Messages in this thread | | | Subject | Re: [PATCHv2] backlight: pwm_bl: disable PWM when 'duty_cycle' is zero | From | Phil Reid <> | Date | Mon, 20 Jun 2016 14:29:12 +0800 |
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On 20/06/2016 14:21, Lothar Waßmann wrote: > Hi, > > On Fri, 17 Jun 2016 15:17:19 +0100 Lee Jones wrote: >> On Sat, 11 Jun 2016, Lothar Waßmann wrote: >>> On Fri, 10 Jun 2016 15:54:49 +0100 Lee Jones wrote: >>>> On Fri, 10 Jun 2016, Lothar Waßmann wrote: >>>>> On Fri, 10 Jun 2016 08:44:49 +0100 Lee Jones wrote: >>>>>> On Fri, 10 Jun 2016, Lothar Waßmann wrote: >>>>>> >>>>>>> Hi, >>>>>>> >>>>>>> On Thu, 9 Jun 2016 14:51:25 +0100 Lee Jones wrote: >>>>>>>> On Tue, 07 Jun 2016, 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. >>>>>>>>> >>>>>>>>> Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de> >>>>>>>>> --- >>>>>>>>> Changes wrt. v1: >>>>>>>>> - update binding docs to reflect the change >>>>>>>>> >>>>>>>>> .../devicetree/bindings/leds/backlight/pwm-backlight.txt | 9 ++++++--- >>>>>>>>> drivers/video/backlight/pwm_bl.c | 4 ++-- >>>>>>>>> 2 files changed, 8 insertions(+), 5 deletions(-) >>>>>>>>> >>>>>>>>> diff --git a/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt b/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt >>>>>>>>> index 764db86..95fa8a9 100644 >>>>>>>>> --- a/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt >>>>>>>>> +++ b/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt >>>>>>>>> @@ -4,10 +4,13 @@ Required properties: >>>>>>>>> - compatible: "pwm-backlight" >>>>>>>>> - pwms: OF device-tree PWM specification (see PWM binding[0]) >>>>>>>>> - 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. >>>>>>>>> + are in the range from 0 to 255, but any range 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). >>>>>>>>> + from these values. 0 means a 0% duty cycle, while the highest value in >>>>>>>>> + the array represents a 100% duty cycle. >>>>>>>>> + The range may be in reverse order (starting with the maximum duty cycle >>>>>>>>> + value) to create a PWM signal with the 100% duty cycle representing >>>>>>>>> + minimum and 0% duty cycle maximum brigthness. >>>>>>>>> - default-brightness-level: the default brightness level (index into the >>>>>>>>> array defined by the "brightness-levels" property) >>>>>>>>> - power-supply: regulator for supply voltage >>>>>>>>> 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) { >>>>>>>> >>>>>>>> How does this work in the aforementioned: >>>>>>>> >>>>>>>> "The range may be in reverse order" >>>>>>>> >>>>>>>> ... case? Surely when duty_cycle is when the screen should be at it's >>>>>>>> brightest? Wouldn't it confuse the user if they turn their brightness >>>>>>>> *up* and the screen goes *off*? >>>>>>>> >>>>>>> Assuming that the PWM output is inactive (LOW) when the duty_cycle is >>>>>>> set to zero, there will be no difference between operating the PWM at >>>>>>> duty_cycle 0 or disabling it. >>>>>>> >>>>>>> Currently, the screen will go bright when it should be off in this >>>>>>> case. >>>>>> >>>>>> It sounds like we need something that lets the framework know if >>>>>> duty_cycle = MAX is the brightest or if duty_cycle = 0 is. Either way >>>>>> someone is going to get screwed by this logic. >>>>>> >>>>> The backlight framework does not (and does not need to) know anything >>>>> about PWM duty cycles. Its 'brightness' values are consistently 0 == >>>>> dark, max == brightest in either case. >>>> >>>> What I'm getting at is; by the look of the documentation, the >>>> brightest setting can either be a duty cycle of 0 or 255. So what >>>> happens with your new semantics when the duty cycle of 0 represents >>>> the brightest setting and you reach 0? Didn't you just turn the >>>> backlight off? >>>> >>> As mentioned earlier, disabling the PWM has generally the same result as >>> setting the duty cycle to 0. The current behaviour is broken in this >>> case, since setting brightness to 0 with a non-zero duty_cycle as the >>> first element of brightness-levels, the PWM will be disabled rather than >>> switched to the given duty cycle. >>> Disabling the PWM should have the same effect as setting the duty cycle >>> to 0, so it is safe to check for duty_cycle == 0 to decide whether to >>> disable the PWM. >> >> I agree with this. BUT, that's not what you're doing is it? >> >> Look at the code you're trying to write: >> >> 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 >> pwm_backlight_power_off(pb); >> >> Let's say duty_cycle == 0. In some cases this can mean "turn >> brightness up to the *maximum*", but with your new logic you just >> turned the backlight *off*. >> > Huh? Please think again! > - duty_cycle == 0 means a CONSTANT LOW level on the PWM output. Agreed? > - Disabling the PWM usually achieves a CONSTANT LOW level on the PWM > output. Agreed? > So duty_cycle == 0 <=> disable the PWM no matter whether the backlight > is darkest or brightest at this duty cycle setting! > > The backlight controller does not know anything about the value of the > 'brightness' variable in the code but only sees the 'duty_cycle' value. > When brightness == 0 translates into max. duty cycle, the original code > will switch the PWM OFF (which is equivalent to a ZERO duty cycle), when > it rather should operate at the max. duty cycle. > When duty_cycle is '0', this is equivalent to the PWM output being at > constant LOW level which is the same as being switched OFF in the usual > cases. > > When the brightness is maximum at duty_cycle == 0, that means, that the > backlight is brightest when the control pin is constantly LOW, which > is usually the case when the PWM is disabled. This is exactly what the > patch does achieve! > With the current code a backlight that is brightest at a constant '0' > level will turn to max. brightness rather than off when selecting > brightness level 0 (max. PWM duty cycle). > >> Conversely, let's say duty_cycle == 255. In some cases this can mean >> "turn the brightness to the *lowest* setting" i.e. *off*. Well your >> logic just turned the backlight *on*. >> > OK. Let's try a sequence of brightness levels and duty cycles: > For simplicity assume a range of brightness levels from 0..100, so > that the 'brightness' value directly represents the duty cycle of the > PWM. So either: brightness == 0 => duty cycle == 0% => constant LOW > Or: brightnes == 0 => duty cycle == 100% => constant HIGH. > > Normal range with current and patched code: > brightness duty_cycle > 0 0 PWM disabled => constant LOW > 1 1 PWM active > ... > 100 100 PWM active => constant HIGH > > Inverted range (backlight brightest at duty cycle 0) > Current code: > brightness duty_cycle > 0 100 PWM disabled (OUTPUT CONSTANT LOW!) > 1 99 PWM active with near full duty cycle > ... > 99 1 PWM active with near ZERO duty cycle > 100 0 PWM active with 0% duty cycle => constant LOW > > With my patch: > brightness duty_cycle > 0 100 PWM active with 100% duty cycle (constant HIGH) > 1 99 PWM active with near full duty cycle > ... > 99 1 PWM active with near ZERO duty cycle > 100 0 PWM disabled => constant LOW > > pwm_backlight_power_off() disables the regulator. So the supply to Backlight disappears, regardless of constant low...
-- Regards Phil Reid
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