[alsa-devel] [PATCH v2] ALSA: ASoC: McASP: add support for 24 bit samples

[Mike Looijmans]

On 10/09/2012 02:21 PM, Clemens Ladisch wrote:
> The three possibilities to store little-endian signed 24-bit samples in
> memory are (where L=LSB, M=MSB):
>
>    0padding LLLLLLLL MMMMMMMM HHHHHHHH  S32_LE
>    LLLLLLLL MMMMMMMM HHHHHHHH 0padding  S24_LE
>    LLLLLLLL MMMMMMMM HHHHHHHH           S24_3LE
>
> The first two require the DMA controller to transfer four bytes, the
> last one, three bytes.
>
> All three use (at least) 24 bits on the wire.
Oh, I always assumed that S32_LE was just a 32-bit format. That's what 
I've been assuming so far, and the resulting data files from "arecord -f 
{S32_LE|S24_LE|S24_3LE}" were perfectly understood by audio programs.
Basically, what you're claiming here is that S24_LE must store the audio 
data in the lower 3 bytes? Because in my experiments so far I concluded 
I had to fill the high order bits for both S24_LE and S32_LE formats.

> >Hence Mike would be right that S24_LE has to use a 4-byte dma transfer
> >size, no?
> Yes.
It's not strictly what I wrote, but it's what I meant, yes.

Now we're on the topic of DMA controllers:
When reading multiple channels (e.g. 2 or more codecs on the serializers 
either using TDM or multiple AXR pins), the DMA controller must always 
read 32 bits per sample from the serializer, regardless of the sample 
format used.
Currently I implement this by enforcing S32_LE or S24_LE formats, and 
denying all others when multiple serializers are involved.
Would it be possible to instruct the davinci's DMA controller to read 32 
bits but write only 8, 16 or 24 bits to memory for each sample?


Met vriendelijke groet / kind regards,

Mike Looijmans

TOPIC Embedded Systems
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