Le jeudi 22 novembre 2018 à 17:31 +0900, Alexandre Courbot a écrit : > On Fri, Nov 16, 2018 at 1:49 AM Nicolas Dufresne <nicolas@xxxxxxxxxxxx> wrote: > > Le mercredi 14 novembre 2018 à 13:12 +0900, Alexandre Courbot a écrit : > > > On Wed, Nov 14, 2018 at 3:54 AM Nicolas Dufresne <nicolas@xxxxxxxxxxxx> wrote: > > > > > > > > Le mar. 13 nov. 2018 04 h 30, Alexandre Courbot <acourbot@xxxxxxxxxxxx> a écrit : > > > > > The last buffer is often signaled by an empty buffer with the > > > > > V4L2_BUF_FLAG_LAST flag set. Such buffers were returned with the > > > > > bytesused field set to the full size of the OPB, which leads > > > > > user-space to believe that the buffer actually contains useful data. Fix > > > > > this by passing the number of bytes reported used by the firmware. > > > > > > > > That means the driver does not know on time which one is last. Why not just returned EPIPE to userspace on DQBUF and ovoid this useless roundtrip ? > > > > > > Sorry, I don't understand what you mean. EPIPE is supposed to be > > > returned after a buffer with V4L2_BUF_FLAG_LAST is made available for > > > dequeue. This patch amends the code that prepares this LAST-flagged > > > buffer. How could we avoid a roundtrip in this case? > > > > Maybe it has changed, but when this was introduced, we found that some > > firmware (Exynos MFC) could not know which one is last. Instead, it > > gets an event saying there will be no more buffers. > > > > Sending buffers with payload size to 0 just for the sake of setting the > > V4L2_BUF_FLAG_LAST was considered a waste. Specially that after that, > > every polls should return EPIPE. So in the end, we decided the it > > should just unblock the userspace and return EPIPE. > > > > If you look at the related GStreamer code, it completely ignores the > > LAST flag. With fake buffer of size 0, userspace will endup dequeuing > > and throwing away. This is not useful to the process of terminating the > > decoding. To me, this LAST flag is not useful in this context. > > Note that this patch does not interfere with DQBUF returning -EPIPE > after the last buffer has been dequeued. It just fixes an invalid size > that was returned for the last buffer. > > Note also that if I understand the doc properly, the kernel driver > *must* set the V4L2_BUF_FLAG_LAST on the last buffer. With Venus the > last buffer is signaled by the firmware with an empty buffer. That's > not something we can change or predict earlier, so in order to respect > the specification we need to return that empty buffer. After that > DQBUF will behave as expected (returning -EPIPE), so GStreamer should > be happy as well. > > Without the proposed fix however, GStreamer would receive the last > buffer with an incorrect size, and thus interpret random data as a > frame. > > So to me this fix seems to be both correct, and needed. Isn't it? Totally, thanks for the extra clarification. > > Cheers, > Alex.
