On Tue, 10 Oct 2006, Christopher "Monty" Montgomery wrote: > On 10/10/06, Alan Stern <[EMAIL PROTECTED]> wrote: > > You're talking about detecting URB completion before a completion IRQ > > arrives. In other words, polling. I would prefer not to see that added > > to the HCDs, but it is a possibility. > > Ah, EHCI (or USB in general) specifies a completion interrupt happens > only at the end of a frame? I was suggesting the interrupt be thrown > as soon as the URB completes, even if that's mid-frame. > > If that's not possible, than I guess it would mean polling, yes. I'm > not enthusiastic about that either.
Both UHCI and OHCI issue IRQs only at frame boundaries. I believe EHCI can issue IRQs at arbitrary uframe boundaries, but I'd have to check the spec. However I think ehci-hcd generally sets up the controller to issue IRQs only at frame boundaries. Dave knows more about that. > > > I will point out that low latency applications generally consider the > > > machine to be an embedded appliance. They own the box. This is > > > accepted. > > > > If they own the box then they also own the OS. > > Yes, at least, they have relatively uncontested priority to whatever > resources they desire. But it's not possible for them to have _fully_ uncontested priority to all resources. Hence some kernel latency is unavoidable. > > Make up your mind. Do you want "as good as other systems that have been > > around a while" or do you want "a reliable 5ms"? The two are not > > obviously compatible. > > I want a reliable 2ms, but it sounds like I'm going to have to take > what I can bloody well get. You certainly won't get far trying to take what you can't get! > You and David have suggested (not stated > explicitly) that I'm probably going to have to be content with 5ms. We haven't stated it explicitly because we don't know what the limits are. At least, not with EHCI. With UHCI or OHCI I think it's clear that 5 ms is the best you can do with a reasonable amount of reliability, although you could push it to 4 ms with occasional dropouts (I don't know _how_ occasional). Alan Stern ------------------------------------------------------------------------- Take Surveys. Earn Cash. Influence the Future of IT Join SourceForge.net's Techsay panel and you'll get the chance to share your opinions on IT & business topics through brief surveys -- and earn cash http://www.techsay.com/default.php?page=join.php&p=sourceforge&CID=DEVDEV _______________________________________________ linux-usb-devel@lists.sourceforge.net To unsubscribe, use the last form field at: https://lists.sourceforge.net/lists/listinfo/linux-usb-devel
