On Wed, 28 Apr 2010, Sarah Sharp wrote: > I've been looking at how EHCI handles errors on split transactions. The > case I'm interested in is when the host has successfully completed the Start > Split and is attempting a Complete Split transfer. If an error occurs, > like a babble or a stall, what does the EHCI driver do with the Complete > Split transfer? > > The hardware guys have told me there's two ways software can recover > from a stall on the Complete Split transfer: > > - a "hard retry" where software flushes the TT pipe, resets the > endpoint, sets the splitXState in the QH to â??startâ?? and then sets the > active bit in the QH to make the host controller retry the start split. I don't see any point in doing this. In general, a STALL response to a Complete Split means that the FS/LS device sent a STALL. So why try to do the same transfer over again? > - a "soft retry" where SW leaves the completion sitting in the TT, > leaves splitXstate in the QH at â??completeâ?? and then sets the active bit > in the QH to make the host controller retry the complete split. By the same reasoning, there's no point doing this either. > Which does EHCI do? When you say "EHCI", do you really mean "ehci-hcd"? EHCI (that is, the controller hardware) doesn't do anything to recover from STALLs. ehci-hcd doesn't follow either of these courses. It simply completes the URB with a -EPIPE error. > I looked at qh_completions() in ehci-q.c, and it Note that qh_completions handles Control, Bulk, and Interrupt transfers but not Isochronous transfers. > seems like it always clears the TT (except in the case of a stall, which This happens only for Control and Bulk transfers, not for Interrupt. > was an interesting thread to read about). So am I correct in thinking > the EHCI driver always does a "hard retry"? Nope. It doesn't retry at all. 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
