Messages in this thread |  | | From | David Brownell <> | Subject | Re: userspace GPIO access (WAS: [patch/rfc 2/4] pcf875x ...) | Date | Fri, 4 Apr 2008 21:05:47 -0700 |
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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
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