Create vendor ops for the USB SND driver: qc_audio_offload: This particular driver has several components associated with it:
- QMI stream request handler
- XHCI interrupter and resource management
- audio DSP memory management
so how does this 'qc_audio_offload' interface with 'q6usb' described above? how are the roles different or complementary?
So in general you can think that the qc_audio_offload is a complement to the USB SND USB class driver, while q6usb is to ASoC. Since the ASoC
Humm, that is far from clear. I don't get how a something that interacts with the USB class driver can also be in charge of the audio DSP memory management.
framework doesn't have any communication with USB SND, the ASoC DPCM USB backend (q6usb) will have to be the entity that maintains what is going on in USB SND. That way, sessions initiated through the ASoC managed sound card can evaluate what is available based on information reported by q6usb.
qc_audio_offload and q6usb will have some interaction between each other. The majority of communication between qc_audio_offload and q6usb is reporting the device connection events.
It's already complicated to figure out how the DSP and USB class driver might interact and probe/timing dependencies, but with two additional drivers in the mix it's really hard to understand.
Maybe ascii-art would help describe the concepts and types of information exchanged. Maintaining a consistent state across multiple drivers is not an easy task.
When the audio DSP wants to enable a playback stream, the request is first received by the ASoC platform sound card. Depending on the selected route, ASoC will bring up the individual DAIs in the path. The Q6USB backend DAI will send an AFE port start command (with enabling the USB playback path), and the audio DSP will handle the request accordingly.
Part of the AFE USB port start handling will have an exchange of control messages using the QMI protocol. The qc_audio_offload driver will populate the buffer information:
- Event ring base address
- EP transfer ring base address
and pass it along to the audio DSP. All endpoint management will now be handed over to the DSP, and the main processor is not involved in transfers.
Overall, implementing this feature will still expose separate sound card and PCM devices for both the platorm card and USB audio device: 0 [SM8250MTPWCD938]: sm8250 - SM8250-MTP-WCD9380-WSA8810-VA-D SM8250-MTP-WCD9380-WSA8810-VA-DMIC 1 [Audio ]: USB-Audio - USB Audio Generic USB Audio at usb-xhci-hcd.1.auto-1.4, high speed
This is to ensure that userspace ALSA entities can decide which route to take when executing the audio playback. In the above, if card#1 is selected, then USB audio data will take the legacy path over the USB PCM drivers, etc...
I already voiced my concerns about exposing two cards, each with their own set of volume controls with the same device. It would be much better to have an additional offloaded PCM device for card0...
But if the consensus is to have two cards, it's still not clear how the routing would be selected. In the case where there are two USB audio devices attached, the offloaded path would only support one of the two. How would userspace know which of the two is selected?
With patch#24: https://lore.kernel.org/linux-usb/20230308235751.495-25-quic_wcheng@quicinc....
Now, userspace can at least choose which device it wants to offload. Part of doing that would mean userspace knows what USB SND card devices are available, so it is aware of which devices are shared (between the offload and USB SND path)
And how would userspace know the difference anyways between two physical devices attached to the platform with no offload, and one physical device with one additional offload path? The names you selected can't be used to identify that card1 is the optimized version of card0.
Is userspace currently able to differentiate between cards that are created by USB SND versus ASoC? How complex can the userspace card discovery be? Can it query kcontrols at this point in time? If so, maybe we can change the names of the newly added ones to reflect that it is an offload device?
SND kcontrol names are currently: Q6USB offload status Q6USB offload SND device select
I must admit I've never seen kcontrols being used to identify what the card is, and in this case it's a pretend-card that's just an improved version of another. It might be easier to use something else, such as the component strings.
Before we review low-level kernel plumbing, it would be good to give a better overview of how userspace applications are supposed to interact with the cards and identify the offloaded path. Testing with tinyplay/tinymix is fine, but that's a developer-level or CI unit test. we've got to see the broader picture of how a sound server would use this USB offload capability.
Sure, I think that is fine. I was hoping that at least adding some of the new kcontrols would help userspace make use of this path in general, but we can add more information if required.
Can I ask if this solution has been used with a complete userspace stack already? I could see how this might be used with a relatively fixed Android HAL, where the platform and routing are relatively controlled. I don't see how a more generic audio server would deal with the discovery and routing.