On Mon, May 21, 2001, Matthew Dharm <[EMAIL PROTECTED]> wrote:
> On Mon, May 21, 2001 at 04:52:58PM -0400, Johannes Erdfelt wrote:
> > On Mon, May 21, 2001, David Brownell <[EMAIL PROTECTED]> wrote:
> > > > That is, I want to move several URBs for one
> > > > endpoint, and _when_they_are_done_, I want to move an URB for another
> > > > endpoint. And, it doesn't matter how the beginning URBs end, I still want
> > > > to move the last one.
> > >
> > > So far as I know, the only way to make that happen is to have the
> > > completion function(s) for that first endpoint trigger the behavior
> > > you want (by submitting urbs for the other endpoint).
> >
> > This could be done with ->next. Or you can do it in the completion
> > handler.
>
> Wait... different answer here.... can this be done with ->next? People
> seem to have different opinions here. Some say ->next is for ISO only.
That's incorrect. From Documentation/usb/URB.txt:
- URBs can be linked. After completing one URB, the next one can be
automatically submitted. This is especially useful for ISO transfers:
You only have read/write the data from/to the buffers in the completion
handler, the continuous streaming itself is transparently done by the
URB-machinery.
It specifically mentions ISO as a place where it useful, but it does not
limit it to specifically ISO.
> What I'd _hope_ for is this: I construct a chain of arbitrary URBs in a
> linked list via the ->next pointer. I send the chain to the API, and now I
> go to sleep while I wait for a completion message from the last URB.
> Meanwhile, the URBs are processed by executing the head of the queue, and
> (when that's finished), executing the next item on the queue.
>
> Is that possible? Yes, I can do this via submitting URBs from the
> completion function. But, then I have to do a lot of management in the
> completion handler, which I don't want to do. I'd like to set everything
> up ahead of time and let it run, so that the time between URBs is
> minimized.
>
> Yes, I realize that I could try to prep the next URB while one is running,
> but then there are synchronization problems. So, if I'm preping them all
> in advance anyway....
I dunno if that's possible. If one of the previous URB's fails, the
behvaiour is poorly defined.
Right now it submits along the chain merrily. I dunno if that's the
desired behaviour or not.
JE
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