RE: [Xen-devel] pvusb performance

["James Harper" <james.harper@xxxxxxxxxxxxxxxx>]

> 
> > Okay maybe I'm a bit confused then.
> >
> > When Windows gave me a URB with 65536 bytes of data, I set
buffer_length
> > to 65536 and filled up the SG registers accordingly, but usbback
gave me
> > an error. When I break it into 512 byte (packet size) chunks it
works
> > okay. I just assumed that you have to give usbback requests with
less
> > that 'max packet size' of data.
> >
> > Maybe the cause of my error was somewhere else? I'll try it again.
> 
> If you transfer 65536 bytes of data, you should change the
> USBIF_MAX_SEGMENTS_PER_REQUESTS value to 16 or more.
> It is defined in include/xen/interface/io/usbif.h
> You need to recompile the drivers.
> 
> Request size and MAX_RING_SIZE are trade-off.
> I defined this value empirically.
> If the value is about up to 20, it doesn't affect the performance
> of memory stick.
> 

D'oh! I feel like a complete fool :)

Yes, 10 * 4096 is much less than 65536 - that will explain it!

I don't think I can tell windows to send me less data, which might be a
bit of a pain.

Still, I can still get it working by breaking the request into 40960
byte chunks.

Thanks for clearing that up for me. Arithmetic has never been my strong
point :)

James


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