On Fri, Nov 16, 2018 at 1:50 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. > It was never the case with the MFC (firmware/driver) we were using on Chrome OS and it doesn't seem to be the case for the current upstream s5p-mfc driver. > 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. Except that -EPIPE is actually signaled by the vb2 core and it happens after the user space dequeues a buffer with the LAST flag set: https://elixir.bootlin.com/linux/v4.20-rc2/source/drivers/media/common/videobuf2/videobuf2-core.c#L1634 https://elixir.bootlin.com/linux/v4.20-rc2/source/drivers/media/common/videobuf2/videobuf2-v4l2.c#L555 Best regards, Tomasz
