[PATCH] drm/ttm: add workaround for some arm hardware issue

[Victor Zhao]

Some Arm based platform has hardware issue which may
generate incorrect addresses when receiving writes from the CPU
with a discontiguous set of byte enables. This affects the writes
with write combine property.

Workaround by change PROT_NORMAL_NC to PROT_DEVICE_nGnRE on arm.
As this is an issue with some specific arm based cpu, adding
a ttm parameter to control.

Signed-off-by: Victor Zhao <Victor.Zhao at amd.com>
---
 drivers/gpu/drm/ttm/ttm_module.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/ttm/ttm_module.c b/drivers/gpu/drm/ttm/ttm_module.c
index e87f40674a4d..b27473cbbd52 100644
--- a/drivers/gpu/drm/ttm/ttm_module.c
+++ b/drivers/gpu/drm/ttm/ttm_module.c
@@ -41,6 +41,12 @@
 
 #include "ttm_module.h"
 
+static int enable_use_wc = 1;
+
+MODULE_PARM_DESC(enable_use_wc,
+	"control write combine usage on arm platform due to hardware issue with write combine found on some specific arm cpu (1 = enable(default), 0 = disable)");
+module_param(enable_use_wc, int, 0644);
+
 /**
  * ttm_prot_from_caching - Modify the page protection according to the
  * ttm cacing mode
@@ -63,7 +69,7 @@ pgprot_t ttm_prot_from_caching(enum ttm_caching caching, pgprot_t tmp)
 #endif
 #if defined(__ia64__) || defined(__arm__) || defined(__aarch64__) || \
 	defined(__powerpc__) || defined(__mips__)
-	if (caching == ttm_write_combined)
+	if (caching == ttm_write_combined && enable_use_wc != 0)
 		tmp = pgprot_writecombine(tmp);
 	else
 		tmp = pgprot_noncached(tmp);
-- 
2.25.1