[CC'ing an USB audio heavyweight - Daniel Mack] On Tue, Sep 4, 2012 at 12:39 AM, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote: > On Mon, 3 Sep 2012, Jassi Brar wrote: > >> On Fri, Aug 31, 2012 at 11:56 PM, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote: >> > Clemens and Laurent (and anyone else who's interested): >> > >> > How should the lower USB layers handle delays in transferring >> > isochronous data? I'm asking you because the most common usages of >> > isochronous transfers are for audio and video. >> > >> > Here's an example to illustrate what I mean. Typically an audio or >> > video driver will keep a queue of around 10 ms of data submitted to an >> > isochronous endpoint. I have seen reports from users where URB >> > completion interrupts were delayed by as much as 50 ms. In one case >> > the delay was caused by a bug in a wireless drivers that left >> > interrupts disabled; in another case the cause was unknown -- it might >> > have been a hardware problem. At any rate, when this happens the >> > endpoint's queue drains completely. >> > >> > Clearly this will cause a glitch in the data stream. The question is: >> > What should we do to recover and re-synchronize? >> > >> How about effectively increasing the queue length from 10ms to 50ms >> (max anticipated latency) ? > > There are two problems with that approach. First, 50 ms isn't really > the max anticipated latency; it's merely the largest that I've seen so > far. (In fact, the max anticipated latency is probably < 10 ms; these > 50-ms delays were definitely exceptional.) > It doesn't have to be hardcoded - maybe USB audio/video code could get the hint via some module parameter? > Second, people involved in real-time programming (such as audio or > video) generally want to keep latency to a minimum. > If we progress the h/w pointer of ALSA ring buffer at URB completion (and not at URB submission), this shouldn't affect the latency. And IIRC, USB isn't anyway recommended for real-time usage. cheers. -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
