Re: [PATCH v30 00/30] Introduce QC USB SND audio offloading support

[Pierre-Louis Bossart <pierre-louis.bossart@linux.dev>]

Sorry to chime in late, I only look at email occasionally.

>> Well, from the sound subsystem side, the only concerns are the design
>> issues: namely, whether the implementations with two cards are
>> acceptable, and whether the current control of PCM mapping is OK from
>> the user POV.  IIRC, there were discussions with Intel people and
>> others, and I haven't followed whether we got consensus.
>> If we reached some agreement, it'd be appreciated if you can put acks
>> from them in the patches, too.

My Reviewed-by tags were added in the last updates. I am not sure if anyone else at Intel had the time to review this large patchset.

> I believe Amadeusz was still against having the two card design, and wants the routing to automatically happen when playback happens on the sound card created by the USB SND layer.  However, even with that kind of implementation, the major pieces brought in by this series should still be relevant, ie soc-usb and the vendor offload driver.  The only thing that would really change is adding a path from the USB SND PCM ops to interact with the ASoC entities.  Complexity-wise, this would obviously have a good amount of changes to the USB SND/ASoC core drivers.  Some things I can think of that we'd need to introduce:

The notion of two cards was agreed inside Intel as far back as 2018, when Rakesh first looked at USB offload. 

Having a single USB card in IMHO more complicated:  what happens for example if you plug-in two or more USB devices? Which of the USB cards will expose an optimized path? The design with an ASoC-based card which exposes as many PCM devices as the SOC can support is simpler conceptually and scales well. This would allow e.g. to allocate these PCM devices with different policies (last plugged, preferred, etc).

Conceptually for the simple case with a single USB device, it does not really matter if there are two cards or not. What matters is that there is a clear mapping visible to userspace so that application can decide to use the optimized PCM device, when enabled, instead of the legacy one. And in the end, the application is *always* in control in terms of routing. It’s really similar to the compress offload path, some application changes will be required. 

> 
> 1.  Exposing some of the ASoC PCM (soc-pcm) APIs to be able to be called by soc-usb (to mimic a FE open from ASoC), so we can trigger ASoC DAI ops when USB SND FE is opened.
> 
> 2.  Proper fallback mechanism in case offload path enablement fails to the legacy USB SND path.
> 
> 3.  Master kcontrol to disable offload logic for each USB SND device.
> 
> IMO, both the points you mentioned correspond to the same topic.  If we go with having offload being operated on one FE, then there is no need for the kcontrol of PCM mapping.  If we have two cards, then we will need the control for offload device mapping.  Can't speak for Pierre, but at least with my discussions with him, I don't think he's against the two card design, just as long as we have the proper kcontrol that notifies userspace of how to utilize the offload path.

Even if there’s a single card you need to have a mapping between a ‘legacy’ PCM device and an ‘optimized’ one. This would be a scalar mapping instead of a (card, device) pair, but it’s a minor change.

-Pierre