Re: [PATCHv2] backlight: pwm_bl: disable PWM when 'duty_cycle' is zero

[Lee Jones <lee.jones@linaro.org>]

On Mon, 20 Jun 2016, Phil Reid wrote:

> 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

You're repleting yourself.  I already told you I agreed with this.

I think you need to re-read my previous reply.  Your code is buggy.

> pwm_backlight_power_off() disables the regulator.
> So the supply to Backlight disappears, regardless of constant low...

Yes exactly.

Look at this again:

> 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*.

When I say that you turned the backlight *off*.  I didn't mean you
turned it right down using the duty_cycle mechanism, I mean you ripped
the socket from the wall.  The IP is trying to turn the backlight to
the highest setting, but it no longer has power.  It's *off*, *off*!

-- 
Lee Jones
Linaro STMicroelectronics Landing Team Lead
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