On Tue, Apr 14, 2020 at 12:07:01PM +0200, Michael Walle wrote:
> Am 2020-04-14 11:50, schrieb Bartosz Golaszewski:

> > Maybe setting the pointer to ERR_PTR(-ENOENT) which will result in
> > IS_ERR() returning true?

> Unfortunatly, its not a pointer, but only a regular unsigned int (ie
> the type the regmap API has for its "reg" property). It could be a
> pointer of course but then the user would have to allocate additional
> memory.

You could define REGMAP_INVALID_ADDR to be (unsigned int)(-1) or some
other suitably implausible address and use that as a value.  It's
possible that there might be a collision with a real address on some
device but it should be sufficiently unlikely to be useful, especially
if it's not something regmap in general goes and evaluates.  For extra
safety we could have an API for allowing users to query the register
validity information regmap has (or can be given) and gpiolib could then
use that to figure out if the value was actually a dummy value but
that's probably overdoing it.