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. For page-aligned buffers, the 5 qt_buffers result in 5 full pages of aligned data. 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. Best regards, Benoît _______________________________________________ U-Boot mailing list U-Boot@lists.denx.de http://lists.denx.de/mailman/listinfo/u-boot
