Hi,
I'm currently building a DIY High speed usb audio device and have come
across some stange behaviour. When an audio stream starts (play), 12 urbs
get sent (very close together) to the main isosynchronous end point, 4 urbs
get sent to the sync endpoint. This is causing a buffer overrun on my DIY
usb audio device. Using wireshark I've captured the traffic against my DIY
device and also against an E-MU 0404 USB ( which handles the condition fine
) and the linux side behaves the same regardless of which device is used.
I enabled debugging on the usb audio module ( Kernel 3.16.7 ) and get the
following output (E-MU 0404):
---cut---
[ 227.363434] [2474] usb 9-1.1.4: Setting params for ep #1 (type 0, 12
urbs), ret=0
[ 227.363440] [2474] usb 9-1.1.4: Setting params for ep #81 (type 1, 4
urbs), ret=0
---cut---
As you can see 12 urbs are allocated for endpoint 1, and 4 urbs for the
sync (81). The wireshark output looks like this, after which it calms down
and sends urbs at the correct interval.
No. Time Source Destination Protocol
Length Info
13 15.783563000 host 6.0 USB
64 SET INTERFACE Request
14 15.783971000 6.0 host USB
64 SET INTERFACE Response
15 15.784085000 host 6.0 USB
65 URB_CONTROL out
16 15.784603000 6.0 host USB
64 URB_CONTROL out
17 15.784707000 host 6.1 USB
344 URB_ISOCHRONOUS out
18 15.784720000 host 6.1 USB
344 URB_ISOCHRONOUS out
19 15.784725000 host 6.1 USB
344 URB_ISOCHRONOUS out
20 15.784729000 host 6.1 USB
344 URB_ISOCHRONOUS out
21 15.784735000 host 6.1 USB
344 URB_ISOCHRONOUS out
22 15.784740000 host 6.1 USB
344 URB_ISOCHRONOUS out
23 15.784744000 host 6.1 USB
344 URB_ISOCHRONOUS out
24 15.784749000 host 6.1 USB
344 URB_ISOCHRONOUS out
25 15.784754000 host 6.1 USB
344 URB_ISOCHRONOUS out
26 15.784758000 host 6.1 USB
344 URB_ISOCHRONOUS out
27 15.784763000 host 6.1 USB
350 URB_ISOCHRONOUS out
28 15.784767000 host 6.1 USB
344 URB_ISOCHRONOUS out
29 15.784775000 host 6.1 USB
80 URB_ISOCHRONOUS in
30 15.784780000 host 6.1 USB
80 URB_ISOCHRONOUS in
31 15.784785000 host 6.1 USB
80 URB_ISOCHRONOUS in
32 15.784789000 host 6.1 USB
80 URB_ISOCHRONOUS in
The payload on these urbs are all zero's. It seems to match up with the
buffers allocated (setting params debug output above). You'll also notice
that they are *very* close together. At this point it's not honouring the
correct interval, but is honouring the correct packet size. It's almost
like a buffer flush of some sort.
So my question is:
1. Why does the driver do this?
2. Is there anything I can adjust in the USB descriptors to convince the
driver NOT to do this with my DIY device. I've done a quick look through
the driver code and I can't easily find where the logic is for where these
12 + 4 urbs are queued out.. As mentioned, it's only when a stream is
initially started, such as right after a cold OS boot etc...
Thanks,
Phil.
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