On Sat, May 01, 2010 at 10:02:56PM -0400, Alan Stern wrote: > > 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!) Well, yeah, that's not ideal :-) > > 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(). Yes, I suppose it would. > > 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. Easy ... if you have the hardware. I need to check my hubs to see if I have one that's USB 1.1 only, so I can force an EHCI controller into UHCI mode. I had to look fairly hard to find an OHCI controller in my machine collection (not too many non-Intel chipsets in my house :-) > > 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. I think the cleanest way to do it is to split qtd_fill into qtd_fill_buf and qtd_fill_sg; use the former for the usb_pipecontrol and one_more cases; and the latter in the main loop. qtd_fill_buf could be quite simple since it's never going to use multiple pages. qtd_fill_sg would be pretty complex though. I have no idea how much improvement we'd see from using multiple pages in a qtd instead of using one qtd per page though. -- Matthew Wilcox Intel Open Source Technology Centre "Bill, look, we understand that you're interested in selling us this operating system, but compare it to ours. We can't possibly take such a retrograde step." -- 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
