GPIO expander user space access(ie. pca9552)

[Andrew Jeffery]

On Mon, 2017-01-23 at 09:58 -0600, Matt Barth wrote:
> Is there a defined process/interface to access GPIO expanders from user 
> space to be able to determine when the state changes? Possibly polling a 
> fd or getting notified on the state change?
> 
> Previously this was an i2c device that was accessed over i2c and the set 
> of interested bits were simply read into the application. An example is 
> the use of a pca9552 for presence detection on chassis fans.

I expect it should be implemented as a gpiochip driver in the kernel
and exposed via the usual methods to userspace (GPIO chardev and sysfs,
though sysfs is considered deprecated).

Andrew
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