Return type of clean_and_invalidate_dcache_va_range

[Nicola Vetrini <nicola.vetrini@bugseng.com>]

Hi all,

In the context of violations of MISRA C:2012 Rule 17.7: "The value 
returned by a function having non-void return type shall be used", I was 
looking at the function "clean_and_invalidate_dcache_va_range". It has 
the following signature on both arm and x86:

static inline int clean_and_invalidate_dcache_va_range
     (const void *p, unsigned long size)

The commit that introduced it for Arm ~9 years ago (71d64afe3e12: "arm: 
return int from *_dcache_va_range") [1] mentions that on Arm it can't 
fail, but supposedly it can on x86.

However, as far as I can tell, for both arch-es the implementation now 
always returns 0 [2][3], so perhaps the mention of -EOPNOTSUPP for x86 
is no longer true (I wasn't able to reconstruct if there was a time at 
which this was true, even in the same commit that changed the return 
type to int).

The question is: should the return type be void, since it appears that 
every user is ignoring the returned value (violating the rule), except 
the one in common/grant_table.c [4]?

The other two resolution paths are either allowing this function's 
result to be ignored or cast all ignored invocations to void, with the 
first being cleaner from a code readability perspective.

[1] These functions cannot really fail on ARM, but their x86 equivalents 
can (-EOPNOTSUPP). Change the prototype to return int.
[2] 
https://gitlab.com/xen-project/xen/-/blob/staging/xen/arch/arm/include/asm/page.h#L218
[3] 
https://gitlab.com/xen-project/xen/-/blob/staging/xen/arch/x86/include/asm/flushtlb.h#L188
[4] 
https://gitlab.com/xen-project/xen/-/blob/staging/xen/common/grant_table.c#L3576

-- 
Nicola Vetrini, BSc
Software Engineer, BUGSENG srl (https://bugseng.com)