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

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

[Lothar Waßmann]

'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 <LW@KARO-electronics.de>
---
 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

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

[Tomi Valkeinen]

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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


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

[Lothar Waßmann]

Hi,

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

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

[Tomi Valkeinen]

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On 07/06/16 09:49, Lothar Waßmann wrote:
> Hi,
> 
> On Mon, 6 Jun 2016 15:02:21 +0300 Tomi Valkeinen wrote:
>> 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).
>>
> 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


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