Re: Confusion about Pinctrl and GPIO

[Stephen Warren]

On 12/25/2013 10:18 PM, æèè wrote:

> I noticed that Stephen<swarren@xxxxxxxxxx> submitted a patch for pinctrl:
> http://www.gossamer-threads.com/lists/linux/kernel/1500890?do=post_view_threaded
> 
> In this patch, Stephen said, "When an SoC muxes pins in a group, it's
> quite possible for the group to contain e.g. 6 pins, but only 4 of
> them actually be needed by the HW module that's mux'd to them. In this
> case, the other 2 pins could be used as GPIOs. However, pinctrl marks
> all the pins within the group as in-use by the selected mux function.
> To allow the expected gpiolib interaction, separate the concepts of
> pin ownership into two parts: One for the mux function and one for
> GPIO usage. Finally, allow those two ownerships to exist in parallel.
> "
> 
> I agree that gpiolib should be able to use the two idle pins as GPIO,
> but after apply this patch, gpiolib can also request the 4 pins used
> by HW module succesfully, and this will override the settings of the 4
> pins for HW module.

Yes, that's true.

The solution is: don't do that.

> Let me talk again about the example described by Stephen. If actually
> only 4 pins of the group which contains 6 pins are needed by HW
> module, then why does the group be defined to contain 6 pins? I think
> the group should be defined only containing 4 pins rather than 6 pins,
> then the other 2 idle pins can be used for any other purpose.

It all depends on what you mean by group...

A lot of HW has a mux setting per pin. In this case, it would make sense
for the pinctrl driver to expose a separate group for each pin, and for
the pinctrl mapping table (or DT content) to contain an entry for each
individual pin setting that pin to whatever mux function was relevant.

So in this case, yes, it'd make sense in most cases to disallow pins
with a defined/selected mux setting from being used as a GPIO. However,
even in this case, we can't ban dual mux/GPIO usage completely, since
e.g. an I2C driver might want the I2C HW module to drive the pins most
of the time, but still need to acquire the pins as GPIO to implement
some kind of manual bit-banging e.g. to implement a "bus unstick" algorithm.

Some other HW has mux settings that affect multiple pins at once. Tegra
is an example. In this case, there's a single register bit that defines
the mux functions for e.g. 6 pins. In this case, there *must* be a
single pinctrl group definition that encompasses all those 6 pins, since
that's how the HW works. However, the GPIO-vs-non-GPIO setting on Tegra
at least is still per pin, hence the need for the patch of mine that you
mentioned above.

Finally, some people want to use pinctrl groups to represent something
higher level than HW that has a mux bit for a group of pins rather than
per-pin. In that case, you also may need GPIO/mux sharing of a pin, for
similar reasons to the case where the HW muxing really does operate in
groups.
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