isochronous support broken?

Duncan Sands baldrick at
Mon Jan 23 07:36:46 EST 2006

Hi Alistair,

> usb 4-1.4: reset full speed USB device using ehci_hcd and address 4

a bug fix for ehci + iso urbs bandwidth allocation just got posted to the
usb mailing list.  So I suggest trying uhci instead (OK, I just saw that
you tried it).


> As you can see, the recv buffer has dropped but nothing has changed. Then I 
> had the idea to remove ehci-hcd, and force it to use uhci-hcd for the USB 1.1 
> device (I have no USB 2.x devices anyway). This was successful:
> I was able to bring pppd up over the interface. /proc/bus/usb/devices looks 
> like this; only 79% utilisation.

What do you mean by "79% utilisation"?
> T:  Bus=01 Lev=00 Prnt=00 Port=00 Cnt=00 Dev#=  1 Spd=12  MxCh= 2
> B:  Alloc=710/900 us (79%), #Int=  2, #Iso=  4
> D:  Ver= 1.10 Cls=09(hub  ) Sub=00 Prot=00 MxPS=64 #Cfgs=  1
> P:  Vendor=0000 ProdID=0000 Rev= 2.06
> S:  Manufacturer=Linux uhci_hcd
> S:  Product=UHCI Host Controller
> S:  SerialNumber=0000:00:10.0
> C:* #Ifs= 1 Cfg#= 1 Atr=e0 MxPwr=  0mA
> I:  If#= 0 Alt= 0 #EPs= 1 Cls=09(hub  ) Sub=00 Prot=00 Driver=hub
> E:  Ad=81(I) Atr=03(Int.) MxPS=   2 Ivl=255m
> My bandwidth is still locked to just under 500KB/s (4MBits). So either a) isoc 
> isn't working or b) you need to do more than just switch from bulk to isoc to 
> get 8Mbits from this device.

This is what I see.  Using bulk I max out at a bit under 4 megabaud.  With isoc
I max out at an even slower rate!

> The Windows workaround does not suggest this, but I suppose it is impossible to tell.

What is "the windows workaround"?  (I don't have windows, which makes comparing with
windows a bit awkward for me).

> If it's a useful datapoint, I used to get sporadic errors and the USB LED 
> would flash RED if the bandwidth was held close to 4Mbit for a long period of 
> time. This no longer seems to happen with isoc, suggesting the bandwidth 
> bottleneck has been lifted. However, I do not see the 6.6 megabits I've 
> synced at.

Are you talking about linux or windows here?  Can you get 6.6 mb with windows?

Anyway, I think it likely that there is something wrong with the driver.  But it
could be a hardware limitation.  Testing with windows should clear that up.  If
you can do some experiments with windows, that would be great.



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