[alsa-devel] How to retrieve the final data being played by HW

Mark Brown broonie at opensource.wolfsonmicro.com
Fri Apr 30 12:51:39 CEST 2010

On Thu, Apr 29, 2010 at 05:46:26PM +0800, Yu Hang wrote:
> On 4/29/2010 5:19 PM, Mark Brown wrote:

> >You need to ensure that the user configures the plugin stack
> >appropriately to ensure that your plugin is the last one that's run, you
> >can't do this purely from within your plugin.  Even if that's done you'd
> >also need to rely on them not configuring any DSP in the hardware which
> >would disrupt things.

> From the pcm_dmix.c, dmix plugin must be followed by HW plugin.
> Which means I cannot insert my own plugin below dmix. Unless I patch
> pcm_dmix.c, I cannot retrieve the final data. Or I can patch
> pcm_hw.c in order to achieve my goal.

Well, you can't assume people are using dmix at all - most modern
desktop systems use PulseAudio, and it's also getting more and more
usage on embedded systems.

> Even if I patch pcm_hw.c, how can I know when the data is actually
> sent to HW?

The information the driver gives you about where it's up to in the
buffer and the latency will tell you this - PulseAudio is probably the
most detailed example out there of this.  Note that "actually sent to
HW" can cover a multitude of things.  There's the data that software can
change, but there's also the potential for additional latency in buffers
and DSP functionality before the audio that's actually being played.

> I have totally no idea about ALSA driver mechanism. A biggest
> question is, how alsa lib (in user space) knows the HW needs new
> data (it must happen by interrupt) and feed HW with new data.

Most ALSA drivers continually process data from a ring buffer, with
their progress through the ring buffer being updated either from
interrupts generated by hardware or by polling from a timer.  The driver
tells userspace what region in the buffer is currently available for
writing, and userspace tells the driver how much data has actually been

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