[PATCH 1/2] pstore-ram: Fix hangs by using write-combine mappings

[Tony Lindgren]

From: Rob Herring <robherring2 at gmail.com>

Currently trying to use pstore on at least ARMs can hang as we're
mapping the peristent RAM with pgprot_noncached().

On ARMs, pgprot_noncached() will actually make the memory strongly
ordered, and as the atomic operations pstore uses are implementation
defined for strongly ordered memory, they may not work. So basically
atomic operations have undefined behavior on ARM for device or strongly
ordered memory types.

Let's fix the issue by using write-combine variants for mappings. This
corresponds to normal, non-cacheable memory on ARM. For many other
architectures, this change does not change the mapping type as by
default we have:

#define pgprot_writecombine pgprot_noncached

The reason why pgprot_noncached() was originaly used for pstore
is because Colin Cross <ccross at android.com> had observed lost
debug prints right before a device hanging write operation on some
systems. For the platforms supporting pgprot_noncached(), we can
add a an optional configuration option to support that. But let's
get pstore working first before adding new features.

Cc: Arnd Bergmann <arnd at arndb.de>
Cc: Anton Vorontsov <cbouatmailru at gmail.com>
Cc: Colin Cross <ccross at android.com>
Cc: Kees Cook <keescook at chromium.org>
Cc: Olof Johansson <olof at lixom.net>
Cc: Tony Luck <tony.luck at intel.com>
Cc: linux-kernel at vger.kernel.org
Signed-off-by: Rob Herring <rob.herring at calxeda.com>
[tony at atomide.com: updated description]
Signed-off-by: Tony Lindgren <tony at atomide.com>
---
 fs/pstore/ram_core.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/pstore/ram_core.c b/fs/pstore/ram_core.c
index 9d7b9a8..24f94b0 100644
--- a/fs/pstore/ram_core.c
+++ b/fs/pstore/ram_core.c
@@ -392,7 +392,7 @@ static void *persistent_ram_vmap(phys_addr_t start, size_t size)
 	page_start = start - offset_in_page(start);
 	page_count = DIV_ROUND_UP(size + offset_in_page(start), PAGE_SIZE);
 
-	prot = pgprot_noncached(PAGE_KERNEL);
+	prot = pgprot_writecombine(PAGE_KERNEL);
 
 	pages = kmalloc_array(page_count, sizeof(struct page *), GFP_KERNEL);
 	if (!pages) {
@@ -422,7 +422,7 @@ static void *persistent_ram_iomap(phys_addr_t start, size_t size)
 	buffer_start_add = buffer_start_add_locked;
 	buffer_size_add = buffer_size_add_locked;
 
-	return ioremap(start, size);
+	return ioremap_wc(start, size);
 }
 
 static int persistent_ram_buffer_map(phys_addr_t start, phys_addr_t size,
-- 
2.1.0