drm/amdgpu:fix race condition

sequence is protected by spinlock so don't access sequence
in paramter seq when invoking this function.

~0 means to get the latest sequence number and 0 means none to
get.

Change-Id: Ib7a03f3cf5594deeb4ad333cc59b47a6bddfd1ad
Signed-off-by: Monk Liu <Monk.Liu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.c

@@ -273,6 +273,9 @@ struct dma_fence *amdgpu_ctx_get_fence(struct amdgpu_ctx *ctx,
 
 	spin_lock(&ctx->ring_lock);
 
+	if (seq == ~0ull)
+		seq = ctx->rings[ring->idx].sequence - 1;
+
 	if (seq >= cring->sequence) {
 		spin_unlock(&ctx->ring_lock);
 		return ERR_PTR(-EINVAL);

include/uapi/drm/amdgpu_drm.h

@@ -295,7 +295,10 @@ union drm_amdgpu_gem_wait_idle {
 };
 
 struct drm_amdgpu_wait_cs_in {
-	/** Command submission handle */
+	/* Command submission handle
+         * handle equals 0 means none to wait for
+         * handle equal ~0ull meanas wait for the latest sequence number
+         */
 	__u64 handle;
 	/** Absolute timeout to wait */
 	__u64 timeout;