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

[Wesley Cheng]

Hi Pierre,

On 3/15/2023 7:30 AM, Pierre-Louis Bossart wrote:
> Hi Wesley,
>
> > Sorry made a mistake on the diagram.  There is no connection from
> > SOC-USB to the APR/GLINK.  The Q6USB driver will be the one that is
> > going to configure some of the Q6AFE ports along withe the Q6AFE DAI
> > driver.
> >
> > |            ASoC
> > ----------------------------------
> > |  _________________________
> > | |sm8250 platform card     |
> > | |_________________________|
> > |         |           |
> > |      ___V____   ____V____
> > |     |Q6USB   | |Q6AFE    |  #5
> > |     |"codec" | |"cpu"    |
> > |     |________| |_________|
> > |         ^  ^        ^
> > |      #6 |  |________|
> > |      ___V____     |
> > |     |SOC-USB |    |
> > #7    |        |    |
> > ----->|________|    |
> > ---                 |
> > | |                 |
> > | |    _____________V________
> > | |   |APR/GLINK             |
> > | |   |______________________|
> > | |              ^
> > | | #8           |
> > | |   ___________V___________
> > | |->|audio DSP              |
> > |    |_______________________|
> > |
> > |
> >
> > > >
>
> Makes sense now, thank you for the clarification.
>
> I'll have to dig more in this 'soc-usb' block, it's clearly a key
> component that will have to maintain a consistent state across two
> different parts of the stack and deal with probe/remove/shutdown.
>
> > > My initial thought was to add a 'DSP offload' PCM to the USB card, you
> > > added a "USB offload" PCM to the DSP card. Nice logical swap!
> > >
> > > Your proposal might be easier in practice since there's typically a
> > > vendor-specific configuration file (UCM or custom) file for the DSP,
> > > where USB information can be added.
> > >
> > > It's more problematic to change a generic USB card as we know it today
> > > and bolt vendor-specific DSP information on top.
> > >
> > > The only open I have with your option is that there are still two
> > > control paths to e.g. set the volume. It would be so much easier for
> > > userspace if there was a single volume control no matter what path is
> > > used for data, or make sure the kcontrols are 'mirrored' somehow. If we
> > > found a way to address this issue that would be ideal.
> >
> > Got it.  Let me look to see if that is something we can address/add.  I
> > think the current implementation is that USB SND will expose some mixer
> > controls based on the UAC descriptor parsing.  Then when they want to
> > change the volume (for example) it will result in a USB SETUP transaction.
> >
> > So USB SND will eventually be the entity controlling these changes.
>
> That's probably ok then, am I getting this right that the the DSP card
> would not expose any USB-related kcontrols then, i.e. the ONLY path to
> change volumes, etc.,  would through the regular USB card kcontrols?
>
> That would limit the changes in the platform sound card to the addition
> of a PCM USB device.

Yes, that's correct.  There won't be any exposed USB volume controls 
from the DSP card.

Thanks
Wesley Cheng