Hello Paul,
On Fri, Nov 18, 2022 at 09:55:40AM +0000, Paul Cercueil wrote:
> Le jeu. 17 nov. 2022 à 14:29:27 +0100, Uwe Kleine-König
> <u.kleine-koenig@xxxxxxxxxxxxxx> a écrit :
> > Hello Paul,
> >
> > On Tue, Oct 25, 2022 at 11:02:00AM +0100, Paul Cercueil wrote:
> > > 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.
> >
> > Given that you set the clk's rate depending on the period to apply, I'd
> > claim that you need to keep the clk on. Maybe it doesn't hurt, because
> > another component of the system keeps the clk running, but it's wrong
> > anyhow. Assumptions like these tend to break on new chip revisions.
>
> If the backlight driver is a module then it will probe before the
> clk_disable_unused initcall, unless something is really wrong.
I'd claim the clk_disable_unused initcall is called before userspace
starts and so before the module can be loaded. Who is wrong here?
> So the backlight would stay ON if it was enabled by the bootloader,
> unless the DTB decides it doesn't have to be.
Don't understand that. How could hte DTB decide the backlight can be
disabled?
> Anyway, I can try your suggestion, and move the trick to force-disable PWM
> pins in the probe(). After that, the clocks can be safely disabled, so I can
> disable them (for the disabled PWMs) at the end of the probe and re-enable
> them again in their respective .request() callback.
I really lost track of the problem here and would appreciate a new
submission of the remaining (and improved?) patches.
Best regards
Uwe
--
Pengutronix e.K. | Uwe Kleine-König |
Industrial Linux Solutions | https://www.pengutronix.de/ |
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