snd_pcm_format_s24_3le and the SSI driver

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snd_pcm_format_s24_3le and the SSI driver

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matthewcampbell
Contributor III

Hi All,

I'm come across a curiosity with the SSI driver in Linux that I was hoping to get some insight on. When opening an ALSA device that uses the fsl_ssi under the hood and you use 24-bit packed data format (snd_pcm_format_s24_3le), it appears that it still treats it like padded 24-bit (snd_pcm_format_s24_le). I think what is happening is that the DMA engine is simply reading the packed 24_3le data and stuffing it right in the ssi fifo (still packed). I'm guessing this is because of one of two things:

  • First, maybe the intent was to have the DMA engine do the unpacking before putting it in the FIFO (via a custom DMA program in the SDMA engine) and this just never got implemented
  • Second, the support for 24_3le format (SNDRV_PCM_FMTBIT_S24_3LE in the fsl_ssi.c driver) was never intended to be supported and should be removed

You can show the effects of this bug by using a tool like aplay, opening the hardware directly avoiding ALSA's re-sample/re-format and playing a wav file that is 24-bit. If you look at the SSI output on a scope, you will see that every fourth byte from the wav file data is dropped because it is effectively getting treated as padding when writing to the 32-bit input of the SSI FIFO. (aplay -D hw:0 24bit_file.wav).

I'm trying to figure out if I'm missing something here, or if this is actually a bug.

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Yuri
NXP TechSupport
NXP TechSupport

Hello,

   Really 24-bit data format without  padding (not aligned) is not supported.


Have a great day,
Yuri

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matthewcampbell
Contributor III

Hi Yuri,

Thanks for the reply. I was pretty sure from experimentation and code reading that the driver doesn't support non-aligned 24-bit data. I guess my real question is if the driver doesn't support SNDRV_PCM_FMTBIT_S24_3LE, why does it present this as a support format? I'd consider this a pretty obvious bug that should be fixed. Do you know if there is anyway of either bug reporting or patch submission to Freescale's Linux repo from the community?

Thanks,

~Matt

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Yuri
NXP TechSupport
NXP TechSupport

Matt, hi !

  I think, it makese sense to apply to FSL Community BSP


Regards,
Yuri.

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