[alsa-devel] [linux-sunxi] Re: [PATCH 2/2] ASoC: sunxi: add support for the on-chip codec on early Allwinner SoCs

[Mark Brown]

On Thu, Sep 17, 2015 at 11:10:41PM +0800, Chen-Yu Tsai wrote:
> On Thu, Sep 17, 2015 at 9:31 PM, Maxime Ripard

> >> > +           .formats        = SNDRV_PCM_FMTBIT_S16_LE |
> >> > +                             SNDRV_PCM_FMTBIT_S32_LE,
> >> > +           .sig_bits       = 24,

> >> So presumably also S24_LE (ie, 24 bits packed into a 32 bit word)?

> > Hmm, probably yes, I'll test that.

> IIRC when Emilio first wrote the driver, we tried 24 bit and no sound
> came out. Turns out it's an alignment issue. The codec's FIFO register
> is 32 bits wide, and takes the higher 24 bits as input when set to 24
> bit mode. The internal FIFO is only 24 bits wide. A20 user manual P174
> describes how the bits are copied.

> So for 24 bit audio, you would actually send it 32 bit audio samples,
> and let it truncate or drop the least significant 8 bits. This is why
> we have SNDRV_PCM_FMTBIT_S32_LE with .sig_bits = 24.

> I don't know if this is just a workaround, but a few other drivers do
> this as well, for example twl3040 and omap-mcpdm.

That's very common - it's essentially what S24_LE mode is.
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