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SubjectRe: [RFC, PATCH] USB: EHCI: Allow users to override 80% max periodic bandwidth
On Thu, 23 Jun 2011, Kirill Smelkov wrote:

> > At 480 Mb/s, each microframe holds 7500 bytes (less if you count
> > bit-stuffing). 4% of that is 300 bytes, which is not enough for a
> > 512-byte bulk packet. I think you'd run into trouble trying to do any
> > serious bulk transfers on such a tight schedule.
>
> Yes, you seem to be right.
>
> I still think 4% is maybe enough for control traffic.

It should be.

> > > @@ -571,6 +579,14 @@ static int ehci_init(struct usb_hcd *hcd)
> > > hcc_params = ehci_readl(ehci, &ehci->caps->hcc_params);
> > >
> > > /*
> > > + * tell user, if using non-standard (80% == 100 usec/uframe) bandwidth
> > > + */
> > > + if (uframe_periodic_max != 100)
> > > + ehci_info(ehci, "using non-standard max periodic bandwith "
> > > + "(%u%% == %u usec/uframe)",
> > > + 100*uframe_periodic_max/125, uframe_periodic_max);
> > > +
> > > + /*
> >
> > Check for invalid values. This should never be less than 100 or
> > greater than 125.
>
> Ok. By the way, why should we limit it to be not less than 100?
> Likewise, a user who knows exactly what he/she is doing could limit
> periodic bandwidth to be less than 80% required by USB specification.

What's the point? If you want to use less than 80% of your bandwidth
for periodic transfers, go ahead and do so. You don't need to change
the limit.

Alan Stern



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