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
