[U-Boot] [PATCH v2 2/5] ehci-hcd: Boost transfer speed

[Benoît Thébaudeau]

On Friday 20 July 2012 16:51:33 Stefan Herbrechtsmeier wrote:
> Am 20.07.2012 15:56, schrieb Benoît Thébaudeau:
> > Dear Marek Vasut,
> >
> > On Friday 20 July 2012 15:44:01 Marek Vasut wrote:
> >>> On Friday 20 July 2012 13:37:37 Stefan Herbrechtsmeier wrote:
> >>>> Am 20.07.2012 13:26, schrieb Benoît Thébaudeau:
> >>>>> +			int xfr_bytes = min(left_length,
> >>>>> +					    (QT_BUFFER_CNT * 4096 -
> >>>>> +					     ((uint32_t)buf_ptr & 4095)) &
> >>>>> +					    ~4095);
> >>>> Why you align the length to 4096?
> >>> It's to guarantee that each transfer length is a multiple of the
> >>> max packet
> >>> length. Otherwise, early short packets are issued, which breaks
> >>> the
> >>> transfer and results in time-out error messages.
> >> Early short packets ? What do you mean?
> > During a USB transfer, all packets must have a length of max packet
> > length for
> > the pipe/endpoint, except the final one that can be a short packet.
> > Without the
> > alignment I make for xfr_bytes, short packets can occur within a
> > transfer,
> > because the hardware starts a new packet for each new queued qTD it
> > handles.
> But if I am right, the max packet length is 512 for bulk and 1024 for
> Interrupt transfer.

There are indeed different max packet lengths for different transfer types, but
it does not matter since the chosen alignment guarantees a multiple of all these
possible max packet lengths.

Best regards,
Benoît