Re: [PATCH v2 1/3] ACPI: osl: Add __force attribute in acpi_os_map_iomem() cast

[Lorenzo Pieralisi]

On Wed, Aug 11, 2021 at 12:40:28PM +0200, Ard Biesheuvel wrote:
> On Tue, 10 Aug 2021 at 18:46, Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
> >
> > On Mon, Aug 02, 2021 at 04:23:57PM +0100, Lorenzo Pieralisi wrote:
> > > Add a __force attribute to the void* cast in acpi_os_map_iomem()
> > > to prevent sparse warnings.
> >
> > Err, no.  These annotation are there for a reason and need to
> > be propagated instead.  And independent of that a __force cast
> > without a comment explaining it is a complete no-go.
> 
> The whole problem we are solving here is that ACPI, being based on
> x86, conflates MMIO mappings with memory mappings, and has been using
> the same underlying infrastructure for either. On arm64, this is not
> sufficient, given that the semantics of uncached memory vs device are
> different (the former permits unaligned accesses and clear cacheline
> instructions, but the latter doesn't). A recent optimization applied
> to memcpy() on arm64 (which now relies more on unaligned accesses for
> performance) has uncovered an issue where firmware tables being mapped
> non-cacheable by the ACPI core will end up using device mappings,
> which causes memcpy() to choke on their contents.
> 
> So propagating the annotation makes no sense, as we are creating a
> memory mapping using the iomem primitive. I wouldn't object to a
> comment being added, but I think the context should have been obvious
> to anyone who had bothered to look at the entire series.

I can add a comment and respin. Basically a __force attribute is
added to ignore a sparse warning that's been ignored for aeons
anyway - I will add the rationale above.

drivers/acpi/osl.c:379:17: warning: cast removes address space '__iomem' of expression