GPIO offsets used by userspace

Andrew Jeffery andrew at
Tue Jan 31 12:15:12 AEDT 2017

On Tue, 2017-01-31 at 10:55 +1030, Joel Stanley wrote:
> > On Mon, Jan 30, 2017 at 12:04 PM, Andrew Jeffery <andrew at> wrote:
> > Hi all,
> > 
> > Just a heads up that when the linked patch is pulled into the OpenBMC
> > kernel it will break our userspace. We will need to synchronise the
> > kernel bump with the fixes for the machine configs to update the magic
> > GPIO offset number.
> > 
> >
> > 
> > The magic offset is essentially 512-(ngpios) (I don't know why, but
> > that's the behaviour), and this now varies between AST2400 and AST2500
> > systems due to the AST2500 having an extra bank, and also because the
> > AST2400 has a "hole" at the end of its GPIO number space.
> Can you suggest a way to detect this from userspace?

Not off the top of my head.

> Would using the chardev api make this easier?

To a degree. I'm not sure if named gpios are exposed by the sysfs
interface, but with the chardev API you can open the devices and
request a GPIO by name. If that fails and you have multiple devices, I
guess you can try the other gpiochips.

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