Re: [RFC PATCH 00/14] Introduce QC USB SND audio offloading support

Wesley Cheng
2022-12-24T08:50:22Z

Hi Greg,

On 12/23/2022 10:45 PM, Greg KH wrote:
> On Fri, Dec 23, 2022 at 03:31:46PM -0800, 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
>>
>> 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
>
> This looks to duplicate a bunch of the same things that a number of
> different google developers have posted recently. Please work with them
> to come up with a unified set of patches that you all can agree with,
> AND get them to sign off on the changes before resubmitting them.
>
> This uncoordinated drip of patches from different people doing the same
> thing is almost impossible to review from our side, as I'm sure you can
> imagine.

I saw some of the Google patchsets submitted awhile back, but didn't
really get a chance to look at them in detail. Let me reach out to
Albert Wang to see if we can come to a solution that works for both
implementations.

From the looks of it (at least from the XHCI HCD changes), it seems
that a different set of resources is required for the Google
implementation to work. I'll need to ask for a bit more details before
I can comment further...

>
> That being said, thank you finally for at least submitting all of the
> needed changes together as one patch set. That's a first, and something
> we had been asking for for years.
>
> Have a good holiday break,

Thanks for the quick in-depth review, and the feedback. Gives me some
more things to think about improving over the break :). Happy holidays!

Thanks
Wesley Cheng