Le mar. 25 oct. 2022 à 08:21:29 +0200, Uwe Kleine-König
<u.kleine-koenig@xxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
>
> 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.
