Re: [PATCH] backlight: pwm_bl: Set pin to sleep state when powered down

[Paul Cercueil]

Le mar. 25 juin 2019 à 5:47, Thierry Reding <thierry.reding@xxxxxxxxx> 
a écrit :
> On Mon, Jun 24, 2019 at 04:31:57PM +0200, Paul Cercueil wrote:
> >  Le lun. 24 juin 2019 à 13:28, Daniel Thompson 
> > <daniel.thompson@xxxxxxxxxx> a
> >  écrit :
> >  > On Fri, Jun 21, 2019 at 03:56:08PM +0200, Thierry Reding wrote:
> >  > >  On Fri, Jun 21, 2019 at 01:41:45PM +0100, Daniel Thompson 
> > wrote:
> >  > >  > On 22/05/2019 17:34, Paul Cercueil wrote:
> >  > >  > > When the driver probes, the PWM pin is automatically 
> > configured
> >  > > to its
> >  > >  > > default state, which should be the "pwm" function.
> >  > >  >
> >  > >  > At which point in the probe... and by who?
> >  > >
> >  > >  The driver core will select the "default" state of a device 
> > right
> >  > > before
> >  > >  calling the driver's probe, see:
> >  > >
> >  > >  	drivers/base/pinctrl.c: pinctrl_bind_pins()
> >  > >
> >  > >  which is called from:
> >  > >
> >  > >  	drivers/base/dd.c: really_probe()
> >  > >
> >  >
> >  > Thanks. I assumed it would be something like that... although 
> > given
> >  > pwm-backlight is essentially a wrapper driver round a PWM I 
> > wondered why
> >  > the pinctrl was on the backlight node (rather than the PWM node).
> >  >
> >  > Looking at the DTs in the upstream kernel it looks like ~20% of 
> > the
> >  > backlight drivers have pinctrl on the backlight node. Others 
> > presumable
> >  > have none or have it on the PWM node (and it looks like support 
> > for
> >  > sleeping the pins is *very* rare amoung the PWM drivers).
> >
> >  If your PWM driver has more than one channel and has the pinctrl 
> > node, you
> >  cannot fine-tune the state of individual pins. They all share the 
> > same
> >  state.
>
> But that's something that could be changed, right? We could for 
> example
> extend the PWM bindings to allow describing each PWM instance via a 
> sub-
> node of the controller node. Pin control states could be described on 
> a
> per-channel basis that way.

There could be an API to dynamically add/remove pin groups to a given
pinctrl state. The PWM driver would start with an empty (no groups)
"default" state, then when enabling e.g. PWM1, the driver would call
a function to add the "pwm1" pin group to the "default" state.

Does that sound like a good idea?

Thanks,
-Paul


> >  > >  > > However, at this
> >  > >  > > point we don't know the actual level of the pin, which may 
> > be
> >  > > active or
> >  > >  > > inactive. As a result, if the driver probes without 
> > enabling the
> >  > >  > > backlight, the PWM pin might be active, and the backlight 
> > would
> >  > > be
> >  > >  > > lit way before being officially enabled.
> >  > >  > >
> >  > >  > > To work around this, if the probe function doesn't enable 
> > the
> >  > > backlight,
> >  > >  > > the pin is set to its sleep state instead of the default 
> > one,
> >  > > until the
> >  > >  > > backlight is enabled. Whenk the backlight is disabled, the 
> > pin
> >  > > is reset
> >  > >  > > to its sleep state.
> >  > >  > Doesn't this workaround result in a backlight flash between
> >  > > whatever enables
> >  > >  > it and the new code turning it off again?
> >  > >
> >  > >  Yeah, I think it would. I guess if you're very careful on how 
> > you
> >  > > set up
> >  > >  the device tree you might be able to work around it. Besides 
> > the
> >  > > default
> >  > >  and idle standard pinctrl states, there's also the "init" 
> > state. The
> >  > >  core will select that instead of the default state if 
> > available.
> >  > > However
> >  > >  there's also pinctrl_init_done() which will try again to 
> > switch to
> >  > > the
> >  > >  default state after probe has finished and the driver didn't 
> > switch
> >  > > away
> >  > >  from the init state.
> >  > >
> >  > >  So you could presumably set up the device tree such that you 
> > have
> >  > > three
> >  > >  states defined: "default" would be the one where the PWM pin is
> >  > > active,
> >  > >  "idle" would be used when backlight is off (PWM pin inactive) 
> > and
> >  > > then
> >  > >  another "init" state that would be the same as "idle" to be 
> > used
> >  > > during
> >  > >  probe. During probe the driver could then switch to the "idle"
> >  > > state so
> >  > >  that the pin shouldn't glitch.
> >  > >
> >  > >  I'm not sure this would actually work because I think the way 
> > that
> >  > >  pinctrl handles states both "init" and "idle" would be the same
> >  > > pointer
> >  > >  values and therefore pinctrl_init_done() would think the driver
> >  > > didn't
> >  > >  change away from the "init" state because it is the same 
> > pointer
> >  > > value
> >  > >  as the "idle" state that the driver selected. One way to work 
> > around
> >  > >  that would be to duplicate the "idle" state definition and
> >  > > associate one
> >  > >  instance of it with the "idle" state and the other with the 
> > "init"
> >  > >  state. At that point both states should be different (different
> >  > > pointer
> >  > >  values) and we'd get the init state selected automatically 
> > before
> >  > > probe,
> >  > >  select "idle" during probe and then the core will leave it 
> > alone.
> >  > > That's
> >  > >  of course ugly because we duplicate the pinctrl state in DT, 
> > but
> >  > > perhaps
> >  > >  it's the least ugly solution.
> >  > >  Adding Linus for visibility. Perhaps he can share some insight.
> >  >
> >  > To be honest I'm happy to summarize in my head as "if it flashes 
> > then
> >  > it's not
> >  > a pwm_bl.c's problem" ;-).
> >
> >  It does not flash. But the backlight lits way too early, so we have 
> > a 1-2
> >  seconds
> >  of "white screen" before the panel driver starts.
>
> I think this always goes both ways. If you set the sleep state for the
> PWM on backlight probe with this patch, you may be able to work around
> the problem of the backlight lighting up too early. But what if your
> bootloader had already enabled the backlight and is showing a splash
> screen during boot? Your patch would turn off the backlight and then 
> it
> would turn on again after everything else was initialized. That's one
> type of flashing.
>
> What we need in this case are explicit pin control states that will
> enable fine-grained control over what happens. Anything implicit is
> bound to fail because it bakes in an assumption (either that the
> backlight is off during boot, or that it has been turned on already).
>
> Ideally we'd need to detect that the backlight is on and if it is we
> just don't do anything with it. Actually, I think that's what we want
> even if the backlight is off. During probe the backlight state should
> not be modified. You only want to modify it when you know that some
> display driver is going to take over. If you can't seamlessly 
> transition
> to the kernel display driver, flashing may be okay. If your display
> driver can take over seamlessly, then the backlight is likely already 
> in
> the desired state anyway.
>
> Thierry