If an optional IRQ is not present, drivers either just ignore it (e.g.
for devices that can have multiple interrupts or a single muxed IRQ),
or they have to resort to polling. For the latter, fall-back handling
is needed elsewhere in the driver.
To me it sounds much more logical for the driver to check if an
optional irq is non-zero (available) or zero (not available), than to
sprinkle around checks for -ENXIO. In addition, you have to remember
that this one returns -ENXIO, while other APIs use -ENOENT or -ENOSYS
(or some other error code) to indicate absence. I thought not having
to care about the actual error code was the main reason behind the
introduction of the *_optional() APIs.

The *_optional() functions return an error code if there has been a
real error which should be reported up the call stack. This excludes
whatever error code indicates the requested resource does not exist,
which can be -ENODEV etc. If the device does not exist, a magic cookie
is returned which appears to be a valid resources but in fact is
not. So the users of these functions just need to check for an error
code, and fail the probe if present.

You seems to be suggesting in binary return value: non-zero
(available) or zero (not available)

This discards the error code when something goes wrong. That is useful
information to have, so we should not be discarding it.

IRQ don't currently have a magic cookie value. One option would be to
add such a magic cookie to the subsystem. Otherwise, since 0 is
invalid, return 0 to indicate the IRQ does not exist.

The request for a script checking this then makes sense. However, i
don't know how well coccinelle/sparse can track values across function
calls. They probably can check for:

   ret = irq_get_optional()
   if (ret < 0)
      return ret;

A missing if < 0 statement somewhere later is very likely to be an
error. A comparison of <= 0 is also likely to be an error. A check for

0 before calling any other IRQ functions would be good. I'm

surprised such a check does not already existing in the IRQ API, but
there are probably historical reasons for that.

      Andrew