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.
Regards, Stefan