[BUG 5.14] arm64/mm: dma memory mapping fails (in some cases)

[Christoph Hellwig]

On Mon, Sep 20, 2021 at 11:57:58AM +0100, Catalin Marinas wrote:
> > As this WARN_ON(pfn_valid()) is only present in dma_map_resource() it's
> > probably safe to drop it entirely. 
> 
> I agree, we should drop it. IIUC dma_map_resource() does not create any
> kernel mapping to cause problems with attribute aliasing. You'd need a
> prior devm_ioremap_resource() if you want access to that range from the
> CPU side. For arm64 at least, the latter ends up with a
> pfn_is_map_memory() check.

It doesn't create any new mappings.  The only real issue is that it
does the wrong thing for RAM in a way that might not be noticed on
simple (x86/PC) platforms.