Re: USB low-speed bulk transfers

Andrew Morton
2007-06-07T00:52:51Z

On Wed, 6 Jun 2007 23:20:46 +0200 "Steinar H. Gunderson" <sgunderson@bigfoot.com> wrote:

> [Please Cc on reply]
>
> Hi,
>
> I recently bought an USB MIDI interface from ESI (called “ESI MIDI Mate”). It
> claims to work with Linux, but doesn't -- I've already asked the manufacturer
> for an explanation, but as I was impatient, I hacked a bit on the drivers to
> actually make it work...
>
> The /proc/bus/usb/devices entry looks like this:
>
> T: Bus=04 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#= 4 Spd=1.5 MxCh= 0
> D: Ver= 2.00 Cls=00(>ifc ) Sub=00 Prot=00 MxPS= 8 #Cfgs= 1
> P: Vendor=0a92 ProdID=1001 Rev= 1.04
> S: Manufacturer=ESI
> S: Product=ESI MIDI Mate
> C:* #Ifs= 2 Cfg#= 1 Atr=80 MxPwr= 20mA
> I:* If#= 0 Alt= 0 #EPs= 0 Cls=01(audio) Sub=01 Prot=00 Driver=snd-usb-audio
> I:* If#= 1 Alt= 0 #EPs= 2 Cls=01(audio) Sub=03 Prot=00 Driver=snd-usb-audio
> E: Ad=81(I) Atr=02(Bulk) MxPS= 4 Ivl=0ms
> E: Ad=01(O) Atr=02(Bulk) MxPS= 4 Ivl=0ms
>
> There are two points worth noting here:
>
> - The device is USB low speed. snd-usb-audio simply checks for full/high
> speed, and refuses any device that isn't. I can hack around this,
> inverting a few checks etc., and it seems to work reasonably well
> (probably since the device has no PCM parts).
> - Both endpoint descriptors are bulk. The HCD driver plain refuses bulk
> transfers for low-speed; it looks like they are disallowed in the USB
> standard somehow. If I comment out the check, the driver works
> (perfectly!), but I guess this isn't acceptable for upstream?
>
> Could the check for low-speed bulk transfers be replaced by a kernel warning
> somehow? I can't see any big harm by allowing them, and obviously, Windows XP
> and Mac OS X does so.
>
> I can supply a patch for the snd-usb-audio specific parts if desired, but I
> can't guarantee it's the correct fix for cards that support PCM. Not that I
> know of any PCM-capable low-speed USB sound cards out there...
>

(added linux-usb-devel to cc)