Hi Alex, On 11/22/18 10:31 AM, Alexandre Courbot wrote: > 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 Small correction, the firmware signals EoS with HFI_BUFFERFLAG_EOS flag with the returned buffer, then we set V4L2_BUF_FLAG_LAST. Usually with v1 and v3 when HFI_BUFFERFLAG_EOS is set the bytesused is zero. > 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? > > Cheers, > Alex. > -- regards, Stan
