On Wed, Nov 11, 2009 at 4:57 PM, Grant Likely <grant.likely@xxxxxxxxxxxx> wrote: > On Wed, Nov 11, 2009 at 2:34 PM, Jon Smirl <jonsmirl@xxxxxxxxx> wrote: >> On Wed, Nov 11, 2009 at 2:24 PM, Grant Likely <grant.likely@xxxxxxxxxxxx> wrote: >>> On Wed, Nov 11, 2009 at 11:37 AM, Mark Brown >>> <broonie@xxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote: >>>> On Wed, Nov 11, 2009 at 11:38:06AM -0500, Jon Smirl wrote: >>>>> > Providing a final valid data point to the driver would possibly even >>>>> > make things worse since if it were used then you'd have the equivalent >>>>> > race where the application has initialized some data but not yet managed >>>>> > to update the driver to tell it it's being handed over; if the driver >>>> >>>>> That's an under run condition. >>>> >>>> Yes, of course - the issue is that this approach encourages them, making >>>> the system less robust if things are on the edge. The mpc5200 seems to >>>> be not just on the edge but comfortably beyond it for some reason. >>> >>> I can't reproduce the issue at all as long at the dev_dbg() statement >>> in the trigger stop path is disabled. With it enabled, I hear the >>> problem every time. The 5200 may not be a speedy beast, but it is >>> plenty fast enough to shut down the audio stream before stale data >>> starts getting played out. >> >> "fast enough" - you just said it is a race. >> I've been saying it is a race too. > > Yes, it is a race; but not the kind that is dangerous. Audio playout > is always a real-time problem; whether in the middle of a stream or at > the end. If the CPU gets nailed with an unbounded latency, then there > will be audible artifacts - Regardless of whether the driver knows > where the end of data is or not. If it does know, then audio will > stutter. If it doesn't know, then there will be repeated samples. > Both are nasty to the human ear. So, making the driver do extra work > to keep the extra data in sync will probably force larger minimum > latencies for playout (trouble for VoIP apps) so the CPU can keep up, > and won't help one iota for making audio better. I don't think it is that much more work for ALSA to provide an accessible field indicating the end of valid data. It's already tracking appl_ptr. Appl_ptr just needs to be translated into a physical DMA buffer address and we've been making mistakes doing that translation. > > The real solution is to fix the worst case latencies. > >> There are two options: >> 1) Eliminate the race by developing a system to deterministically flag >> the end of valid data. >> 2) Fudge everything around making it almost impossible to lose the >> race, but the race is still there. > > 3) eliminate the unbounded latencies (fix the PSC driver and/or use a > real time kernel) > 4) make sure userspace fills all the periods with silence before > triggering stop. Gstreamer seems to already do this. I suspect > pulseaudio does the same. > >> The dev_dbg() aggravates the race until it is obviously visible every >> time. A deterministic solution would not be impacted by the dev_dbg(). > > But it still wouldn't help a bit when the same latency occurs in the > middle of playback. The deterministic solution of tracking the end of valid data ensures that under run will be silent instead of playing invalid data. > > g. > > -- > Grant Likely, B.Sc., P.Eng. > Secret Lab Technologies Ltd. > -- Jon Smirl jonsmirl@xxxxxxxxx _______________________________________________ Alsa-devel mailing list Alsa-devel@xxxxxxxxxxxxxxxx http://mailman.alsa-project.org/mailman/listinfo/alsa-devel
