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On some ARMs the memory can be mapped pgprot_noncached() and still be working for atomic operations. As pointed out by Colin Cross <ccross@android.com>, in some cases you do want to use pgprot_noncached() if the SoC supports it to see a debug printk just before a write hanging the system. On ARMs, the atomic operations on strongly ordered memory are implementation defined. So let's provide an optional kernel parameter for configuring pgprot_noncached(), and use pgprot_writecombine() by default. Cc: Arnd Bergmann <arnd@arndb.de> Cc: Rob Herring <robherring2@gmail.com> Cc: Randy Dunlap <rdunlap@infradead.org> Cc: Anton Vorontsov <anton@enomsg.org> Cc: Colin Cross <ccross@android.com> Cc: Olof Johansson <olof@lixom.net> Cc: Russell King <linux@arm.linux.org.uk> Cc: stable@vger.kernel.org Acked-by: Kees Cook <keescook@chromium.org> Signed-off-by: Tony Lindgren <tony@atomide.com> Signed-off-by: Tony Luck <tony.luck@intel.com>