On Mon, 11 Nov 2013, David Laight wrote: > > Suppose, for example, the MBP is 1024. If you have a TD with length > > 1500, and if it had only one fragment, the last (and only) fragment's > > length would not less than the MBP and it would not be an exact > > multiple of the MBP. > > That doesn't matter - eg example 2 in figure 25 You're right. I do wish the spec had been written more clearly. > Reading it all again makes me think that a LINK trb is only > allowed on the burst boundary (which might be 16k bytes). > The only real way to implement that is to ensure that TD never > contain LINK TRB. That's one way to do it. Or you could allow a Link TRB at an intermediate MBP boundary. It comes down to a question of how often you want the controller to issue an interrupt. If a ring segment is 4 KB (one page), then it can hold 256 TRBs. With scatter-gather transfers, each SG element typically refers to something like a 2-page buffer (depends on how fragmented the memory is). Therefore a ring segment will describe somewhere around 512 pages of data, i.e., something like 2 MB. Since SuperSpeed is 500 MB/s, you'd end up getting in the vicinity of 250 interrupts every second just because of ring segment crossings. Using larger ring segments would help. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
