What was your intention for the jack.channelsCapture/Playback variables in the context? It's just that it's very unusual to have a channel count at the context level rather than the device level. Regardless, this code will need to change:

    if (pContext->backend == ma_backend_jack) {
        pContext->jack.channelsCapture = pConfig->capture.channels;
        pContext->jack.channelsPlayback = pConfig->playback.channels;
    }

Any backend-specific logic like that needs to be within the backend itself, not in the cross-platform section.

Just to clarify the rules. When you specify a channel count in the device config, the caller is specifying what they want. The backend (jack in this case), will use that as a hint. If it can exactly represent that channel count, it should do so, and if not, it should get as close as possible and then report via pPlayback/CaptureDescriptor the actual channel count. Then from that information miniaudio will do all the necessary conversion for you. From the perspective of the caller, they must always have the channel count they request in the device config, unless they pass in 0 in which case the backend should choose the "native" channel count.

Oh about jack.channelsCapture/Playback, yes that was a bit of stop gap. I wasn't sure how to solve this but just wanted to get a proof of concept to see it working. Details:

The number of channels set in the ma_device_config struct is then stored in these locations:

pDevice->capture.channels            = pConfig->capture.channels;
pDevice->playback.channels           = pConfig->playback.channels;
descriptorPlayback.channels                 = pConfig->playback.channels;
descriptorCapture.channels                  = pConfig->capture.channels;

and while ma_device_get_info() is passed ma_device* pDevice in the first arg, when it calls
ma_context_get_device_info(pDevice->pContext, type, pDevice->playback.pID, pDeviceInfo); which calls ma_context_get_device_info__jack(ma_context* pContext, ma_device_type deviceType, const ma_device_id* pDeviceID, ma_device_info* pDeviceInfo) it no longer has access to pDevice so doesn't know the requested number of channels. (But there's a good chance I'm missing some method to access it)

Since I saw pContext->jack.tryStartServer = pConfig->jack.tryStartServer; having a ma_device_config member being copied into pContext, I copied that as a way to have that info be avail to the jack implementation.

Oh and I think there was a reason I couldn't put pContext->jack.channelsCapture = pConfig->capture.channels; etc in ma_context_init__jack because ma_context_get_device_info__jack is called earlier? I'd have to take another look.

Also re: the channel count config rules, I do feel it wasn't easy to understand the different .channels floating around in this patch, so may need some more rewriting to be super clear as to how the jack implementation is getting/setting. number of channels. Like the code around using ppPortsCapture could use some pondering

OK just stepping through the calls to jack backend to trace things out for a better understanding:

• ma_device_init
  • ma_context_init
    • ma_context_init__jack: pConfig here is context config. We don't have device config if we wanted to store requestd number of channels somewhere
      • ma_context_open_client__jack (temp dummy client)
  • ma_device_init_jack: here we use pDevice->playback->channels or (device config) pConfig->playback->channels. Oh maybe here they can be stored in pDevice->pContext so it can be available to ma_context_get_device_info__jack later
    • ma_context_open_client__jack
  • ma_device_post_init -> ma_device_get_info -> ma_context_get_device_info
    • ma_context_get_device_info__jack: here we need num channels but only have pContext. pDeviceID is null and pDeviceInfo is zero initialized.
      • ma_context_open_client__jack
• ma_device_start
  • ma_device_start__jack: here we use pDevice->playback->channels or pConfig->playback->channels

So does it make sense to have to store pContext->jack->channels{Playback,Capture}? Or else is there a way in ma_context_get_device_info__jack() to have the device pointer? The issue is the ma_context_get_device_info() function prototype does not pass a pDevice arg. But if this pDevice->pContext->callbacks.onDeviceGetInfo callback exists it is passed pDevice and that result used instead of calling ma_context_get_device_info(), so maybe that is where a ma_context_get_device_info__jack() with a matching signature should be. Otherwise a global change to the ma_context_get_device_info() function prototype to add an arg for pDevice?