On Sat, 1 May 2010, Matthew Wilcox wrote: > > You're right that the performance hit will be less with UAS than with > > BOT, but it might still be larger than one would like. It all depends > > on how the speed of the device compares to the speed of the USB bus. > > If the bus speed is the bottleneck then adding extra transfers will > > slow things down, otherwise it won't (or at least, not as much). > > Yup. It's probably also more overhead on the host side. As we've > discussed before, most UAS devices probably still do BOT on a different > interface configuration, and I'm quite happy for usb-storage to outperform > uas in these circumstances. So whatever scheme we use to select a driver, it should default to UAS if the HCD has SG support and to BOT otherwise? (Boy, I hardly ever get to use so many acronyms in a single sentence!) > > So now that we're both on the same page... What improved support do > > you mean? > > Only what I've done so far -- making it possible for a driver to directly > populate urb->sg and send it down without using the usb_sg_wait() > interface. If the scatterlist is allowed to contain more than one element, this would be tantamount to enhancing usb_sg_* to work asynchronously and folding it into usb_submit_urb(). If the scatterlist contains only one element then all that's needed is set urb->num_sgs to 1, as your UAS code does. > The patch you did to EHCI (as1300 / commit 40f8db8f8f) does look quite > straightforward to port to the uhci-q.c and ohci-q.c file. On the one > hand, that's a good thing, because I can just do that (er, if I can figure > out a way to test either of them). Testing is easy -- just see if your mass storage devices still work. usb-storage calls usb_sg_*, which will use the HCD's native SG support. > On the other hand, it's not taking > advantage of EHCI's native ability to do scatter-gather in a single qtd if > the pages are virtually contiguous (see pages 86-88 of the EHCI 1.0 spec). > > So EHCI could be further enhanced ... if it's worth it. Hmm, maybe... If it's not too hard to do. I'll have to look at the code. 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
