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).
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.
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