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My configuration:
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Host: Freescale i.MX512 with ARM Cortex A8 (USB 2.0 host controller) Linu=

x

kernel: 2.6.31, using EHCI USB driver
Hub: 4-PORT USB 1.1 HUB (Texas Instruments PN: tusb2046b)
Devices: 4 USB 1.1 audio codecs (Texas Instruments PN: pcm2901)
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Note: each codec is being used in R/W access, so with 4 codecs, I have
4 playback and 4 capture streams.
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My problem:
-----------
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I have usb urb leaks when connecting more than 1 codec to the USB 1.1 Hub=

.

(the result is that some of the audio data is not transferred, part of th=

e sound is

simply missing) No problem when using only 1 of the 4 codecs connected to=

 the

hub; When I connect a second codec, the sound quality starts to degrade. =

With

3 codecs, we just cannot recognize a speach.
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Tests and observations:
-----------------------
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Since I have 3 usb ports available on the i.MX512, I tried to connect
3 codecs directly on USB ports: the sound is perfect on each of the three=

 ports.

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I bought a consumer USB 2.0 Hub: no problem when using 3 codecs connected
to that Hub, however, the audio will completly stop on all channels when
connecting the 4th codec.
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I checked the communication between the Hub (USB 1.1) and the Host
controller (USB 2.0) with a scope and concluded that the communication sp=

eed

is 1.5 MBytes/s has expected (so the communication is downgraded to USB 1=

.1,

since codecs and hub are USB
1.1 devices).
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Also, I know that there is physically enough bandwidth to transfer the da=

ta for

two reasons:
1) I have an older CPU with a USB 1.1 host controller (using the OHCI dri=

ver),

using the same hub and the same codecs: works like a champ, using less th=

an

50% of the available bandwidth (observed with a
scope)
2) 1 audio stream is 32khz-mono, 16 bits =3D 64 kB/s,
4 codecs =3D 8 streams(R/W) x 64 kB/s =3D 512 kB/s (out of 1.5MB/s)
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I noticed that my sound problem starts happening with only 2 codecs
(4 streams, 256 kB/s). I first thought that it was a bandwidth limitation=

, so I

decided to connect only 1 codec using more bandwidth.
I configured it to 48khz-stereo (16-bits), using 384 kB/s for both read a=

nd write

streams: no problem. With that configuration, the scope shows about 30% o=

f

total bandwidth usage (300us used out of 1ms periods). Then, I added a se=

cond

codec (48khz-stereo-16bits): very strange, now the total bandwidth usage =

felt

down to about 200us, which seems to keep the same, whatever the number of
codec I add (I also tried 3 and 4...). So it looks like the scheduler is =

not able to

properly allocate Isochronous time slots when more than one device is
connected to the hub. However, without the hub, it works perfectly.
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I am wonder if it is similar problem I met when using multiple interrupt tr=
ansfers,
when you find you lose the data, try to run 'top' to show cpu utilization, =
if it is
close to 100%, it means the ehci can't queue request in time, so the host c=
an't send IN
token in time.

Using a USB bus analyzer can also verify it.

Peter

Another interresting fact is that at application level, the Read and Writ=

e

operations are returning the good amount of bytes read/written.
This is not the case at kernel level: I noticed that function "usb_submit=

_urb"

(from /drivers/usb/core/urb.c) will only tranfer part of the "urbs" when =

the

sound is degraded. I tried to figure out where the leak comes from withou=

t

success. Also, there are no error messages from kernel so everything appe=

ars

to work well, excepted that part of the sound is missing!
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I can't change my hardware (this is in the hand of customers), so the onl=

y

possible solution for me is to correct the software.
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I tried to change my ehci driver with the one from kernel 2.6.39.4 but di=

d not

work, same problem.
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Question:
---------
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Before attempting to upgrade to an earlier kernel driver (this is a fairl=

y big

amount of work), I would really like to know if this problem would still =

be in the

3.x kernels. Has anyone seen that issue in 3.x kernels?
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I am pretty new to USB driver debugging, so any ideas of where/how to fin=

d

solutions will be appreciated. Thank you very much in advance for the sup=

port.

Also don't hesitate to redirect me if I'm not at the right place to ask t=

hese

questions. I can also provide some code if someone need it to help.
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Attached is a dump of my "dmesg" after startup.
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Michael Tessier
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