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.
Best regards, Benoît