Re: GPIOs not correctly exported via sysfs on ATSAMA5D2

[Russell King - ARM Linux admin <linux@xxxxxxxxxxxxxxx>]

On Mon, Mar 02, 2020 at 05:34:19PM +0100, Linus Walleij wrote:
> On Mon, Mar 2, 2020 at 10:36 AM Russell King - ARM Linux admin
> <linux@xxxxxxxxxxxxxxx> wrote:
> > On Mon, Mar 02, 2020 at 09:39:59AM +0100, Uwe Kleine-König wrote:
> 
> > >  - Sometimes it is useful to make use of a GPIO and a dedicated function
> > >    on the same pin in a driver (e.g. an i2c driver might need to switch
> > >    to gpio to do a bus recovery). The automatic pinmuxing then has
> > >    strange side effect because you have to remux the pins after
> > >    requesting the GPIOs even if you didn't drive the pins as GPIO and
> > >    there is a short time where the pin function isn't the dedicated
> > >    one.
> >
> > It's worse than that for the i2c driver.  The pins are muxed to the i2c
> > function when the driver binds.  When the i2c driver claims the GPIOs
> > corresponding with those pins, they get switched to GPIO mode behind
> > the back of pinctrl.  You then have to _explicitly_ switch pinctrl to
> > GPIO mode and back to I2C mode to get them back to I2C mode.
> 
> That's especially annoying. I would consider adding a specific
> consumer flag for GPIOs used this way, in additon to
> GPIOD_ASIS, something like GPIOD_ASIS_NOMUX
> (thinking of better names).

It's very annoying, and I believe something I did point out in my email
about it when I discovered it towards the end of last year.

Having a way to avoid the muxing would be a very good idea, as there
are cases where we really should not be taking the I2C pins away from
the controller during driver initialisation.  In the case of i2c-pxa,
when the pins are taken away, the controller sees the disconnected SCL
and SDA lines go low, and it can assume that the bus is busy as a
result, or worse see it as a START condition if SDA goes low while SCL
is high.

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