RE: [alsa-devel] [PATCH] ASoC: bdw-rt5677: enable runtime channel merge

[Lu, Brent]

> >> In the DAI link "Capture PCM", the FE DAI "Capture Pin" supports
> >> 4-channel capture but the BE DAI supports only 2-channel capture. To
> >> fix the channel mismatch, we need to enable the runtime channel merge
> for this DAI link.
> >>
> >
> > Hi Pierre,
> >
> > This patch is for the same issue discussed in the following thread:
> > https://patchwork.kernel.org/patch/11134167/
> >
> > We enable the runtime channel merge for the DMIC DAI instead of adding
> > a machine driver constraint. It's working good on chrome's 3.14 branch
> > (which requires some backport for the runtime channel merge feature).
> > Please let me know if this implementation is correct for the FE/BE mismatch
> problem.
> 
> Sorry, I don't fully understand your points, and it's the first time I see anyone
> use this .dpcm_merged_chan field for an Intel platform.
> 
> If I look at the code I see that the core would limit the number of channels to
> two. But that code needs the CPU_DAI to use 2 channels, which I don't see.
> So is this patch self-contained or do we need an additional constraint on the
> FE?
> 
> Thanks
> -Pierre

Hi Pierre,

We don't need constraint on FE because dpcm_runtime_merge_chan() modifies
the channel number of snd_pcm_hardware structure directly. The structure will
be used to initialize the snd_pcm_hw_constraints structure later in the
snd_pcm_hw_constraints_complete() function. Since the channel number is already
modified, we don't need a constraint to install an extra rule for it.

The result of using dpcm_merged_chan flag and machine driver constraint should
be the same when user space programs calling HW_REFINE ioctl call but I think the
flag is more elegant for machine driver code.


Regards,
Brent