"> > " == Roland King "> " == Vojtech Pavlik
> > This is a little similar to the discussion we were having last > > week about how to do interrupt transfers on the usbdevfs > > interface. The answer seemed to end up being that you should > > just do bulk transfers. "Seemed" -- if you can arrange that nothing is going to ever be able to remember the endpoint being used in a different transfer mode. I've tripped over problems trying to use that approach. (No, I wasn't using UHCI.) > > What is an interrupt transfer really? Is it just a transfer which is > > sent again and again by the driver, ...? By the host controller driver. It's a "periodic" transfer, which _by design_ is going to be happening over and over and over again. > > How is that supposed to work with output transfers then? If you > > schedule an interrupt transfer and the driver continues to resubmit > > it, doesn't that just mean that, as the initial writer in this thread > > noted, you get the same data sent to the endpoint again and again? Yes, which is why an interrupt OUT transfer has to modify the buffer. And why there's that issue I mentioned (reported in Janary by Wolfgang Mües) about changing the _length_ of an OUT transfer. I've some comments I hope to send out, but can't do so much before Wednesday. > > What are interrupt outputs supposed to do? If you have just one > > thing to send are you better off always just sending a bulk one even > > to an interrupt endpoint? "Better off" is a tradeoff. One factor is guaranteed bandwidth: > As far as I know the only difference between one-shot out interrupt (which is a UHCI-ism :) > and > an out bulk is that the interrupt should have guaranteed bandwidth - the > USB core should allocate enough bandwidth on the bus to allow it to go > through immediately or fail with a 'not enough bandwidth' error if it > isn't possible. With bulk it simply waits until the bandwidth is > available and then sends it. Another factor is when the transfer happens ... if your device is only able to handle data every 16 msec, you're not doing anyone a favor by trying to send/recv it more often, it'll just choke. So use interrupt transfers, not bulk transfers. Then there's the fact that the transfer sizes are different. For high speed (480 MBit/sec) interrupts max out at 64 bytes by default, but single transfers go up to 1KByte and there's also a "high bandwidth" mode where one endpoint can do 3KBytes in each microframe (12 KByte/msec, 12 MByte/sec). Bulk maxpacketsize at High speed is fixed at 512 bytes. - Dave _______________________________________________ [EMAIL PROTECTED] To unsubscribe, use the last form field at: https://lists.sourceforge.net/lists/listinfo/linux-usb-devel
