Re: [PATCH v16 00/50] Introduce QC USB SND audio offloading support

[Wesley Cheng]

On 2/15/2024 7:58 PM, Wesley Cheng wrote:
> Several Qualcomm based chipsets can support USB audio offloading to a
> dedicated audio DSP, which can take over issuing transfers to the USB
> host controller.  The intention is to reduce the load on the main
> processors in the SoC, and allow them to be placed into lower power modes.
> There are several parts to this design:
>    1. Adding ASoC binding layer
>    2. Create a USB backend for Q6DSP
>    3. Introduce XHCI interrupter support
>    4. Create vendor ops for the USB SND driver
>
>        USB                          |            ASoC
> --------------------------------------------------------------------
>                                     |  _________________________
>                                     | |sm8250 platform card     |
>                                     | |_________________________|
>                                     |         |           |
>                                     |      ___V____   ____V____
>                                     |     |Q6USB   | |Q6AFE    |
>                                     |     |"codec" | |"cpu"    |
>                                     |     |________| |_________|
>                                     |         ^  ^        ^
>                                     |         |  |________|
>                                     |      ___V____    |
>                                     |     |SOC-USB |   |
>     ________       ________               |        |   |
>    |USB SND |<--->|QC offld|<------------>|________|   |
>    |(card.c)|     |        |<----------                |
>    |________|     |________|___     | |                |
>        ^               ^       |    | |    ____________V_________
>        |               |       |    | |   |APR/GLINK             |
>     __ V_______________V_____  |    | |   |______________________|
>    |USB SND (endpoint.c)     | |    | |              ^
>    |_________________________| |    | |              |
>                ^               |    | |   ___________V___________
>                |               |    | |->|audio DSP              |
>     ___________V_____________  |    |    |_______________________|
>    |XHCI HCD                 |<-    |
>    |_________________________|      |
>
>
> Adding ASoC binding layer:
> soc-usb: Intention is to treat a USB port similar to a headphone jack.
> The port is always present on the device, but cable/pin status can be
> enabled/disabled.  Expose mechanisms for USB backend ASoC drivers to
> communicate with USB SND.
>
> Create a USB backend for Q6DSP:
> q6usb: Basic backend driver that will be responsible for maintaining the
> resources needed to initiate a playback stream using the Q6DSP.  Will
> be the entity that checks to make sure the connected USB audio device
> supports the requested PCM format.  If it does not, the PCM open call will
> fail, and userpsace ALSA can take action accordingly.
>
> Introduce XHCI interrupter support:
> XHCI HCD supports multiple interrupters, which allows for events to be routed
> to different event rings.  This is determined by "Interrupter Target" field
> specified in Section "6.4.1.1 Normal TRB" of the XHCI specification.
>
> Events in the offloading case will be routed to an event ring that is assigned
> to the audio DSP.
>
> 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
>
> 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...
>
> This feature was validated using:
> - tinymix: set/enable the multimedia path to route to USB backend
> - tinyplay: issue playback on platform card
>
> Changelog
> --------------------------------------------
> Changes in v16:
> - Modified some code layer dependencies so that soc usb can be split as a separate
>    module.
>    - Split the kcontrols from ASoC QCOM common layer into a separate driver
> - Reworked SOC USB kcontrols for controlling card + pcm offload routing and status
>    so that there are individual controls for card and pcm devices.
> - Added a kcontrol remove API in SOC USB to remove the controls on the fly.  This
>    required to add some kcontrol management to SOC USB.
> - Removed the disconnect work and workqueue for the QC USB offload as it is not
>    required, since QMI interface driver ensures events are handled in its own WQ.

Sorry for sending a bad series.  Please ignore this version, as its 
missing some changes made.  Will resend as v17.

Thanks
Wesley Cheng