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

Benoît Thébaudeau
2012-07-20T15:35:26Z

On Friday 20 July 2012 17:15:13 Stefan Herbrechtsmeier wrote:
> Am 20.07.2012 17:03, schrieb Beno?t Th?baudeau:
> > On Friday 20 July 2012 16:51:33 Stefan Herbrechtsmeier wrote:
> >> Am 20.07.2012 15:56, schrieb Beno?t Th?baudeau:
> >>> 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.
> >> 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.
> But thereby you limit the transfer to 4 qT buffers for unaligned
> transfers.

Not exactly. The 5 qt_buffers are used for page-unaligned buffers, but that
results in only 4 full pages of unaligned data, requiring 5 aligned pages.

For page-aligned buffers, the 5 qt_buffers result in 5 full pages of aligned
data.

The unaligned case could be a little bit improved to always use as many packets
as possible per qTD, but that would over-complicate things for a very negligible
speed and memory gain.

Best regards,
Beno?t