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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