Re: [RFC] gpio: expose pull-up/pull-down line flags to userspace

[Bartosz Golaszewski <brgl@bgdev.pl>]

pt., 4 paź 2019 o 09:22 Bartosz Golaszewski <brgl@bgdev.pl> napisał(a):
>
> czw., 3 paź 2019 o 14:47 Linus Walleij <linus.walleij@linaro.org> napisał(a):
> >
> > On Mon, Sep 23, 2019 at 10:38 AM Bartosz Golaszewski <brgl@bgdev.pl> wrote:
> >
> > > I remember discussing it with Linus some time ago. This may not be as
> > > straightforward as simply adding new flags. Since PULL-UP/DOWN
> > > resistors can - among other parameters - also have configurable
> > > resistance, we'll probably need some kind of a structure for this
> > > ioctl() to pass any additional information to the kernel. Since we
> > > can't change ABI this may require adding a whole new ioctl() for
> > > extended configuration. This in turn has to be as future-proof as
> > > possible - if someone asks for user-space-configurable drive-strength,
> > > the new ioctl() should be ready for it.
> > >
> > > I should have some bandwidth in the coming days, so I'll try to give it a try.
> >
> > What we did for the in-kernel API and the Device Tree ABI
> > was to simply say that if you need such elaborate control over
> > the line, it needs to be done with a proper pin control driver.
> >
> > So for lines that just have the GPIO_PULL_UP or
> > GPIO_PULL_DOWN set as a (one-bit) flag, what you will
> > get is "typical" pull down/up (whatever the hardware default
> > is, or what the driver thinks is default, which should be safe
> > so the highest possible pull resistance I suppose).
> >
> > So one option is to just go with these flags and explicitly
> > say that it will give a "system default (high resistance)
> > pull up/down".
> >
> > That said, the pin controller back-end is fully capable of
> > accepting more elaborate configuration, so if we prefer then
> > we can make the more complex userspace ABI that can
> > set it to a desired-or-default resistance.
> >
> > I lean toward simplicity here. I haven't seen that these
> > userspace consumers need very elaborate control of this
> > resistance, they are for one-off hacks and as such should
> > be fine with just default pull up/down I think?
> >
> > I  think that specifying "this line will use pull up/down"
> > at request time and having the driver set a safe default
> > pull-up/down as response, (and pretty much what this
> > patch does) and then add another explicit
> > ioctl to refine it the day we need it is a viable way forward.
> >
> >  in the future something like:
> > #define GPIOHANDLE_SET_LINE_CONFIG_IOCTL _IOWR(0xB4, 0x0a, struct
> > gpiohandle_config)
> >
> > And then, when we need it, try to come up with some
> > really flexible ABI for the config, based on
> > include/linux/pinctrl/pinconf-generic.h
> >
>
> Thanks for your input Linus. I'm good with that. The config ioctl (or
> something similar) you're mentioning may appear sooner actually -
> users of libgpiod have been requesting a way of changing the direction
> of a line without releasing it - something that's possible in the
> kernel, but not from user-space at the moment. I'll submit something
> that allows to change the configuration of a requested line soon.
>
> Bart

Cc'ing Kent, as he might be interested in this.

I'll be travelling tomorrow and will have a couple hours on the plane,
so I'll try to cook up an additional ioctl for configuration changes
of requested lines with additional padding for future extension.

Bart

>
> > But no upfront code for that right now as it is not needed.
> > A practical usecase must come first.
> >
> > Yours,
> > Linus Walleij