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Message-Id: <CVZAKR.06MA7BGA170W3@crapouillou.net>
Date: Tue, 25 Oct 2022 11:02:00 +0100
From: Paul Cercueil <paul@...pouillou.net>
To: Uwe Kleine-König
<u.kleine-koenig@...gutronix.de>
Cc: Thierry Reding <thierry.reding@...il.com>, od@...ndingux.net,
linux-pwm@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-mips@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH 1/5] pwm: jz4740: Fix pin level of disabled TCU2 channels,
part 1
Le mar. 25 oct. 2022 à 08:21:29 +0200, Uwe Kleine-König
<u.kleine-koenig@...gutronix.de> a écrit :
> Hello,
>
> On Mon, Oct 24, 2022 at 09:52:09PM +0100, Paul Cercueil wrote:
>> The "duty > cycle" trick to force the pin level of a disabled TCU2
>> channel would only work when the channel had been enabled
>> previously.
>>
>> Address this issue by enabling the PWM mode in jz4740_pwm_disable
>> (I know, right) so that the "duty > cycle" trick works before
>> disabling
>> the PWM channel right after.
>>
>> This issue went unnoticed, as the PWM pins on the majority of the
>> boards
>> tested would default to the inactive level once the corresponding
>> TCU
>> clock was enabled, so the first call to jz4740_pwm_disable() would
>> not
>> actually change the pin levels.
>>
>> On the GCW Zero however, the PWM pin for the backlight (PWM1, which
>> is
>> a TCU2 channel) goes active as soon as the timer1 clock is enabled.
>> Since the jz4740_pwm_disable() function did not work on channels not
>> previously enabled, the backlight would shine at full brightness
>> from
>> the moment the backlight driver would probe, until the backlight
>> driver
>> tried to *enable* the PWM output.
>>
>> With this fix, the PWM pins will be forced inactive as soon as
>> jz4740_pwm_apply() is called (and might be reconfigured to active if
>> dictated by the pwm_state). This means that there is still a tiny
>> time
>> frame between the .request() and .apply() callbacks where the PWM
>> pin
>> might be active. Sadly, there is no way to fix this issue: it is
>> impossible to write a PWM channel's registers if the corresponding
>> clock
>> is not enabled, and enabling the clock is what causes the PWM pin
>> to go
>> active.
>>
>> There is a workaround, though, which complements this fix: simply
>> starting the backlight driver (or any PWM client driver) with a
>> "init"
>> pinctrl state that sets the pin as an inactive GPIO. Once the
>> driver is
>> probed and the pinctrl state switches to "default", the regular PWM
>> pin
>> configuration can be used as it will be properly driven.
>>
>> Fixes: c2693514a0a1 ("pwm: jz4740: Obtain regmap from parent node")
>> Signed-off-by: Paul Cercueil <paul@...pouillou.net>
>> Cc: stable@...r.kernel.org
>
> OK, understood the issue. I think there is another similar issue: The
> clk is get and enabled only in the .request() callback. The result is
> (I
> think---depends on a few further conditions) that if you have the
> backlight driver as a module and the bootloader enables the backlight
> to
> show a splash screen, the backlight goes off because of the
> clk_disable_unused initcall.
I will have to verify, but I'm pretty sure disabling the clock doesn't
change the pin level back to inactive.
-Paul
> So the right thing to do is to get the clock in .probe(), and ensure
> it
> is kept on if the PWM is running already. Then you can also enable the
> counter in .probe() and don't care for it in the enable and disable
> functions.
>
> The init pinctrl then has to be on the PWM then, but that's IMHO ok.
>
> Best regards
> Uwe
>
> PS: While looking into the driver I noticed that .request() uses
> dev_err_probe(). That's wrong, this function is only supposed to be
> used
> in .probe().
>
> --
> Pengutronix e.K. | Uwe Kleine-König
> |
> Industrial Linux Solutions |
> https://www.pengutronix.de/ |
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