[alsa-devel] [PATCH 11/20] OMAP: McBSP: Add link DMA mode selection

Peter Ujfalusi peter.ujfalusi at nokia.com
Thu Aug 13 08:01:24 CEST 2009

On Wednesday 12 August 2009 14:48:33 Nurkkala Eero.An (EXT-Offcode/Oulu) wrote:
> On Wed, 2009-08-12 at 13:45 +0200, ext Jarkko Nikula wrote:
> > The threshold based transfer will cause that omap_pcm_pointer will
> > loose a bit its accuracy. Probably irrelevant but still better to play
> > safe at least over one kernel release before making it default.
> No, it doesn't loose accuracy =) It's as accurate with both modes.
> The difference is, that the other does things in bursts;

In element mode it is kind of easy to estimate where the hardware actually in 
the playback case (aplay -f dat /dev/zero):

omap_pcm_pointer returns 669, than the HW is around
669-512=157 (plus few samples).

In threshold mode you only know that the HW is playing in between 0-512, 
512-1024 somewhere.

I know neither of these are accurate and these examples are quite 
oversimplified, but there is a difference and that difference is quite 

> that's called evolution rather than regression =)

Note that evolution can also introduce regression...

> That's how things will be in the future =)

I agree with you on this, since the threshold mode provides quite good power 
saving benefits.
But on the other hand it would be still better to keep the element mode as 
default for at least one release cycle.
If no report is coming about problems, than we can make the threshold mode for 
McBSP2 as the default

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