On Mon, 18 May 2009, Steve Calfee wrote: > Hi Alan, > > It has been a really long time since I used the Arc core (around > 2003?), and I no longer have the ARC ehci document, which may have > been under NDA. > > However, It is my impression that the ARC/TD core was done the "right" > way, with respect to non-HS transfers. I think it just uses the ehci > dma transfer machine to send on FS/LS. So I think it is the case that > there is no transaction translator at all. All that split transfer > stuff was just to implement a new protocol between HS controllers and > HS hubs with FS/LS connections. Why would the ARC core do the TT hack > internally for known speed connections? Because if you don't buffer FS/LS transfers separately from HS transfers, you end up delaying the HS bus unnecessarily. Hence there has to be special buffering hardware for FS/LS transfers -- and once you've done that, you're 50% of the way toward implementing a TT. > I always thought that the uhci/ohci "companion" controller addition to > ehci was an intel hardware hack because they ran out of time when > releasing the earliest ehci hardware/documentation. Because of that > hack, Intel did not address the FS/LS directly connected device issue. > I really hope Sarah can push back on the Intel XHCI hardware guys and > prevent a similar hack for superspeed and lower speed devices. It may be too late for that; their hardware is already developed (though perhaps not finalized). However the overall design _is_ finalized. USB-3.0 actually contains two separate buses, one running at 1.5/12/480 Mbs and the other running at 4 Gbs. > Anyway, if you do have the doc, look at how the FS isoc schedule is > done for directly connected FS devices. If it is handled the same as > HS devices, there probably is no ARC/TD EHCI TT on the internal > controller. No, the Isoc schedule is handled the same as it is with regular EHCI spec. As far as the driver is concerned, very few changes have to be made to accomodate the built-in TT -- mainly connected with port handover, not changes to the schedulers. 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
