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.
Best regards,
Stefan
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