dma-buf: add a warning when drv try to reserve 0 fence slots

When dma_resv_reserve_fences() is called with num_fences=0 it usually
means that a driver or other component messed up its calculation how
many fences are needed. Warn in that situation.

When no fence are needed the function shouldn't be called in the first
place.

Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240529084322.2284-1-christian.koenig@amd.com
Reviewed-by: Matthew Auld <matthew.auld@intel.com>

drivers/dma-buf/dma-resv.c

@@ -186,6 +186,13 @@ int dma_resv_reserve_fences(struct dma_resv *obj, unsigned int num_fences)
 
 	dma_resv_assert_held(obj);
 
+	/* Driver and component code should never call this function with
+	 * num_fences=0. If they do it usually points to bugs when calculating
+	 * the number of needed fences dynamically.
+	 */
+	if (WARN_ON(!num_fences))
+		return -EINVAL;
+
 	old = dma_resv_fences_list(obj);
 	if (old && old->max_fences) {
 		if ((old->num_fences + num_fences) <= old->max_fences)