[PATCH 03/10] tty: pxa: configure pin

[linus.walleij@linaro.org (Linus Walleij)]

On Fri, Oct 19, 2012 at 12:20 AM, Stephen Warren <swarren@wwwdotorg.org> wrote:

> On 10/18/2012 03:06 AM, Haojian Zhuang wrote:
>> Configure pins by pinctrl driver.
>
> In general, it seems better to use pinctrl "hogs" if the driver is only
> going to statically set up one pinctrl state and never change it. The
> alternative is make every single driver execute these pinctrl calls,
> which could be tedious.

True. And each platform has to choose how to go ahead with this.

I always imagined that any system of the "power socket in wall"
type would just use hogs to configure its pins and be done with
it, and there appear to be some platforms like that. (e.g. MIPS and
various power-inaware references come to mind).

For the Ux500 drivers we have found that all of those that have pin
resources actually have to be handled by the driver. The reason is
that all of them have idle and/or sleep states that need to be
handled at runtime.

As an additional complication our drivers are used also by
the Versatile and SPEAr family of platforms, so the control
need to be tolerant (accept missing pinctrl handles and states)
as well. (I can see that other drivers take shortcuts by being less
elaborate since there is a 1:1 driver<->platform mapping.)

An alternative to embedding the pin handling code into
the drivers is to use bus notifiers as is done with the
shmobile clocking by drivers/base/power/clock_ops.c
I could easily imagine pinctrl_ops.c like that for some
platforms. This mandates still binding the pin table entries
to a device but avoids adding any code to the drivers.

However this latter approach does not work for us (Ux500) -
the three resources we have: clocks, pins and power domains
are dependent on state switch ordering (sometimes you need
to switch off the clock then set pin state, sometimes the
other way around) and it is not even
the same for all drivers - the notifier approach mandates
that all drivers do the clock, power domain and pin handling
uniformly.

> However, that does raise one question: If all the pinctrl setup is done
> by hogs, then how do we ensure that the pinctrl driver is probed first,
> and hence sets up the pins before any driver relies on them being set
> up? If a driver explicitly enables a pinctrl state (as in this patch),
> then deferred probe ensures that, but without any explicit pinctrl
> action, it'll only work by luck.

Yes, since there are no explicit dependencies with hogs
it is implicitly decoupled and you only know that the hogging will
happen whenever the pin controller driver is probed.

We have many such pieces of code in the kernel for sure,
but I agree it's not always very elegant :-/

If using the bus nofifier approach I described above the
listener can just listen to BUS_NOTIFY_BIND_DRIVER
and BUS_NOTIFY_UNBOUND_DRIVER and hog/unhog
the drivers' pins at this point, but as described this approach
has other drawbacks.

Yours,
Linus Walleij