On Mon, 15 Jul 2013, James Stone wrote:
> >> James, did you ever provide a usbmon trace for 3.8 doing playback only
> >> and using 64 frames/period? I don't recall seeing any. It might help.
> >>
> >
> > OK - will send it to you off-list.
I got it. It explains a lot.
The audio-out stream uses a pipeline of only 2 URBs. The URBs start
out alternating between 8 and 7 packets apiece. This yields a total
latency around 1.9 ms (equivalent to 2 periods of about 41 frames at
44.1 KHz), which is smaller than I would expect (2 periods of 64 frames
works out to 2.9 ms).
Anyway, this sort of works. Every so often (roughly at intervals of 15
ms) there is an underrun. Evidently the computer's EHCI hardware
sometimes requires URBs to be submitted more than 7 microframes in
advance. This means that the minimum safe pipeline length is 2 ms, and
a little more would be better. Indeed, there was one example in the
usbmon trace with an IRQ latency over 65 us, which is half a
microframe. (Alternatively, a pipeline containing 3 URBs with 5
packets each should be okay, although it would cause increased
interrupt overhead.)
After a second of this, the URBs suddenly changed. They began to
alternate between 7 and 5 packets; one of them had only 4. I have no
idea why this happened. It led to a situation where ehci-hcd refused
to accept an URB submission because it wasn't far enough in advance,
and at that point everything came to a halt.
Try the patch below; it should eliminate the "everything stops"
problem. But it will not do anything about the ongoing underruns. To
fix that, the latency has to be increased. Or rather, the latency
between the ALSA and USB stacks needs to increase to match the
latency expected by JACK. A pipeline of 2.9 ms (23 packets total)
ought to be okay, whereas a pipeline of 15 packets -- what you're
getting now -- is too short.
And if the pipeline was long enough, the patch below wouldn't be
needed.
Alan Stern
Index: usb-3.10/drivers/usb/host/ehci-sched.c
===================================================================
--- usb-3.10.orig/drivers/usb/host/ehci-sched.c
+++ usb-3.10/drivers/usb/host/ehci-sched.c
@@ -1441,14 +1441,13 @@ iso_stream_schedule (
/*
* Not ASAP: Use the next slot in the stream. If
- * the entire URB falls before the threshold, fail.
+ * the URB is too late, some of the packets won't
+ * be handled by the hardware.
*/
else if (start + span - period < next) {
ehci_dbg(ehci, "iso urb late %p (%u+%u < %u)\n",
urb, start + base,
span - period, next + base);
- status = -EXDEV;
- goto fail;
}
}
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