Re: [PATCH v2] ALSA: ASoC: McASP: add support for 24 bit samples

[Daniel Mack <zonque@gmail.com>]

On 09.10.2012 12:24, Mark Brown wrote:
> On Tue, Oct 09, 2012 at 11:47:57AM +0200, Daniel Mack wrote:
>> On 09.10.2012 11:41, Mike Looijmans wrote:
>>> Sorry for the lack of quoting, but I onle get the digest.
> 
>>> These are wrong:
> 
>>> +       case SNDRV_PCM_FORMAT_U24_LE: +       case
>>> SNDRV_PCM_FORMAT_S24_LE:
> 
>>> These pack a 24-bit sample value in a 32-bit word. The codec will
>>> send 32 bits to the McASP, and you should transfer 32 bits to the
>>> user, not just 24. Hence, SNDRV_PCM_FORMAT_S24_LE must be treated
>>> just like SNDRV_PCM_FORMAT_S32_LE.
> 
>>> I've tested that on a DA850-alike board with several TLV320AIC3256
>>> codecs, treating them as 3-byte samples will reasult in invalid
>>> data.
> 
>> Ok, thanks for reporting this. Would like to send a patch or want me to
>> fix it?
> 
> The above explanation isn't quite right.  For 24 bit the CODEC should be
> working with 24 bits on the wire (though obviously extra BCLKs are
> allowed) and the AP should be working with 32 bit words in memory.  The
> issue here is probably that you're working with real 24 bit data in RAM
> too.

Hmm, I don't understand. I thought S24_3LE exists to denote a 3-byte
representation, because S24_LE uses 32 bits? Hence Mike would be right
that S24_LE has to use a 4-byte dma transfer size, no?

I'm using S24_3LE here, hence that didn't hit me.

Where's the confusion? :)

Daniel