Re: [PATCH 0/2] gpio: Allow userspace export from DT

[Johan Hovold <johan@kernel.org>]

On Wed, May 06, 2015 at 07:56:05PM +0200, Fuzzey, Martin wrote:
> On 6 May 2015 at 15:06, Johan Hovold <johan@kernel.org> wrote:
> > On Wed, May 06, 2015 at 09:22:22AM +0200, Linus Walleij wrote:
> > > On Mon, May 4, 2015 at 10:49 AM, Johan Hovold <johan@kernel.org> wrote:
> > > > On Mon, Apr 13, 2015 at 01:05:15PM +0200, Martin Fuzzey wrote:
> > >
> > > >> The above means that, in order to export the GPIO to userspace via
> > > >> /sys/class/gpio/export the userspace code must know the exact hardware and
> > > >> kernel version information.
> > > >
> > > > Not quite. You can still determine the gpio number in the above cases by
> > > > walking the sysfs tree to find the chip and it's base. This is the only
> > > > way to do this for dynamic buses such as USB.
> > >
> 
> A problem with that is that the "label" sysfs attribute is not
> necessarilly unique (and is even documented as being non unique).
> Many drivers just use the chip name as the label so if you have more
> than one of them...
> 
> Of course if you know the bus and bus address it is possible to find
> the associated GPIO base but why should userspace have to know that?
> Isn't the whole point of the DT to describe such hardware details?

Yes, eventually, but the current gpio sysfs-interface does not allow for
that information to be confined to DT. It has always required knowledge
of the hardware (e.g. the pin numbers), but people eventually discovered
that those numbers may actually change.

I'm only pointing out that *that* particular quirk can be worked around
by providing different information about the hardware (e.g. the bus
topology).

IMO, the current interface should remain to be a pain to use so that
people either fix the drivers or start working on a replacement
user-space interface.

> > > Maybe we should start providing something like an
> > > "lsgpio" utility in tools/gpio to do this, just as a hint
> > > to userspace people on how things should be done.
> >
> > I think we should focus on defining a new user-space interface rather
> > than make it easier to use the current one (it should really only be
> > used for development or one-off hacks IMO).
> >
> Doesn't the ability to name GPIOs, fix their direction, and "pre
> export" them  in the device tree remove most of the warts of the
> current interface? (for GPIOs for which it is used in any case)

I agree with you that such properties (e.g. direction and function)
should be defined in DT. A gpio should not be exported until requested
by userspace though. This allow for the case where a userspace driver
can be used as a fall back if a kernel driver isn't available or loaded.

> > > >> This patch series solves both problems by performing the external
> > > >> signal => GPIO mapping in the device tree.
> > > >
> > > > As Rob already mentioned, what we want is some way to declare pin
> > > > functions. These could then be requested from userspace (or used in DT,
> > > > something which should allow for further refactoring there as well)
> > > > unless a driver has already claimed them.
> > >
> > > We have the ability to name the GPIO lines (I usually refer to
> > > lines rather than pins, as pins are physical and not all GPIOs
> > > are, actually) using the array "names" in struct gpio_chip,
> > > however this has no DT binding, so maybe people should
> > > work on that. These names appear as named line files
> > > in sysfs IIRC. Or maybe you're thinking of something else?
> >
> > Yes, something like that. As you mention above, if it was possibly to
> > define those names in firmware, even the current sysfs interface would
> > expose the pins as
> >
> >         /sys/class/gpio/<function>
> >
> > rather than say /sys/class/gpio/gpio279, thereby solving the gpio-number
> > look-up issue. Well, to actually make the pin available *from* userspace
> > you'd currently still need the number...
> >
> Indeed that  doesn't solve the how to export it problem.
> AFAICT nothing is visible in sysfs until the export has been done.

That will be solved by the new userspace interface. ;)

Johan