ASoC TDM channel re-mapping

[Edward Cragg <edward.cragg@codethink.co.uk>]

Hi,

We have an ASoC driver in which we have added support for TDM over an I2S
interface.

We would like to have the capability of re-mapping the channel ordering, for
which i know there is a callback in place to start this process,
set_channel_map.

The only backend implementation of this is using the ALSA API and the .copy
handler in an ALSA PCM driver to mutate data copied out of ALSA before it's
DMA'd to hardware. This is in sound/soc/blackfin/bf5xx-i2s-pcm.c

The problem we have is that the driver we're using currently uses a dmaengine
based PCM driver, and i'm not sure whether this is compatible with using the
.copy callback, without reimplementing the driver entirely as an ALSA PCM
driver.

Since there aren't very many examples in mainline, I was hoping to ask for
advice for the best way of doing this.

I wondered:

 - Would it be possible to add a functional copy callback using a dmaengine
   based PCM driver?
 - Otherwise, would there be a good way to implement channel remapping in a
   TDM frame using the dmaengine based PCM driver?
   or
 - Would the best way indeed be to reimplement the PCM driver as a normal ALSA
   PCM driver?

Thanks,

Ed Cragg