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From: Bartosz Golaszewski <brgl@bgdev.pl>
To: Linus Walleij <linus.walleij@linaro.org>
Cc: Drew Fustini <drew@pdp7.com>,
	"open list:GPIO SUBSYSTEM" <linux-gpio@vger.kernel.org>,
	Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Subject: Re: [RFC] gpio: expose pull-up/pull-down line flags to userspace
Date: Fri, 4 Oct 2019 09:22:33 +0200
Message-ID: <CAMRc=McH=ui1c9yTMtuMwVUT2-yBHhV=r0VGsKY0KbYMLHJhPA@mail.gmail.com> (raw)
In-Reply-To: <CACRpkdZjswY4zW232ahSQSGfprbgBx8YL4Wb0i3ebegT00v3jQ@mail.gmail.com>

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

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

  reply index

Thread overview: 35+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-09-21 10:25 Drew Fustini
2019-09-23  8:38 ` Bartosz Golaszewski
2019-10-02 11:10   ` Drew Fustini
2019-10-03 12:47   ` Linus Walleij
2019-10-04  7:22     ` Bartosz Golaszewski [this message]
2019-10-04 12:46       ` Bartosz Golaszewski
2019-10-05 17:02       ` Linus Walleij
2019-10-06  3:12         ` Kent Gibson
2019-10-06 21:06           ` Linus Walleij
2019-10-07  4:37             ` Kent Gibson
2019-10-08  6:15 ` Kent Gibson
2019-10-08 20:56   ` Bartosz Golaszewski
2019-10-08 23:21     ` Kent Gibson
2019-10-08 23:30       ` Bartosz Golaszewski
2019-10-08 23:56         ` Kent Gibson
2019-10-09  0:03           ` Bartosz Golaszewski
2019-10-09  0:22             ` Kent Gibson
2019-10-09  6:55               ` Kent Gibson
2019-10-09 12:57                 ` Drew Fustini
2019-10-09 13:23                   ` Kent Gibson
2019-10-09 13:30                 ` Drew Fustini
2019-10-09 14:11                   ` Kent Gibson
2019-10-09 15:50                     ` Bartosz Golaszewski
2019-10-09 16:19                       ` Kent Gibson
2019-10-09 23:59                   ` Kent Gibson
2019-10-10  7:47                     ` Drew Fustini
2019-10-10 10:14                       ` Kent Gibson
2019-10-10 11:17                         ` Kent Gibson
2019-10-11 13:04                           ` Drew Fustini
2019-10-11 13:06                         ` Drew Fustini
2019-10-11 13:49                           ` Kent Gibson
2019-10-09 11:32               ` Drew Fustini
2019-10-09 13:57       ` Drew Fustini
2019-10-09 14:01         ` Kent Gibson
2019-10-09 11:46   ` Drew Fustini

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