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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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