Hi All,
I am currently looking into getting the asymmetric sample rate converters (ASRC) found on some i.MX SoCs to do something useful.
The ASRC units are completely independent units, i.e. independent of the rest of the audio subsystem. They can read from memory using the SDMA engine, convert sample rates and/or audio formats and write back to memory also using the SDMA engine. The ASRC on the i.MX8MN has four contexts to convert up to four streams simultanously. I am not aware of any other non i.MX SoCs having such a unit, but I am pretty sure they exist on other SoCs as well.
There are likely two usecases for such a unit. First would be to offload sample rate and format conversions to hardware. The other would be to synchronize different audio sources/sinks with different master clocks to each other when the master clocks drift away.
How would such units be integrated into ASoC? I can think of two ways. First would be to create an separate audio card from them which records on one end and plays back with a different sample rate / format on the other end, in the v4l2 world that would be a classical mem2mem device. Is Alsa/ASoc prepared for something like this? Would it be feasible to go into such a direction? I haven't found any examples for this in the tree.
The other way is to attach the ASRC to an existing audio card. That is done with the existing in-tree sound/soc/fsl/fsl_asrc.c and sound/soc/fsl/fsl_easrc.c drivers. This approach feels somehow limited as it's not possible to just do conversions without playing/recording something. OTOH userspace is unaffected which might be an advantage. What nags me with that approach is that it's currently not integrated into the simple-audio-card or audio-graph-card bindings. Currently the driver can only be used in conjunction with the fsl,imx-audio-* card driver. It seems backward to integrate such a generic ASRC unit into a special purpose audio card driver. The ASoC core is fully unaware of the ASRC with this approach currently which also doesn't look very appealing. OTOH I don't know if ASoC could handle this. Can ASoC handle for example a chain of DAIs when there are different sample rates and formats in that chain?
Currently I don't really know how to proceed. It would be great if you could share some thoughts to this topic.
Thanks, Sascha