On 22 February 2016 at 20:05, Dan Williams [off-list ref] wrote:

On Mon, Feb 22, 2016 at 6:02 AM, Ard Biesheuvel
[off-list ref] wrote:

quoted

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 <redacted>

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?