[PATCH] backlight: pwm_bl: disable PWM when 'duty_cycle' is zero

'brightness' is usually an index into a table of duty_cycle values,
where the value at index 0 may well be non-zero
(tegra30-apalis-eval.dts and tegra30-colibri-eval-v3.dts are real-life
examples).
Thus brightness = 0 does not necessarily mean that the PWM output
will be inactive.
Check for 'duty_cycle = 0' rather than 'brightness = 0' to decide
whether to disable the PWM.

Signed-off-by: Lothar Waßmann <redacted>
---
 drivers/video/backlight/pwm_bl.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
index b2b366b..80b2b52 100644
--- a/drivers/video/backlight/pwm_bl.c
+++ b/drivers/video/backlight/pwm_bl.c

@@ -103,8 +103,8 @@ static int pwm_backlight_update_status(struct backlight_device *bl)
 	if (pb->notify)
 		brightness = pb->notify(pb->dev, brightness);
 
-	if (brightness > 0) {
-		duty_cycle = compute_duty_cycle(pb, brightness);
+	duty_cycle = compute_duty_cycle(pb, brightness);
+	if (duty_cycle > 0) {
 		pwm_config(pb->pwm, duty_cycle, pb->period);
 		pwm_backlight_power_on(pb, brightness);
 	} else

-- 
2.1.4

Hi,

On 06/06/16 13:44, Lothar Waßmann wrote:

'brightness' is usually an index into a table of duty_cycle values,
where the value at index 0 may well be non-zero
(tegra30-apalis-eval.dts and tegra30-colibri-eval-v3.dts are real-life
examples).
Thus brightness == 0 does not necessarily mean that the PWM output
will be inactive.
Check for 'duty_cycle == 0' rather than 'brightness == 0' to decide
whether to disable the PWM.

The binding doc does say:

  - brightness-levels: Array of distinct brightness levels. Typically these
      are in the range from 0 to 255, but any range starting at 0 will do.
      The actual brightness level (PWM duty cycle) will be interpolated
      from these values. 0 means a 0% duty cycle (darkest/off), while the
      last value in the array represents a 100% duty cycle (brightest).

 Tomi

Hi,

On Mon, 6 Jun 2016 15:02:21 +0300 Tomi Valkeinen wrote:

Hi,

On 06/06/16 13:44, Lothar Waßmann wrote:

quoted

'brightness' is usually an index into a table of duty_cycle values,
where the value at index 0 may well be non-zero
(tegra30-apalis-eval.dts and tegra30-colibri-eval-v3.dts are real-life
examples).
Thus brightness = 0 does not necessarily mean that the PWM output
will be inactive.
Check for 'duty_cycle = 0' rather than 'brightness = 0' to decide
whether to disable the PWM.

The binding doc does say:

  - brightness-levels: Array of distinct brightness levels. Typically these
      are in the range from 0 to 255, but any range starting at 0 will do.
      The actual brightness level (PWM duty cycle) will be interpolated
      from these values. 0 means a 0% duty cycle (darkest/off), while the
      last value in the array represents a 100% duty cycle (brightest).

So, what should I do, when I need a range of levels that doesn't start
at 0? E.g. if the brightness is inverse proportional to the PWM duty
cycle.



Lothar Waßmann

On 07/06/16 09:49, Lothar Waßmann wrote:

Hi,

On Mon, 6 Jun 2016 15:02:21 +0300 Tomi Valkeinen wrote:

quoted

Hi,

On 06/06/16 13:44, Lothar Waßmann wrote:

quoted

'brightness' is usually an index into a table of duty_cycle values,
where the value at index 0 may well be non-zero
(tegra30-apalis-eval.dts and tegra30-colibri-eval-v3.dts are real-life
examples).
Thus brightness == 0 does not necessarily mean that the PWM output
will be inactive.
Check for 'duty_cycle == 0' rather than 'brightness == 0' to decide
whether to disable the PWM.

The binding doc does say:

  - brightness-levels: Array of distinct brightness levels. Typically these
      are in the range from 0 to 255, but any range starting at 0 will do.
      The actual brightness level (PWM duty cycle) will be interpolated
      from these values. 0 means a 0% duty cycle (darkest/off), while the
      last value in the array represents a 100% duty cycle (brightest).

So, what should I do, when I need a range of levels that doesn't start
at 0? E.g. if the brightness is inverse proportional to the PWM duty
cycle.

That's a question to the PWM/backlight maintainers, but I think in the
addition of your patch, the binding doc needs to be changed, as it
doesn't hold true after your patch.

 Tomi