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Re: [Qemu-devel] usb descriptor parsing, why bother?
From: |
Gerd Hoffmann |
Subject: |
Re: [Qemu-devel] usb descriptor parsing, why bother? |
Date: |
Mon, 14 May 2012 09:30:47 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:10.0.4) Gecko/20120422 Thunderbird/10.0.4 |
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 <address@hidden>
> 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