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 _______________________________________________ U-Boot mailing list U-Boot@lists.denx.de http://lists.denx.de/mailman/listinfo/u-boot
