[PATCH 6/6] [RESEND] drm/amdgpu: work around llvm bug #42576

[Arnd Bergmann]

Code in the amdgpu driver triggers a bug when using clang to build
an arm64 kernel:

/tmp/sdma_v4_0-f95fd3.s: Assembler messages:
/tmp/sdma_v4_0-f95fd3.s:44: Error: selected processor does not support `bfc w0,#1,#5'

I expect this to be fixed in llvm soon, but we can also work around
it by inserting a barrier() that prevents the optimization.

Link: https://bugs.llvm.org/show_bug.cgi?id=42576
Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
---
Apparently this bug is still present in both the released clang-9
and the current development version of clang-10.
I was hoping we would not need a workaround in clang-9+, but
it seems that we do.
---
 drivers/gpu/drm/amd/amdgpu/sdma_v4_0.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/gpu/drm/amd/amdgpu/sdma_v4_0.c b/drivers/gpu/drm/amd/amdgpu/sdma_v4_0.c
index 78452cf0115d..39459cd8ddef 100644
--- a/drivers/gpu/drm/amd/amdgpu/sdma_v4_0.c
+++ b/drivers/gpu/drm/amd/amdgpu/sdma_v4_0.c
@@ -961,6 +961,7 @@ static uint32_t sdma_v4_0_rb_cntl(struct amdgpu_ring *ring, uint32_t rb_cntl)
 	/* Set ring buffer size in dwords */
 	uint32_t rb_bufsz = order_base_2(ring->ring_size / 4);
 
+	barrier(); /* work around https://bugs.llvm.org/show_bug.cgi?id=42576 */
 	rb_cntl = REG_SET_FIELD(rb_cntl, SDMA0_GFX_RB_CNTL, RB_SIZE, rb_bufsz);
 #ifdef __BIG_ENDIAN
 	rb_cntl = REG_SET_FIELD(rb_cntl, SDMA0_GFX_RB_CNTL, RB_SWAP_ENABLE, 1);
-- 
2.20.0