Re: [PATCH] iio: core: make iio_device_claim_direct_mode() __must_check

[Dan Williams <dan.j.williams@intel.com>]

kernel test robot wrote:
> Hi David,
> 
> kernel test robot noticed the following build warnings:
> 
> [auto build test WARNING on 431c39f6d3edbab14f48dbf37a58ccdc0ac3be1e]
> 
> url:    https://github.com/intel-lab-lkp/linux/commits/David-Lechner/iio-core-make-iio_device_claim_direct_mode-__must_check/20241002-233644
> base:   431c39f6d3edbab14f48dbf37a58ccdc0ac3be1e
> patch link:    https://lore.kernel.org/r/20241002-iio-must-check-claim-direct-v1-1-ab94ce728731%40baylibre.com
> patch subject: [PATCH] iio: core: make iio_device_claim_direct_mode() __must_check
[..]
> >> include/linux/iio/iio.h:669:50: warning: ignoring return value of function declared with 'warn_unused_result' attribute [-Wunused-result]
>      669 | DEFINE_GUARD(iio_claim_direct, struct iio_dev *, iio_device_claim_direct_mode(_T),

So I think this points to the fact that iio_device_claim_direct_mode()
should not be using DEFINE_GUARD() in the first instance. I think
iio_claim_direct() really wants to be using DEFINE_CLASS() directly.
Skip usage of DEFINE_GUARD() which I now see is unable to interoperate
with a __must_check locking function.

Perhaps the new class can be something like:

    DEFINE_GUARD_EXCL_COND()

...which creates a guard that is exclusively conditional and has no
unconditional flavor. However, maybe that only lives in iio unless and
until another user arrives.