drm/syncobj: Deal with signalled fences in drm_syncobj_find_fence.

commit b19926d4f3a660a8b76e5d989ffd1168e619a5c4 upstream.

dma_fence_chain_find_seqno only ever returns the top fence in the
chain or an unsignalled fence. Hence if we request a seqno that
is already signalled it returns a NULL fence. Some callers are
not prepared to handle this, like the syncobj transfer functions
for example.

This behavior is "new" with timeline syncobj and it looks like
not all callers were updated. To fix this behavior make sure
that a successful drm_sync_find_fence always returns a non-NULL
fence.

v2: Move the fix to drm_syncobj_find_fence from the transfer
    functions.

Fixes: ea569910cbab ("drm/syncobj: add transition iotcls between binary and timeline v2")
Cc: stable@vger.kernel.org
Signed-off-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Christian König <christian.koenig@amd.com>
Acked-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211208023935.17018-1-bas@basnieuwenhuizen.nl
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c
index 3491460498491d0d8f5f96058b5adbf6a7ef6c80..993898a2c7791a1ee9f145d2b1216d37fac32a46 100644 (file)
--- a/drivers/gpu/drm/drm_syncobj.c
+++ b/drivers/gpu/drm/drm_syncobj.c
@@ -391,8 +391,17 @@ int drm_syncobj_find_fence(struct drm_file *file_private,
 
        if (*fence) {
                ret = dma_fence_chain_find_seqno(fence, point);
-               if (!ret)
+               if (!ret) {
+                       /* If the requested seqno is already signaled
+                        * drm_syncobj_find_fence may return a NULL
+                        * fence. To make sure the recipient gets
+                        * signalled, use a new fence instead.
+                        */
+                       if (!*fence)
+                               *fence = dma_fence_get_stub();
+
                        goto out;
+               }
                dma_fence_put(*fence);
        } else {
                ret = -EINVAL;