Hi Pierre,
On 3/9/2023 9:13 AM, Pierre-Louis Bossart wrote:
On 3/8/23 17:57, Wesley Cheng wrote:
Changes in v3:
- Changed prefix from RFC to PATCH
- Rebased entire series to usb-next
- Updated copyright years
You may want to move the per-version changes after the description of
this patchset.
Thanks for reading through this :).
Sure, will do that going forward.
XHCI:
- Rebased changes on top of XHCI changes merged into usb-next, and only added
changes that were still under discussion.
- Added change to read in the "num-hc-interrupters" device property.
ASoC:
- qusb6 USB backend
- Incorporated suggestions to fetch iommu information with existing APIs
- Added two new sound kcontrols to fetch offload status and offload device
selection.
- offload status - will return the card and pcm device in use
tinymix -D 0 get 1 --> 1, 0 (offload in progress on card#1 pcm#0)
- device selection - set the card and pcm device to enable offload on. Ex.:
tinymix -D 0 set 1 2 0 --> sets offload on card#2 pcm#0
(this should be the USB card)
USB SND:
- Fixed up some locking related concerns for registering platform ops.
- Moved callbacks under the register_mutex, so that
- Modified APIs to properly pass more information about the USB SND device, so
that the Q6USB backend can build a device list/map, in order to monitor offload
status and device selection.
Changes in v2:
XHCI:
- Replaced XHCI and HCD changes with Mathias' XHCI interrupter changes
in his tree:
https://git.kernel.org/pub/scm/linux/kernel/git/mnyman/xhci.git/log/?h=feature_interrupters
Adjustments made to Mathias' changes:
- Created xhci-intr.h to export/expose interrupter APIs versus exposing xhci.h.
Moved dependent structures to this file as well. (so clients can parse out
information from "struct xhci_interrupter")
- Added some basic locking when requesting interrupters.
- Fixed up some sanity checks.
- Removed clearing of the ERSTBA during freeing of the interrupter. (pending
issue where SMMU fault occurs if DMA addr returned is 64b - TODO)
- Clean up pending events in the XHCI secondary interrupter. While testing USB
bus suspend, it was seen that on bus resume, the xHCI HC would run into a command
timeout.
- Added offloading APIs to xHCI to fetch transfer and event ring information.
ASoC:
- Modified soc-usb to allow for multiple USB port additions. For this to work,
the USB offload driver has to have a reference to the USB backend by adding
a "usb-soc-be" DT entry to the device saved into XHCI sysdev.
- Created separate dt-bindings for defining USB_RX port.
- Increased APR timeout to accommodate the situation where the AFE port start
command could be delayed due to having to issue a USB bus resume while
handling the QMI stream start command.
USB SND:
- Added a platform ops during usb_audio_suspend(). This allows for the USB
offload driver to halt the audio stream when system enters PM suspend. This
ensures the audio DSP is not issuing transfers on the USB bus.
- Do not override platform ops if they are already populated.
- Introduce a shared status variable between the USB offload and USB SND layers,
to ensure that only one path is active at a time. If the USB bus is occupied,
then userspace is notified that the path is busy.
---> start of the cover letter:
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
clarify what 'backend' means. I would guess you are referring to the
ASoC DPCM concept of Back-End, which typically exposes an audio
interface controlled by a DSP?
If yes, the description should be
"Create a Q6DSP ASoC DPCM BackEnd for USB audio playback/record".
You're correct. "backend" was referring to a Q6DSP ASoC DPCM backend.
3. Introduce XHCI interrupter support
4. Create vendor ops for the USB SND driver
Add an introduction as to why this would be needed? which 'vendor' are
we talking about, is this on the SOC side or the USB device side?
This would be referring to the SoC side. For example, QCOM can
potentially have a different implementation of offloading USB SND to the
audio DSP versus another OEM. This is to allow those other entities to
be able to define their own offload methodology.
For the most part, the common operations across all offloading
implementations have so far is being able to handle USB SND
connect/disconnect events.
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.
port == backend?
Using different words for the same concept is a sure way to lose the
reviewers, and not describing different concepts isn't much better.
Yes, port is corresponding to the
Create a USB backend for Q6DSP:
q6usb: Basic backend driver that will be responsible for maintaining the
What does this mean? Is this a module that registers a set of ASoC DAIs?
It's not clear to me what a 'backend driver' means either in a USB or
ASoC context.
Will fix this up next time. It refers to a Q6DSP DPCM ASoC backend, as
you mentioned above.
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.
If it's an ASoC backend, there is no PCM open call. You're probably
referring to the hw_params, not the open/startup anyways.
Correct. Will fix this.
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
so how does this 'qc_audio_offload' interface with 'q6usb' described
above? how are the roles different or complementary?
So in general you can think that the qc_audio_offload is a complement to
the USB SND USB class driver, while q6usb is to ASoC. Since the ASoC
framework doesn't have any communication with USB SND, the ASoC DPCM USB
backend (q6usb) will have to be the entity that maintains what is going
on in USB SND. That way, sessions initiated through the ASoC managed
sound card can evaluate what is available based on information reported
by q6usb.
qc_audio_offload and q6usb will have some interaction between each
other. The majority of communication between qc_audio_offload and q6usb
is reporting the device connection events.
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...
I already voiced my concerns about exposing two cards, each with their
own set of volume controls with the same device. It would be much better
to have an additional offloaded PCM device for card0...
But if the consensus is to have two cards, it's still not clear how the
routing would be selected. In the case where there are two USB audio
devices attached, the offloaded path would only support one of the two.
How would userspace know which of the two is selected?
With patch#24:
https://lore.kernel.org/linux-usb/20230308235751.495-25-quic_wcheng@xxxxxxxxxxx/T/#u
Now, userspace can at least choose which device it wants to offload.
Part of doing that would mean userspace knows what USB SND card devices
are available, so it is aware of which devices are shared (between the
offload and USB SND path)
And how would userspace know the difference anyways between two physical
devices attached to the platform with no offload, and one physical
device with one additional offload path? The names you selected can't be
used to identify that card1 is the optimized version of card0.
Is userspace currently able to differentiate between cards that are
created by USB SND versus ASoC? How complex can the userspace card
discovery be? Can it query kcontrols at this point in time? If so,
maybe we can change the names of the newly added ones to reflect that it
is an offload device?
SND kcontrol names are currently:
Q6USB offload status
Q6USB offload SND device select
Before we review low-level kernel plumbing, it would be good to give a
better overview of how userspace applications are supposed to interact
with the cards and identify the offloaded path. Testing with
tinyplay/tinymix is fine, but that's a developer-level or CI unit test.
we've got to see the broader picture of how a sound server would use
this USB offload capability.
Sure, I think that is fine. I was hoping that at least adding some of
the new kcontrols would help userspace make use of this path in general,
but we can add more information if required.
Thanks
Wesley Cheng
