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Date: Mon, 14 May 2012 09:30:47 +0200
From: Gerd Hoffmann <kraxel@redhat.com>
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Subject: Re: [Qemu-devel] usb descriptor parsing, why bother?
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To: David Fries <david@fries.net>
Cc: qemu-devel@nongnu.org

On 05/12/12 03:32, David Fries wrote:
> I was bit in kvm-qemu (Debian qemu-kvm-1.0+dfsg-11) with the usb
> descriptor parsing code.  I was enhancing a driver in the guest and
> found that I could talk to usb alt 0, but not alt 3, I made a local
> fix and I see there is an upstream fix (listed below) in qemu.
> 
> commit 96dd9aac37d30f3425088f81523942e67b2d03ac
> Author: Gerd Hoffmann <kraxel@redhat.com>
> Date:   Thu Mar 29 16:06:28 2012 +0200
> usb-host: rewrite usb_linux_update_endp_table

Yea, that one should fix it ;)

> I'm curious why qemu/qemu-kvm even bothers?  As far as I could tell
> parsing the descriptor table is only used to deny the guest from
> submitting urbs on the wrong pipe.

No.  It uses the wMaxPacketSize to pick a useful buffer size for iso
endpoints.  In master / upcoming 1.1 usb-host enables pipelining for
bulk endpoints.

Beside that enabling the usb_host_parse_* tracepoints gives a useful
trace of guest activities (you should see your guest picking alt #3).

cheers,
  Gerd