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From: David Brownell <david-b@pacbell.net>
To: Ben Nizette <bn@niasdigital.com>
Cc: Trent Piepho <tpiepho@freescale.com>,
	Jean Delvare <khali@linux-fr.org>,
	Linux Kernel list <linux-kernel@vger.kernel.org>,
	Mike Frysinger <vapier.adi@gmail.com>
Subject: Re: userspace GPIO access (WAS: [patch/rfc 2/4] pcf875x ...)
Date: Fri, 4 Apr 2008 21:05:47 -0700	[thread overview]
Message-ID: <200804042105.47687.david-b@pacbell.net> (raw)
In-Reply-To: <1207285076.4082.75.camel@moss.renham>

On Thursday 03 April 2008, Ben Nizette wrote:
> David, you're kinda the gatekeeper here; any input from you on which
> approach is to be preferred, essential features etc?

I won't much care about /dev/... vs /sys/... though I'd
probably have used sysfs myself (just because it's much
simpler and doesn't need to imply mdev/udev and classes).

The configuration part of each driver bothers me:

 - Mike's simple_gpio requires manual kernel config
   to set up the platform_device nodes ... and thus
   rules out the first usage scenarios I ever heard
   of for such a userspace mechanism.

 - Trent's gpio_class exposes all GPIOs, even ones
   that are claimed by kernel drivers ... and thus
   makes it easy to clobber kernel driver state.
   (Plus it won't work on most built-in GPIOs, since
   they by and large don't have parent devices.)

What I'd like to see is userspace config commands to
cause the gpio_request() ... *maybe* something like

    echo 42 foo 0 > .../gpio_config

	... causing error-checked versions of:

    gpio_request(42, "foo")
    gpio_direction_output(42, 0)

	... then some .../gpio42 file, read/write, appears

and

    echo 84 bar in > .../gpio_config

	... causing error-checked versions of:

    gpio_request(84, "bar")
    gpio_direction_input(84)

	... then some .../gpio84 file, read-only, appears

Though arguably the label could just always be "userspace"
(it's mostly for /sys/kernel/debug/gpio), and the default could
be to configure as an input (unless an output value was given).
Plus, there should be some way to cause gpio_free() too.

A potential advantage of the /dev/... node approach would be
that it's easier to support an IRQ-backed poll() mechanism
for inputs.

- Dave


  reply	other threads:[~2008-04-05  4:05 UTC|newest]

Thread overview: 60+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <200710291809.29936.david-b@pacbell.net>
2007-10-30  1:51 ` [patch/rfc 1/4] GPIO implementation framework David Brownell
2007-11-05 21:05   ` David Brownell
2007-11-13  2:28     ` eric miao
2007-11-13 19:06       ` David Brownell
2007-11-14  0:57         ` eric miao
2007-11-14  1:00           ` eric miao
2007-11-14  1:02             ` eric miao
2007-11-14  1:03               ` eric miao
2007-11-14  1:04                 ` eric miao
2007-11-14  1:04                   ` eric miao
2007-11-14  4:36                     ` David Brownell
2007-11-14  6:51                       ` eric miao
2007-11-14  7:19                         ` David Brownell
2007-11-14  7:36                           ` eric miao
2007-11-17 10:38                       ` Jean Delvare
2007-11-17 17:36                         ` David Brownell
2007-11-20 15:20                           ` Jean Delvare
2007-11-14  4:18                 ` David Brownell
2007-11-14  6:46                   ` eric miao
2007-11-14  3:28               ` David Brownell
2007-11-14  3:25             ` David Brownell
2007-11-14  3:53               ` David Brownell
2007-11-14  6:37               ` eric miao
2007-11-14  3:30           ` David Brownell
2007-11-14  6:40             ` eric miao
2007-11-14  7:08               ` David Brownell
2007-11-27  1:46                 ` David Brownell
2007-11-27 10:58                   ` eric miao
2007-11-27 17:26                     ` David Brownell
2007-11-27 19:03                     ` David Brownell
2007-11-27 19:29                     ` David Brownell
2007-11-28  5:11                       ` eric miao
2007-11-28  3:15                     ` [patch/rfc 2.6.24-rc3-mm] gpiolib grows a gpio_desc David Brownell
2007-11-28  9:10                       ` eric miao
2007-11-28  9:53                         ` David Brownell
2007-10-30  1:51 ` [patch/rfc 2/4] pcf875x I2C GPIO expander driver David Brownell
2007-11-30 12:32   ` Jean Delvare
2007-11-30 13:04     ` Bill Gatliff
2007-11-30 13:36       ` Jean Delvare
2007-11-30 14:09         ` Bill Gatliff
2007-11-30 18:40     ` David Brownell
2007-11-30 20:13       ` Jean Delvare
2007-11-30 20:59         ` David Brownell
2008-04-04  2:06           ` Trent Piepho
2008-04-04  2:45             ` Ben Nizette
2008-04-04  3:33               ` Trent Piepho
2008-04-04  4:57                 ` Ben Nizette
2008-04-05  4:05                   ` David Brownell [this message]
2008-04-07 17:56                     ` userspace GPIO access (WAS: [patch/rfc 2/4] pcf875x ...) Trent Piepho
2008-04-04  8:09             ` [patch/rfc 2/4] pcf875x I2C GPIO expander driver Jean Delvare
2008-04-04 19:07               ` Trent Piepho
2008-04-04 19:36                 ` Jean Delvare
2008-04-04 20:18                   ` Trent Piepho
2008-04-05  2:51                 ` David Brownell
2008-04-05  2:53               ` David Brownell
2007-12-06  3:03       ` [patch/rfc 2/4] pcf857x " David Brownell
2007-12-06 23:17         ` Jean Delvare
2007-12-07  4:02           ` David Brownell
2007-10-30  1:53 ` [patch/rfc 3/4] DaVinci platform uses new GPIOLIB David Brownell
2007-10-30  1:54 ` [patch/rfc 4/4] DaVinci EVM uses pcf857x GPIO driver David Brownell

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