[alsa-devel] Clock Rate Clock Rate Adjustment API

[Takashi Iwai]

On Fri, 04 Dec 2015 01:32:41 +0100,
Chas Leichner wrote:
> 
> I would like to be able to make small adjustments the playback clock rate
> of a PCM device as it is playing in order to accurately synchronize music
> playback across several computers. Looking at the kernel source, this
> functionality has been previously implemented in both the aloop driver and
> ST's uniperif driver where the clock rate adjustment was exposed to the
> user space as a control element.
> 
> Although the control interface can be used to communicate this information,
> the naming and properties of the clock adjustment controls don't appear to
> be standardized or documented and I don't see anything related to clock
> rate adjustment in Documentation/sound/alsa/ControlNames.txt.

Right, the document hasn't been updated much, and it doesn't mean to
stop you inventing a new one.  Just propose the de facto standard.

> In order to make a standardized interface, do you think it makes more sense
> to standardize on a clock rate adjustment control or introduce a new
> interface other than the control interface because the control interface is
> only for mixer controls rather than arbitrary runtime changes to the driver?

It's not clear what kind of interface you're referring to.  Is the new
IFACE_XXX of kctl element, or a new API or whatever?  IMO, we can keep
using the alsa ctl API, but yes, we can standardize the control name.

The IFACE_* can be set to IFACE_PCM, for example, if it's dedicated to
a certain stream.  Or it can be IFACE_CARD if it's global to all.  In
both cases, it won't appear in the mixer list, at least.


thanks,

Takashi