Le 20/05/2022 à 14:35, Måns Rullgård a écrit : > Christophe Leroy <christophe.le...@csgroup.eu> writes: > >> Le 19/05/2022 à 21:24, Mans Rullgard a écrit : >>> The dma_sync_single_for_cpu() call must precede reading the received >>> data. Fix this. >> >> See original commit 070e1f01827c. It explicitely says that the cache >> must be invalidate _AFTER_ the copy. >> >> The cache is initialy invalidated by dma_map_single(), so before the >> copy the cache is already clean. >> >> After the copy, data is in the cache. In order to allow re-use of the >> skb, it must be put back in the same condition as before, in extenso the >> cache must be invalidated in order to be in the same situation as after >> dma_map_single(). >> >> So I think your change is wrong. > > OK, looking at it more closely, the change is at least unnecessary since > there will be a cache invalidation between each use of the buffer either > way. Please disregard the patch. Sorry for the noise. >
I also looked deeper. Indeed it was implemented in kernel 4.9 or 4.8. At that time dma_unmap_single() was a no-op, it was not doing any sync/invalidation at all, invalidation was done only at mapping, so when we were reusing the skb it was necessary to clean the cache _AFTER_ the copy as if it was a new mapping. Today a sync is done at both map and unmap, so it doesn't really matter whether we do the invalidation before or after the copy when we re-use the skb. Christophe
