[alsa-devel] Using codecs with AND without cpu_dai

Mike Looijmans mike.looijmans at topic.nl
Fri Oct 25 08:56:56 CEST 2013

I've run into a second project where we'll probably want to use an audio codec 
in multiple ways. In this case, it means that we want to activate the codec 
without actually capturing data, but we alsa want to "peek" at the data while 
the codec is running.

More in detail, the I2S data from the codec is processed by, for example, an 
FPGA. The I2C bus is also connected to the CPU, so that the CPU can also 
aquire the data via Alsa.

So far, I've been using a piece of hackish code like this to turn on a codec 
without capturing from it (kernel 2.6.37 based code for now):

	struct snd_soc_codec *codec = codec_dai->codec;
	struct snd_soc_pcm_runtime fake_rtd;
	fake_rtd.codec_dai = tacho_dai;
	fake_rtd.codec = codec;
	snd_soc_dapm_stream_event(&fake_rtd, capture_stream_name, 

This works well, it neatly uses DAPM to switch on the proper ADC paths. One 
can now also use the regular Alsa interfaces to start sampling from that 
codec. Of course, when you stop the capture stream, the codec will be shut 
down because there's no reference counting or anything similar on the codec usage.

It would be neater to have a "dummy" dai that connects to the codec, but when 
I do that, the regular cpu dai cannot be used and I cannot capture the stream 
and process it elsewhere at the same time.

A similar situation in pure audio components would be a second codec that 
outputs the I2S data to SPDIF, and a primary codec that samples analog audio 
to the I2S bus. Quite similar, you don't need to have a capture running, but 
both codecs must be powered on an actively run in order to have a 
analog-to-spdif function without involving the CPU. And the CPU should be able 
to "plug in" at will to grab the audio data, without disturbing the two.

Anyone any experience or ideas into a CORRECT way to handle these kind of setups?

Met vriendelijke groet / kind regards,

Mike Looijmans

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