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

[Benoît Thébaudeau]

On Monday 23 July 2012 15:35:25 Stefan Herbrechtsmeier wrote:
> Am 20.07.2012 17:35, schrieb Benoît Thébaudeau:
> > On Friday 20 July 2012 17:15:13 Stefan Herbrechtsmeier wrote:
> >> Am 20.07.2012 17:03, schrieb 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.
> >> 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.
> Sorry I mean 4 full pages of unaligned data.
> >
> > For page-aligned buffers, the 5 qt_buffers result in 5 full pages
> > of aligned
> > data.
> Sure.
> >
> > 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.
> In my use case (fragmented file on usb storage)  the gain would be
> nearly 20%. The reason is that the data are block aligned (512) and
> could be aligned to 4096 with the first transfer (5 qt_buffers).

Can you explain where this gain would come from? In both cases, the data in USB
transfers would be organized in the same way, and it would be accessed in memory
also in the same way (regarding bursts). The only difference would be the fetch
time of a little bit more qTDs, which is extremely fast and insignificant
compared to the transfer time of the payload, which remains unchanged.

Moreover, in your use case, if you are e.g. using FAT, on the one hand, the
buffers in fat.c are never aligned to more than the DMA min alignment, and on
the other hand, if you can align your user buffers to 512 bytes, you can also
align them directly to 4 kB.

> My suggestion would be to truncate the xfr_bytes with the max
> wMaxPacketSize (1024) and for the qtd_count use:
> 
> if ((uint32_t)buffer & 1023)    /* wMaxPacketSize unaligned */
>      qtd_count += DIV_ROUND_UP(((uint32_t)buffer & 4095) +
>              length, (QT_BUFFER_CNT - 1) * 4096);
> else                /* wMaxPacketSize aligned */
>      qtd_count += DIV_ROUND_UP(((uint32_t)buffer & 4095) +
>              length, QT_BUFFER_CNT * 4096);
> 
> This allows 50% of unaligned block data (512) to be transferred with
> min
> qTDs.

That would also require a realignment-to-page stage. This is specific code for
specific buffer alignment from the upper layers. We could also skip the
realignment to page and always keep the same qTD transfer size except for the
last one, by adding as many packets as possible for the buffer alignment.

But I still don't see a significant reason to complicate code to do that.

BTW, the 15x speed gain that I gave in my patch description was compared to an
older version of the original code that used 20 blocks per transfer in
usb_storage.c. This is now 40 blocks per transfer with a page-aligned buffer, so
the speed gain compared to the current code should be rather about 7x. I should
update that.

Best regards,
Benoît