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SubjectRe: [RFC, PATCH] USB: EHCI: Allow users to override 80% max periodic bandwidth
On Wed, Jun 22, 2011 at 10:35:44AM -0700, matt mooney wrote:
> On 10:30 Wed 22 Jun , matt mooney wrote:
> > On 20:02 Wed 22 Jun , Kirill Smelkov wrote:
> > > There are cases, when 80% max isochronous bandwidth is too limiting.
> > >
> > > For example I have two USB video capture cards which stream uncompressed
> > > video, and to stream full NTSC + PAL videos we'd need
> > >
> > > NTSC 640x480 YUV422 @30fps ~17.6 MB/s
> > > PAL 720x576 YUV422 @25fps ~19.7 MB/s
> > >
> > > isoc bandwidth.
> > >
> > > Now, due to limited alt settings in capture devices NTSC one ends up
> > > streaming with max_pkt_size=2688 and PAL with max_pkt_size=2892, both
> > > with interval=1. In terms of microframe time allocation this gives
> > >
> > > NTSC ~53us
> > > PAL ~57us
> > >
> > > and together
> > >
> > > ~110us > 100us == 80% of 125us uframe time.
> > >
> > > So those two devices can't work together simultaneously because the'd
> > > over allocate isochronous bandwidth.
> > >
> > > 80% seemed a bit arbitrary to me, and I've tried to raise it to 90% and
> > > both devices started to work together, so I though sometimes it would be
> > > a good idea for users to override hardcoded default of max 80% isoc
> > > bandwidth.
> >
> > There is nothing arbitrary about 80%. The USB 2.0 Specification constrains
> > periodic transfers for high-speed endpoints to 80% of the microframe. See
> > section 5.6.4.
> >
>
> Looking at the patch, I see that you probably already knew that.
>
> So I digress and defer to the USB experts ;)

Yes, it was meant as 80% being arbitrary chosen by USB 2.0
specification. Notes taken - I'll clarify patch description.


Thanks for commenting,
Kirill


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