Re: [fedora-arm] Accessing GPIOs from userspace using recent kernels

>> We have several pieces of software that make use of the
>> /sys/class/gpio userspace interface to the GPIO subsystem.
>>
>> One important prerequisite to being able to use the userspace GPIO
>> interface is to be able to put the corresponding package pins into
>> GPIO mode, by modifying the chip's mux settings.
>>
>> With Fedora 19 on the Pandaboard, we were able to achieve this by
>> poking around in /sys/kernel/debug/omap_mux. However, when running
>> Fedora 20, this directory is empty. My assumption is that this
>> functionality has been subsumed by the pinctrl subsystem in recent
>> kernels. Unfortunately, there doesn't seem to be any documented
>> userspace API.
>>
>> I'd greatly appreciate it if someone could give me some pointers about
>> how to deal with this issue!
>
>
> I'm still looking for a solution to this problem!
>
> Is there any way that I can simply amend the device tree in order to change
> a couple of mux settings?

Not at the moment. Basically as it stands you'll need to patch the
appropriate .dts and recompile the device tree. There's been a number
of proposals surrounding DT overlays but to date none have made them
upstream.

There was one for the BeagleBone for support of different cape
combinations and I've seen at least one other. It involves both kernel
and DTC (compiler) changed but until something stable lands upstream
it's not something that's easy for us to support in Fedora.

> I've figured out that I would need a stanza that looks something like:
>
> &omap4_pmx_core
> {
> foo_pins: pinmux_foo_pins {
> pinctrl-single,pins = <
> 0x5e (PIN_INPUT | MUX_MODE3) /* gpmc_ad15.gpio_39 */ /* IRQ */
> 0x12e (PIN_INPUT | MUX_MODE3) /* i2c4_scl.gpio_132 */ /* RST */
> };
> };
>
> But I can't work out what else I need to modify in order for these mux
> settings to take effect.

Basically you'll need to work out where it needs to go in the existing
.dts/.dtsi files, patch it and rebuild the .dtb

Peter
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