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Re: [Qemu-devel] Re: ehci update

From: Jan Kiszka
Subject: Re: [Qemu-devel] Re: ehci update
Date: Wed, 14 Apr 2010 21:38:40 +0200
User-agent: Mozilla/5.0 (X11; U; Linux i686 (x86_64); de; rv: Gecko/20080226 SUSE/ Thunderbird/ Mnenhy/

David S. Ahern wrote:
> On 04/13/2010 07:20 PM, Alexander Graf wrote:
>>>>> It still needs a lot of love, but definitely an improvement from the
>>>>> last version. The biggest difference for the performance boost and
>>>>> stability is discovering that the usbfs in linux limits transactions to
>>>>> 16k versus the EHCI spec which allows 20k per qTD. I added a hack to
>>>>> submit which detects 20k requests from a guest and breaks it up into 2
>>>>> requests through the host (a 16k and then a 4k).
>>>> Did someone already bring this up on LKML or wherever usbfs is
>>>> discussed? Should be fixable, I naively guess.
>>> I submitted the patch to linux-usb and it was nack'ed. The response was
>>> that memory is allocated in powers of 2 so trying to up the limit from
>>> 16k to 20k means it will actually want to find 32k of contiguous memory.
>>> The suggestion was to handle it with multiple requests within qemu. I
>>> guess libusb does that.
>> Any reason we're not using libusb?
> Good question. I was wondering the same. I was going to look at
> converting usb-linux to use libusb1 when I get some time.

Will that overcome the 16k limit or just push the split-up into libusb?


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