[RFC PATCH] drm/ttm: force cached mappings for system RAM on ARM

[Ard Biesheuvel]

ARM systems do not permit the use of anything other than cached
mappings for system memory, since that memory may be mapped in the
linear region as well, and the architecture does not permit aliases
with mismatched attributes.

So short-circuit the evaluation in ttm_io_prot() if the flags include
TTM_PL_SYSTEM when running on ARM or arm64, and just return cached
attributes immediately.

This fixes the radeon and amdgpu [TBC] drivers when running on arm64.
Without this change, amdgpu does not start at all, and radeon only
produces corrupt display output.

Cc: Christian Koenig <christian.koenig at amd.com>
Cc: Huang Rui <ray.huang at amd.com>
Cc: Junwei Zhang <Jerry.Zhang at amd.com>
Cc: David Airlie <airlied at linux.ie>
Reported-by: Carsten Haitzler <Carsten.Haitzler at arm.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
---
 drivers/gpu/drm/ttm/ttm_bo_util.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/gpu/drm/ttm/ttm_bo_util.c b/drivers/gpu/drm/ttm/ttm_bo_util.c
index 046a6dda690a..0c1eef5f7ae3 100644
--- a/drivers/gpu/drm/ttm/ttm_bo_util.c
+++ b/drivers/gpu/drm/ttm/ttm_bo_util.c
@@ -530,6 +530,11 @@ pgprot_t ttm_io_prot(uint32_t caching_flags, pgprot_t tmp)
 	if (caching_flags & TTM_PL_FLAG_CACHED)
 		return tmp;
 
+#if defined(__arm__) || defined(__aarch64__)
+	/* ARM only permits cached mappings of system memory */
+	if (caching_flags & TTM_PL_SYSTEM)
+		return tmp;
+#endif
 #if defined(__i386__) || defined(__x86_64__)
 	if (caching_flags & TTM_PL_FLAG_WC)
 		tmp = pgprot_writecombine(tmp);
-- 
2.17.1