linux-kernel - [PATCH] drm/amd: work around llvm bug #42576

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Date:   Fri, 12 Jul 2019 11:34:57 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Alex Deucher <alexander.deucher@....com>,
        Christian König <christian.koenig@....com>,
        "David (ChunMing) Zhou" <David1.Zhou@....com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>
Cc:     Arnd Bergmann <arnd@...db.de>, Huang Rui <ray.huang@....com>,
        Evan Quan <evan.quan@....com>, xinhui pan <xinhui.pan@....com>,
        Rex Zhu <Rex.Zhu@....com>, Oak Zeng <Oak.Zeng@....com>,
        Philip Yang <Philip.Yang@....com>,
        amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: [PATCH] drm/amd: work around llvm bug #42576

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@...db.de>
---
Sending this for completeness, please decide for yourselves whether to
apply it or not, given that it's a trivial workaround but probably not
needed in the long run.
---
 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 4428018672d3..154416a626df 100644
--- a/drivers/gpu/drm/amd/amdgpu/sdma_v4_0.c
+++ b/drivers/gpu/drm/amd/amdgpu/sdma_v4_0.c
@@ -773,6 +773,7 @@ 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