[alsa-devel] [Q] ASoC: What's the real meaning of SND_SOC_DAIFMT_NB_NF?

[Peter Ujfalusi]

On 02/24/2012 09:53 AM, javier Martin wrote:
> If TFSI bit is 1, frame clock is active high.
> If TFSI bit is 0, frame clock is active low.
> 
> In the piece of code I've shown to you, when someone configures
> (SND_SOC_DAIFMT_I2S | SND_SOC_DAIFMT_NB_NF) he's really selecting an
> active high frame clock when the standard says just the opposite
> (wm8974 datasheet p50).

We had similar issues on OMAP McBSP in the past. If you take a look at
the omap-mcbsp.c: omap_mcbsp_dai_set_dai_fmt() function we have inv_fs
variable to handle such a case.
We do the FS/BCLK configuration only in DAIFMT_INV config phase. The FS
active level is different in these formats. We invert the FS based on
the used format (LEFT/RIGHT just, DSP). The DAIFMT_INV switch configures
according to I2S, but we invert the FS if we are in DSP, Justified modes
afterwards.

>>> there is at least one platform relying on this misleading behavior
>>> that I can't test:
>>> http://lxr.linux.no/#linux+v3.2.7/sound/soc/imx/wm1133-ev1.c#L81
>>
>> That was correct at the time it was written, though the code has changed
>> since then and I don't know when it was last retested.  Of course
>> depending on the signal it may interoperate well even if the setup isn't
>> actually correct.  I'd just fix the driver and if it works for yours
>> it's probably OK for that board too.
> 
> It is not one but two boards that are affected:
> http://lxr.linux.no/#linux+v3.2.7/sound/soc/imx/wm1133-ev1.c#L81
> http://lxr.linux.no/#linux+v3.2.7/sound/soc/imx/eukrea-tlv320.c#L32
> 
> If I fixed the bug in the SSI I would break both of them, since I
> would be changing frame clock polarity.

I don't think it is going to break. I would guess that the channels are
swapped on these machines at the moment. If you correct the CPU dai
driver the only thing that will happen is that the channels will jump in
their correct place.

-- 
Péter