Re: Return type of clean_and_invalidate_dcache_va_range

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Fri, 9 Feb 2024, Nicola Vetrini wrote:
> 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]?

Looking at the implementation on both ARM and x86, I am in favor of
changing the return type to void



> 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)
>