Re: [RFC PATCH] pwm: Add the concept of an "active" pinctrl

[Doug Anderson]

Linus,

On Thu, Sep 25, 2014 at 12:31 AM, Linus Walleij
<linus.walleij@xxxxxxxxxx> wrote:
> On Wed, Sep 24, 2014 at 1:17 AM, Doug Anderson <dianders@xxxxxxxxxxxx> wrote:
>
>> There are cases where you really don't want a PWM's pinctrl to take
>> effect until you know that the PWM is driving at the proper rate.  A
>> good example of this is a PWM-controlled regulator, where the boot
>> state of the pin (maybe a pulled down input) could cause a vastly
>> different voltage to take effect than having the pin setup as a PWM
>> but not having the PWM enabled yet.
>>
>> Let's add a feature where you can define an "active" pinctrl state.
>> This state (if it exists) will be applied after the PWM has actually
>> been enabled.  If the PWM is ever disabled we'll go back to "default"
>> state.
>>
>> Signed-off-by: Doug Anderson <dianders@xxxxxxxxxxxx>
>> ---
>> I'm sure Linus W or Thierry or Mark will tell me this is an idiotic
>> idea (or totally the wrong way to do it), but I figured I'd float it
>> out there and maybe someone can tell me something better to do.  ;)
>
> Hey it's not like this is obvious or anything....

;)


> The device core will handle three states for you: "default", "sleep", "idle".

Except it will handle only one state unless "CONFIG_PM" is defined,
right?  In other words "sleep" and "idle" totally don't exist in the
"!CONFIG_PM" case.

Maybe I shouldn't worry about the "!CONFIG_PM" case, but in my use
case this extra state was needed for correctness (so we didn't glitch
the voltage), so it seemed unwise to rely on the "sleep" and "idle"
states.  Normally it should be considered "safe" (though high power)
to disable CONFIG_PM, right?  ...of course I could say that anyone
using this particular feature should just put some hard dependency on
"PM" in their Kconfig and call it done...  ...or I I could make "idle"
present always.


> What I worry about in this patch is that you redefine the meaning
> of "default" from what it used to be into "sleep", basically, that
> you want the device to come up in a relaxed mode, whereas
> the "default" state is usually defined as "ready to go", i.e. what
> you refer to as "active".

Yeah, I didn't like that either.  That's mostly why I put the RFC and
"I'm gonna get yelled at" comment.  ;)

Hrm, I wonder if another reasonable option is to add a boolean
property somewhere saying that "pinctrl starts idle" would fix things
for me.


> If you choose this approach, it is better to not define the "default"
> state *at all*, just define "active" and "sleep" and/or "idle".
> (The difference between the two latter is basically the same
> semantic as suspend vs runtime suspend.)
>
> We have some helpers in the device core to help getting
> things right.
>
> Inspecting the helper code in drivers/base/pinctrl.c you will find
> that if the "default" state is defined it will be retrieved and
> activated on boot, so you don't have to do this yourself. Then
> if "sleep" and "idle" exist, these will be grabbed and made
> accessible using special helper functions.

How I deal with this would depend on whether I require CONFIG_PM.
What do you think?


> I would actually prefer, if it is not causing any trouble, to have
> the same semantics as the core expects, then have the driver
> do something like this in probe():
>
> probe():
>    pinctrl_pm_select_sleep_state(dev);
>
> This means the device will be pushed to "default" before
> it's probed, then set to sleep quickly after that. It requires that
> CONFIG_PM is enabled for the platform, too. (As you can see
> in drivers/base/pinctrl.c)
>
> Then when you want it back into "active", i.e. "default" mode
> use the pinctrl_pm_select_default_state(dev);
>
> I realize that this means that you will end up with a transient
> state where the pins are in "default" mode before probe
> commence. If this is a big problem, yeah, then you have to
> go for custom states.

In this case I think it could be a big problem.  When you're just
talking about a PWM backlight then it doesn't matter much, but I don't
like the idea of letting a regulator glitch to a random voltage during
probe.


> But then the "default" state should not
> be defined at all, instead call them something unique like
> "active" and "relaxed" or so, so you don't overrun states already
> used by the pinctrl core. Then all confusion is gone.
>
> The "default" state was added with the assumption that most
> drivers want their pins to be in "ready to go" mode by default.

What do you think about the "default to idle" boolean, then (and
corresponding device tree property)?  It would be a pretty small patch
to pinctrl_bind_pins(), I think.  I could make sure that "idle" is
defined even if no CONFIG_PM...


-Doug

P.S. Some of this might be moot point on my particular board.  It
looks likely that the firmware will want to leave this PWM running
when it starts the kernel and they kernel could really just get by
with not touching the pinctrl at all.  ...however, I think that the
points made in this patch are still valid and it would still be worth
addressing them (and you never know, someone might be running old
firmware).
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