and in fact my first solution was to implement simulated polarity inversion
in the pwm driver, but that was shot down because polarity inversion is not
actually supported by the pwm hardware.  My inverter is external in the path
between the pwm output and the backlight.  Not sure of the reason for its presence.

Thanks,
Mike

On Thu, Sep 12, 2013 at 11:35:52AM -0700, Mike Dunn wrote:

Currently the driver assumes that the values specified in the brightness-levels
device tree property increase as they are parsed from left to right.  But boards
that invert the signal between the PWM output and the backlight will need to
specify decreasing brightness-levels.  This patch removes the assumption that
the last element of the array is the max value, and instead searches the array
for the max value and uses that as the normalizing value when determining the
duty cycle.

"maximum value", "... and uses that as the scale to normalize the duty
cycle"?

Also please wrap commit messages at 72 characters.

Perhaps call this "scale"? Otherwise there some potential to mix it up
with max_brightness.

i should be unsigned int to match the type of data->max_brightness.

+		for (i = 0; i <= data->max_brightness; i++) {

There should be a blank line above this one to increase readability.

+			if (data->levels[i] > max_value) {
+				max_value = data->levels[i];
+				max_idx = i;
+			}
+		}
+		pb->max_level = max_idx;

Some here.

Also I suggest to just drop the max_ prefix from the local variables.
Perhaps also simplify all of it to something like:

	for (i = 0; i <= data->max_brightness; i++)
		if (data->levels[i] > pb->scale)
			pb->scale = data->levels[i];

And get rid of the index altogether. That way you can use pb->scale
directly during the computation of the duty cycle and don't have to
index the levels array over and over again.

Thierry

On 09/19/2013 04:56 AM, Thierry Reding wrote:

On Thu, Sep 12, 2013 at 11:35:52AM -0700, Mike Dunn wrote:

quoted

Currently the driver assumes that the values specified in the brightness-levels
device tree property increase as they are parsed from left to right.  But boards
that invert the signal between the PWM output and the backlight will need to
specify decreasing brightness-levels.  This patch removes the assumption that
the last element of the array is the max value, and instead searches the array
for the max value and uses that as the normalizing value when determining the
duty cycle.

"maximum value", "... and uses that as the scale to normalize the duty
cycle"?

It's been a while since my last math class... is "normalizing value" not the
correct term?  Maybe just "uses that in the duty cycle calculation"?

Also please wrap commit messages at 72 characters.

OK.  Sorry, didn't know.

quoted

diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
index 1fea627..d66aaa0 100644
--- a/drivers/video/backlight/pwm_bl.c
+++ b/drivers/video/backlight/pwm_bl.c

@@ -27,6 +27,7 @@ struct pwm_bl_data {
 	unsigned int		period;
 	unsigned int		lth_brightness;
 	unsigned int		*levels;
+	unsigned int		max_level;

Perhaps call this "scale"? Otherwise there some potential to mix it up
with max_brightness.

Yes, this name is thorny.  The code was somewhat confusing to me until I
realized that for the DT case, brightness and max_brightness are indices into
the levels[] array, whereas they are actual values for the platform_data case.
I'll go with "scale" if you prefer.

quoted

@@ -195,7 +196,15 @@ static int pwm_backlight_probe(struct platform_device *pdev)
 	}
 
 	if (data->levels) {
-		max = data->levels[data->max_brightness];
+		int i, max_value = 0, max_idx = 0;

i should be unsigned int to match the type of data->max_brightness.

Yes, thanks.  I'm surprised there's no warning from the compiler.  I'm also
assigning an unsigned to a signed.

quoted

+		for (i = 0; i <= data->max_brightness; i++) {

There should be a blank line above this one to increase readability.

quoted

+			if (data->levels[i] > max_value) {
+				max_value = data->levels[i];
+				max_idx = i;
+			}
+		}
+		pb->max_level = max_idx;

Some here.

Also I suggest to just drop the max_ prefix from the local variables.
Perhaps also simplify all of it to something like:

	for (i = 0; i <= data->max_brightness; i++)
		if (data->levels[i] > pb->scale)
			pb->scale = data->levels[i];

And get rid of the index altogether. That way you can use pb->scale
directly during the computation of the duty cycle and don't have to
index the levels array over and over again.

Ok, if you prefer.  The reason I made max_level an index is for consistency.
For the DT case, brightness and max_brightness are indices, and I had already
been confused by the value-versus-index issue.

Thanks much for the review!  I'll ready a v2 patch.

Mike