[FFmpeg-devel] [PATCH] libavdevice: JACK demuxer

[Michael Niedermayer]

On Wed, Mar 04, 2009 at 02:33:47PM +0100, Olivier Guilyardi wrote:
[...]
> >> +
> >> +    timeout.tv_sec = av_gettime() / 1000000 + 2;
> >> +    if (sem_timedwait(&self->packet_signal, &timeout)) {
> >> +        if (errno == ETIMEDOUT) {
> >> +            av_log(context, AV_LOG_ERROR,
> >> +                   "Input error: timed out when waiting for JACK process callback output\n");
> >> +        } else {
> >> +            av_log(context, AV_LOG_ERROR, "Error while waiting for audio packet: %s\n",
> >> +                   strerror(errno));
> >> +        }
> >> +        if (!self->client) {
> >> +            av_log(context, AV_LOG_ERROR, "Input error: JACK server is gone\n");
> >> +        }
> >> +        return AVERROR(EIO);
> >> +    }
> >> +
> >> +    if (self->overrun) {
> >> +        av_log(context, AV_LOG_WARNING, "Packet ringbuffer overrun\n");
> >> +        self->overrun = 0;
> >> +    }
> >> +
> >> +    if (self->xrun) {
> >> +        av_log(context, AV_LOG_WARNING, "JACK xrun\n");
> >> +        self->xrun = 0;
> >> +    }
> >> +
> > 
> >> +    self->reader_index = (self->reader_index + 1) % RING_NBUFFERS;
> >> +
> > 
> >> +    memcpy(pkt, self->ringbuffer + self->reader_index, sizeof(*pkt));
> > 
> > *pkt= self->ringbuffer[self->reader_index];
> > 
> > also it seems 1 entry in the ring buffer cant be used with your
> > implementation
> > 
> > maybe the following is better:
> > process_callback(){
> >     if((uint8_t)(writer_index - reader_index) < RING_SIZE){
> >         write at buffer[writer_index & (RING_SIZE-1)];
> >         writer_index++;
> >     }
> > }
> > 
> > read_packet(){
> >     if(writer_index == reader_index)
> >         return AVERROR(EAGAIN);
> > 
> >     take packet from buffer[reader_index & (RING_SIZE-1)];
> >     buffer[reader_index & (RING_SIZE-1)]= av_new_packet();
> >     reader_index++;
> > }
> > 
> > note, both writer_index and reader_index must be unsigned for above
> > to work, and probably its best if they are uint8_t to exclude any issues
> > with the writes to them not being done in one piece.
> 
> Okay, I've completely changed this. My experience with lock free ringbuffers
> shows that it's hard to tell what is safe and what is not from a theoretical
> point of view (and there are many such point of views...).
> 
> As of JACK 0.116, the jack ringbuffer has been thoroughly unit tested and fixed
> (see the dozens of posts on LAD, LAU and jack-devel), on single/multi cpu, and
> especially on weakly memory ordered ones (such as PowerPC SMP).
> 
> So I found a way to avoid superfluous copy operations, using 2 jack ringbuffers.
> The idea is simple: audio_read_packet() sends newly allocated empty packets
> through one ringbuffer. Then process_callback() retrieve one, directly copy and
> interleave data into it, and send it back through another ringbuffer.

Iam sure this works but it is certainly not the simplest solution, that is i
belive my suggestion needs less code also mine needs just one buffer not 2


[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Complexity theory is the science of finding the exact solution to an
approximation. Benchmarking OTOH is finding an approximation of the exact
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