On Mon, Feb 22, 2016 at 11:17 AM, Ard Biesheuvel <ard.biesheu...@linaro.org> wrote: > On 22 February 2016 at 20:05, Dan Williams <dan.j.willi...@intel.com> wrote: >> On Mon, Feb 22, 2016 at 6:02 AM, Ard Biesheuvel >> <ard.biesheu...@linaro.org> wrote: >>> Currently, the memremap code serves MEMREMAP_WB mappings directly from >>> the kernel direct mapping, unless the region is in high memory, in which >>> case it falls back to using ioremap_cache(). However, the semantics of >>> ioremap_cache() are not unambiguously defined, and on ARM, it will >>> actually result in a mapping type that differs from the attributes used >>> for the linear mapping, and for this reason, the ioremap_cache() call >>> fails if the region is part of the memory managed by the kernel. >>> >>> So instead, implement an optional hook 'arch_memremap_wb' whose default >>> implementation calls ioremap_cache() as before, but which can be >>> overridden by the architecture to do what is appropriate for it. >>> >> >> Acked-by: Dan Williams <dan.j.willi...@intel.com> >> >> I still have patches pending to delete ioremap_cache() from ARM and >> require memremap() to be used for cacheable mappings. Do you see any >> use for ioremap_cache() on ARM after this change? > > I am not exactly sure why ioremap_cache() does not use MT_MEMORY_RW > attributes, but the ARM architecture simply does not allow mismatched > attributes, so we cannot simply replace each instance of > ioremap_cache() with memremap() > > Perhaps Russell can explain?
>From the x86 perspective mismatched mappings are also disallowed. The goal of deprecating ioremap_cache() in favor of memremap() is simply that the __iomem annotation is misplaced [1], and to clean up the calling convention to be explicit without silent fallbacks to different mapping types. [1]: https://lwn.net/Articles/653585/