Hi Pierre,
On 10/17/2023 1:58 PM, Pierre-Louis Bossart wrote:
It's been a very long time since I reviewed earlier versions, and I am
still lost on terminology and concepts. The explanations below should
really be added as a .rst file in Documentation for reference, not just
as a cover letter.
Thanks for the review!
Sure, maybe I can write a more comprehensive documentation that saves
these details somewhere. Will add a RST documentation for material
where necessary.
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
"backend" is a loaded terms for ASoC. Can you clarify which part of the
ascii art below is a 'backend'?
This would be the Q6USB entity which is the DPCM backend for this
particular audio path.
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.
What is a 'port'? USB refers to "interfaces" and "endpoints". Is a
"port" a 1:1 mapping to "endpoint"?
Below I read "AFE port" so not sure what concepts refer to what.
"Port" in this explanation refers to the USB port. So the audio device
connected. You are right that a USB device can enumerate w/ multiple
interfaces (ie UAC + HID + ...) so the closest relation to "port" is
"interface." It is not a 1:1 mapping w/ the number of endpoints exposed
by a device.
"AFE port" is just something that has been termed from the audio DSP
end, so that concept of port is not related to the port where USB
devices are connected to. This is something that is defined within the
audio DSP.
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
The asciiart above suggests that q6usb exposes a codec DAI - but the
backend dailink is created by the platform card?
That is my understanding, at least. The machine driver (sm8250.c) has
an API that parses our DT node for the DAI relationships
(qcom_snd_parse_of()), and seems to also create the dailinks.
resources needed to initiate a playback stream using the Q6DSP. Will
is capture supported? there's explicit references to "num_capture" in
following patches.
We (QCOM) do not support capture at the moment, but the num_capture was
added to the soc-usb driver, which is platform independent. This is in
case other vendors want to add this support, but have capture included
as well.
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.
what about capture, e.g. for a headset?
Not supported yet at the moment.
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:
typo: platform
Thanks, will fix this.
0 [SM8250MTPWCD938]: sm8250 - SM8250-MTP-WCD9380-WSA8810-VA-D
SM8250-MTP-WCD9380-WSA8810-VA-DMIC
How do you plan on exposing the USB PCM device?
The lines above are really cryptic, and with no USB reference in any of
the short/long card names it's not obvious that this card is different
from the no-offload case, is it?
In the end, since the offload case is handled by the audio DSP, it would
have to go through the platform/machine sound card. That is the sm8250
device above.
1 [Audio ]: USB-Audio - USB Audio
Generic USB Audio at usb-xhci-hcd.1.auto-1.4, high speed
likewise some sort of qualifier would be useful to show that card 0 and
card 1 can target the same USB endpoints.
Do userspace entities look at this card string? Assuming there is only
one platform card, there are situations where maybe multiple USB audio
devices are connected to the same USB root hub, so offloading can happen
on any one of them (not at the same time).
Thanks
Wesley Cheng
