Re: [PATCH v7 3/5] pwm: kona: Fix incorrect config, disable, and polarity procedures

[Jonathan Richardson]

On 15-05-17 09:53 PM, Tim Kryger wrote:
> On Tue, May 12, 2015 at 4:28 PM, Jonathan Richardson
> <jonathar@xxxxxxxxxxxx> wrote:
> 
>> The polarity procedure no longer applies the settings to change the
>> output signal because it can't be called when the pwm is enabled anyway.
>> The polarity is only updated in the control register. The correct
>> polarity will be applied on enable. The old method of applying changes
>> would result in no signal when the polarity was changed. The new
>> apply_settings function would fix this problem but it isn't required
>> anyway.
> 
> Thanks for incorporating some of my suggestions in your latest version.
> 
> I'm still concerned about delaying when polarity changes take effect.
> 
> Since backlight is a common use of PWM, consider the following situation.
> 
> backlight {
>         compatible = "pwm-backlight";
>         pwms = <&pwm 0 5000000 PWM_POLARITY_NORMAL>;
>         brightness-levels = <0 4 8 16 32 64 128 255>;
>         default-brightness-level = <0>;
> };
> 
> The Kona PWM hardware starts in inversed mode so it will drive output high
> once its clock is enabled during the probe.
> 
> Polarity is not adjusted during probe so it stays high and it registers with
> the PWM core using the new pwmchip_add_inversed() function.
> 
> Next, the pwm-backlight driver probe executes and it calls devm_pwm_get()
> which then calls pwm_set_period() and most importantly pwm_set_polarity().
> 
> The output would change to constant low at this point in the original driver
> but with your proposed change it will remain high.
> 
> The driver sets bl->props.brightness and calls backlight_update_status() but,
> since in this case the default brightness is zero, it assumes it doesn't need
> to enable the PWM.
> 
> The backlight driver probe then returns and the PWM output is incorrect.

Thanks for the detailed use case. You are right - the problem does
occur. I assumed if the pwm signal was being changed at all that it
should first be enabled. Since the bl driver can't know what 0
brightness means with different polarities, shouldn't it always enable
the pwm first, config 0 period, then disable (when brightness is 0)?
Then the polarity would have been updated properly also. 0 can mean full
brightness or off depending on polarity.

It seems odd that changing the polarity should affect the output signal
when the pwm is disabled. If using sysfs and you change the polarity,
you can't defer the signal change to when it's enabled.

If this is correct - polarity changes affect the output signal
immediately, then I can change our driver. Could you confirm first this
is what we want? This would cause polarity changes to affect all devices
immediately which is what I thought enable was for. The pwm core wanting
the pwm disabled to change the polarity implied to me that it shouldn't
cause a change in the signal until it was enabled.

Thanks.

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