GPIO Centralized Control Daemon

[Patrick Venture]

I apologize if this already exists or is in the works.

I propose we create a daemon that centralizes userspace GPIO access.

>From a high level, the daemon will implement interfaces to export or
unexport GPIOs.  Each GPIO exported will exist on the dbus:

/xyz/openbmc_project/gpio/53 and implement an interface with the following
properties:
value, direction, active_low, etc.

Some doubts, should the gpio name on the dbus be the relative name (53),
should it be the system specific name (G5) or the absolute name?  I'm
thinking the relative name and let the daemon handle internalizing the
adjustment from relative to absolute.

I'm working on adding GPIO support within phosphor-hwmon so that I can
access a voltage sensor that's gated by a GPIO, and I know there have been
conversations and implementations of this for IPMI OEM -- and I think it
could easily be centralized.

Thoughts?

Patrick
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