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

[Paul Cercueil <paul@crapouillou.net>]

Le mar. 25 juin 2019 à 5:47, Thierry Reding <thierry.reding@gmail.com> 
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@linaro.org> 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