[alsa-devel] BUG: USB audio discontinuities with 'UHCI: implement new semantics for URB_ISO_ASAP'

Alan Stern stern at rowland.harvard.edu
Tue Apr 23 17:23:37 CEST 2013

On Tue, 23 Apr 2013, Clemens Ladisch wrote:

> Alan Stern wrote:
> > On Tue, 23 Apr 2013, Clemens Ladisch wrote:
> >> Why is having a queue of two URB with one packet each suddenly not
> >> allowed?
> >
> > It _is_ allowed when URB_ISO_ASAP is clear.  I have never fully
> > understood why the audio driver sets that flag.  By setting it, you are
> > telling the host controller driver that you are willing to give up
> > reduced latency in order to avoid underruns.
> This flag was needed to avoid having to set urb->start_frame.

Ah, okay.  It is now unnecessary to set urb->start_frame; in fact that 
field is now output-only.

(To be fair, I haven't checked _all_ the HCDs in this regard, just 
uhci-hcd, ohci-hcd, and ehci-hcd.  However, if any other HCD requires 
urb->start_frame to be set then that HCD should be considered buggy and 
should be fixed.)

> With the changed queueing API, the audio driver needs to change too.
> I'll look into this ...

Let me know if you have any questions.

Alan Stern

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