[Sound-open-firmware] What is the correct loopback configuration for ASoC machine driver DAI links?

[Thomas Preston]

Ah, no attachments. I will paste the relevant bits.

On 30/05/2019 16:14, Thomas Preston wrote:
> Hi,
> I have a question about configuring a physical loopback on a custom
> board for the Intel Skylake platform driver. I have also asked on
> freenode/#alsa-dev but they directed me here.
> 
> We have the following physical SSP layout:
> 
> A2B axillary audio
> + SSP0 Tx (connected to SSP5 Rx, for debug)
> + SSP0 Rx
> 
> Main speaker output + loopback from A2B
> + SSP5 Tx
> + SSP5 Rx (connected to SSP0 Tx)
> 
> We have a board-level hardware loopback where the output signals and
> clock (slave) on SSP0 are connected to the input signals and clock
> (master) on SSP5.
> 
> I'm trying to configure the DAI links in the machine driver so that they
> link against SSP0 Pin CPU DAI correctly but I have three problematic
> configurations:
> 
> 1. Separate BE DAI links for SSP5 playback and capture, linking against
> the same Skylake CPU DAI "SSP5 Tx". This crashes the kernel with a NULL
> pointer dereference (I'm looking into this). Presumably this is an
> invalid configuration because I have multiple DAI links linking against
> one CPU DAI.  See machine1.c
> 

machine1.c  https://paste.gnome.org/pllakypau

> 2. One BE DAI link for SSP5 playback + capture, linking against the same
> Skylake CPU DAI. This silences my SSP5 Tx main audio output (which
> appears to be muted) when I try to arecord on the SSP5 Rx A2B debug
> loopback. The recording is a perfect copy of SSP0 Tx so the loopback
> part works. I can recover by restarting my whole audio stack.  See
> machine2.c
> 

machine2.c  https://paste.gnome.org/pxdam0y3j

> 3. Separate BE DAI links for SSP playback and loopback, linking against
> a separated SSP5 Skylake CPU DAI (SSP5 Pin with ssp5 Tx, SSP5 Pin
> Loopback with ssp5 Rx). This configuration works but requires a
> modification to the Skylake platform driver - which I'd rather avoid.
> See machine3.c, skylake.diff
> 

machine3.c   https://paste.gnome.org/peoip440a
skylake.diff https://paste.gnome.org/pctai1nxh

> Configuration 2 is closest to what I want, and configuration 3 is my
> work-around for now.
> 
> What is the correct way to link CPU DAIs for this kind of loopback? I
> have attached example code.