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From: Linus Walleij <linus.walleij@linaro.org>
To: Bartosz Golaszewski <brgl@bgdev.pl>
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: Thu, 3 Oct 2019 14:47:41 +0200
Message-ID: <CACRpkdZjswY4zW232ahSQSGfprbgBx8YL4Wb0i3ebegT00v3jQ@mail.gmail.com> (raw)
In-Reply-To: <CAMRc=Me=6JeOOv_SRhKt+vOsd3p5yOVkWyNu4Oo+DeCwMJHmaA@mail.gmail.com>

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

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

Yours,
Linus Walleij

  parent 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 [this message]
2019-10-04  7:22     ` Bartosz Golaszewski
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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