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Date:   Tue, 17 Oct 2023 15:58:20 -0500
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Subject: Re: [PATCH v9 00/34] Introduce QC USB SND audio offloading support
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To:     Wesley Cheng <quic_wcheng@quicinc.com>, mathias.nyman@intel.com,
        gregkh@linuxfoundation.org, lgirdwood@gmail.com,
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        agross@kernel.org, andersson@kernel.org, konrad.dybcio@linaro.org,
        robh+dt@kernel.org, krzysztof.kozlowski+dt@linaro.org,
        conor+dt@kernel.org, srinivas.kandagatla@linaro.org,
        bgoswami@quicinc.com, Thinh.Nguyen@synopsys.com
Cc:     linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org,
        alsa-devel@alsa-project.org, linux-arm-msm@vger.kernel.org,
        devicetree@vger.kernel.org
References: <20231017200109.11407-1-quic_wcheng@quicinc.com>
From:   Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
In-Reply-To: <20231017200109.11407-1-quic_wcheng@quicinc.com>
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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.

> 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'?

>   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.

> 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?

> resources needed to initiate a playback stream using the Q6DSP.  Will

is capture supported? there's explicit references to "num_capture" in
following patches.

> 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?

> 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

>  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?

>  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.

> 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