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

From: Clemens Ladisch <clemens@ladisch.de>
To: alsa-devel@alsa-project.org
Cc: tova-dev@tovacompany.com, linux-usb@vger.kernel.org,
	Joe Rayhawk <jrayhawk@fairlystable.org>,
	alsa-devel@vger.kernel.org
Subject: Re: [alsa-devel] BUG: USB audio discontinuities with 'UHCI: implement new semantics for URB_ISO_ASAP'
Date: Fri, 19 Apr 2013 18:25:46 +0200	[thread overview]
Message-ID: <5171700A.50308@ladisch.de> (raw)
In-Reply-To: <Pine.LNX.4.44L0.1304191132130.21363-100000@netrider.rowland.org>

Alan Stern wrote:
> ...
> This trace shows that the frame numbers do not increase sequentially:
> 1057125, 1057126, 1057128, 1057129, 1053131, ...  This is because the
> new driver is a little more conservative than the old driver,
> requiring latencies to be larger than 1 ms.  You can see this
> explicitly in one of the new lines added by the commit you found:
>
> +		next = uhci->frame_number + 2;
>
> That 2 is the minimum latency, in frames (one frame per ms).

One frame worked fine with the old driver.  What is the reason for
this regression?

> Anyway, the solution is simple: The audio driver needs either to submit
> buffers that are at least 2 ms long, or to use more than two buffers
> (or both).

The audio driver already has the infrastructure to cope for hardware
restrictions like this, it just needs to *know* about them.  How can
it detect that it runs on the OHCI/UHCI drivers?

Regards,
Clemens
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