On 9/25/2024 2:59 AM, Wesley Cheng wrote:
Requesting to see if we can get some Acked-By tags, and merge on usb-next.
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 userspace 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 platform 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...
The current limitation is that the latest USB audio device that is identified
will be automatically selected by the Q6USB BE DAI for offloading. Future
patches can be added to possibly add for more flexibility, but until the userpace
applications can be better defined, having these mechanisms will complicate the
overall implementation.
USB offload Kcontrols
=====================
Part of the vendor offload package will have a mixer driver associated with it
(mixer_usb_offload.c). This entity will be responsible for coordinating with
SOC USB and the Q6USB backend DAI to fetch information about the sound card
and PCM device indices associated with the offload path. The logic is done
based on the current implementation of how paths are controlled within the QC
ASoC implementation.
QC ASoC Q6Routing
-----------------
Within the Q6 ASOC design, the registered ASoC platform card will expose a set
of kcontrols for enabling the BE DAI links to the FE DAI link. For example:
tinymix -D 0 contents
Number of controls: 1033
ctl type num name value
...
1025 BOOL 1 USB Mixer MultiMedia1 Off
1026 BOOL 1 USB Mixer MultiMedia2 Off
1027 BOOL 1 USB Mixer MultiMedia3 Off
1028 BOOL 1 USB Mixer MultiMedia4 Off
1029 BOOL 1 USB Mixer MultiMedia5 Off
1030 BOOL 1 USB Mixer MultiMedia6 Off
1031 BOOL 1 USB Mixer MultiMedia7 Off
1032 BOOL 1 USB Mixer MultiMedia8 Off
Each of these kcontrols will enable the USB BE DAI link (q6usb) to be connected
to a FE DAI link (q6asm). Since each of these controls are DAPM widgets, when
it is enabled, the DAPM widget's "connect" flag is updated accordingly.
USB Offload Mapping
-------------------
Based on the Q6routing, the USB BE DAI link can determine which sound card and
PCM device is enabled for offloading. Fetching the ASoC platform sound card's
information is fairly straightforward, and the bulk of the work goes to finding
the corresponding PCM device index. As mentioned above, the USB BE DAI can
traverse the DAPM widgets to find the DAPM path that is related to the control
for the "USB Mixer." Based on which "USB Mixer" is enabled, it can find the
corresponding DAPM widget associated w/ the FE DAI link (Multimedia*). From there
it can find the PCM device created for the Multimedia* stream.
Only one BE DAI link can be enabled per FE DAI. For example, if the HDMI path is
enabled for Multimedia1, the USB Mixer will be disabled and switched over.
Examples of kcontrol
--------------------
tinymix -D 0 contents
Number of controls: 1033
ctl type num name
...
1025 BOOL 1 USB Mixer MultiMedia1 Off
1026 BOOL 1 USB Mixer MultiMedia2 On
1027 BOOL 1 USB Mixer MultiMedia3 Off
1028 BOOL 1 USB Mixer MultiMedia4 Off
1029 BOOL 1 USB Mixer MultiMedia5 Off
1030 BOOL 1 USB Mixer MultiMedia6 Off
1031 BOOL 1 USB Mixer MultiMedia7 Off
1032 BOOL 1 USB Mixer MultiMedia8 Off
tinymix -D 2 contents
Number of controls: 7
ctl type num name value
0 INT 2 Playback Channel Map 0, 0 (range 0->36)
1 BOOL 2 MDR-1ADAC Playback Switch On, On
2 BOOL 1 MDR-1ADAC Playback Switch On
3 INT 2 MDR-1ADAC Playback Volume 127, 127 (range 0->127)
4 INT 1 MDR-1ADAC Playback Volume 127 (range 0->127)
5 BOOL 1 Sony Internal Clock Validity On
6 INT 2 USB Offload Playback Route PCM#0 0, 1 (range -1->255)
The example highlights that the userspace/application can utilize the offload path
for the USB device on card#0 PCM device#1.
When dealing with multiple USB audio devices, only the latest USB device identified
is going to be selected for offload capable.
tinymix -D 1 contents
Number of controls: 9
ctl type num name value
0 INT 2 Capture Channel Map 0, 0 (range 0->36)
1 INT 2 Playback Channel Map 0, 0 (range 0->36)
2 BOOL 1 Headset Capture Switch On
3 INT 1 Headset Capture Volume 1 (range 0->4)
4 BOOL 1 Sidetone Playback Switch On
5 INT 1 Sidetone Playback Volume 4096 (range 0->8192)
6 BOOL 1 Headset Playback Switch On
7 INT 2 Headset Playback Volume 20, 20 (range 0->24)
8 INT 2 USB Offload Playback Route PCM#0 -1, -1 (range -1->255)
"-1, -1" shows that this device has no route to the offload path.
This feature was validated using:
- tinymix: set/enable the multimedia path to route to USB backend
- tinyplay: issue playback on platform card
Just a reminder that I'm still not a fan of this design. Two seemingly
unrelated Front Ends end up accessing same resource and blocking each
other, that's why there is a need for above kcontrol interface.
In my opinion the fact that audio is routed through different HW paths,
should not require separate Front Ends. It should start on same FE as it
ends on one device. It would be a lot more user friendly that when end
user opens USB FE it will just happen automatically, with current
implementation user space needs to be aware of two separate front ends
and internal dependency between them based on kcontrol value. If there
is need to selectively disable it due to problems with some HW or in
testing, just provide simple kcontrol on whole USB device with values
"auto"(default), "force" (to always force offload), "off" (to disable it).
