[alsa-devel] [PATCH] ASoC: tlv320aic3x: Correct S24_3LE support

[Lars-Peter Clausen]

On 12/13/2013 02:29 PM, Peter Ujfalusi wrote:
> On 12/13/2013 03:04 PM, Mark Brown wrote:
>> On Fri, Dec 13, 2013 at 02:58:19PM +0200, Peter Ujfalusi wrote:
>>
>>> -	case SNDRV_PCM_FORMAT_S24_LE:
>>> +	case SNDRV_PCM_FORMAT_S24_3LE:
>>>  		data |= (0x02 << 4);
>>>  		break;
>>
>> This should be adding the case for the new format rather than replacing
>> the old one shouldn't it?  They ought to turn out the same on the AIF so
>> the CODECs shouldn't care about the difference, ideally the core would
>> hide the difference from them.
> 
> Not really since the codec has only field to specify the data format. The
> codec can not support S24_LE (S24_LE is basically S32_LE msbits==24) since we

S24_LE is a 24 bit sample in a 32 bit word, but it is right aligned, so
there are 8 bits of leading padding, while with S32_LE msbits=24 you'd have
8bits of trailing padding.

> can not say to the codec to ignore the 8bit over the 24 bits of real data.
> In case of S24_3LE the I2S bus will have 24 clocks/per channel which can not
> be used to stream S24_LE either.
> 

Normally you'd expect the I2S core to only put the 24 bits of data onto the
bus for both S24_3LE and S24_LE (it might add necessary trailing padding if
the bit clocks per frame is > 24). A CODEC driver should really not have to
care about the in memory layout of the data since all it will see is a
serialized bit stream. I think ideally we wouldn't check for params_format()
but rather for snd_pcm_format_width(params_format()).

- Lars