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

Benoît Thébaudeau
2012-07-20T13:56:18Z

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 Beno?t Th?baudeau:
> > > > + 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.

Best regards,
Beno?t