Re: [PATCH v3 00/28] Introduce QC USB SND audio offloading support

[Pierre-Louis Bossart]

>>> 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@xxxxxxxxxxx/T/#u
> 
> 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.