Message-ID: <0810f951-f4a6-a51d-97e3-43691b05f702@quicinc.com>
Date:   Mon, 13 Mar 2023 18:42:54 -0700
From:   Wesley Cheng <quic_wcheng@...cinc.com>
To:     Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>,
        <srinivas.kandagatla@...aro.org>, <mathias.nyman@...el.com>,
        <perex@...ex.cz>, <broonie@...nel.org>, <lgirdwood@...il.com>,
        <krzysztof.kozlowski+dt@...aro.org>, <agross@...nel.org>,
        <Thinh.Nguyen@...opsys.com>, <bgoswami@...cinc.com>,
        <andersson@...nel.org>, <robh+dt@...nel.org>,
        <gregkh@...uxfoundation.org>, <tiwai@...e.com>
CC:     <linux-kernel@...r.kernel.org>, <linux-arm-msm@...r.kernel.org>,
        <alsa-devel@...a-project.org>, <devicetree@...r.kernel.org>,
        <linux-usb@...r.kernel.org>, <quic_jackp@...cinc.com>,
        <quic_plai@...cinc.com>
Subject: Re: [PATCH v3 00/28] Introduce QC USB SND audio offloading support

Hi Pierre,

On 3/13/2023 5:42 PM, Pierre-Louis Bossart wrote:
> 
> 
> On 3/13/23 18:43, Wesley Cheng wrote:
>> Hi Pierre,
>>
>> On 3/9/2023 4:37 PM, Pierre-Louis Bossart wrote:
>>>
>>>>>> 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
>>>
>>> Humm, that is far from clear. I don't get how a something that interacts
>>> with the USB class driver can also be in charge of the audio DSP memory
>>> management.
>>>
>>
>> This is because the USB class driver is the entity which is going to
>> work with the USB HCD (XHCI) in this case to fetch the required
>> addresses, and map that into memory accessible by the audio DSP.  It
>> would be odd to be doing that from the q6usb end, which is part of the
>> ASoC layer.
>>
>>>> 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.
>>>
>>> It's already complicated to figure out how the DSP and USB class driver
>>> might interact and probe/timing dependencies, but with two additional
>>> drivers in the mix it's really hard to understand.
>>>
>>
>> I did test some cases based on existence of both these drivers
>> (qc_audio_offload and q6usb).  If either one doesn't exist in the
>> system, then the offload path would not work.  I did improve some of
>> these potential sequences in the latest revision, such as patch#28. This
>> would address scenarios where the q6usb ASoC DPCM backend wasn't probed,
>> while the USB SND (and qc_audio_offload) were still detecting device
>> connections.
>>
>> Once the Q6USB driver is probed, then the offload snd kcontrols would be
>> created, and devices would be properly identified with the rediscover api.
>>
>>> Maybe ascii-art would help describe the concepts and types of
>>> information exchanged. Maintaining a consistent state across multiple
>>> drivers is not an easy task.
>>>
>>
>> Hopefully this might help?  I know its a lot to read through.
> 
> It's very helpful! Thanks for spending the time to illustrate the
> different building blocks.
> 
>>
>>       USB                          |            ASoC
>> --------------------------------------------------------------------
>>                                    |  _________________________
>>                                    | |sm8250 platform card     |
>>                                    | |_________________________|
>>                                    |         |           |
>>                                    |      ___V____   ____V____
>>                                    |     |Q6USB   | |Q6AFE    |  #5
>>                                    |     |"codec" | |"cpu"    |
>>                                    |     |________| |_________|
>>                                    |         ^
>>                                    |         |  #6
>>                                    |      ___V____
>>                                    |     |SOC-USB |
>>    ________   #1  ________         #7    |        |
>>   |USB SND |<--->|QC offld|<------------>|________|
>>   |(card.c)|     |        |<----------       ^
>>   |________|     |________|___ #4  | |       |
>>       ^               ^       |    | |    ___V__________________
>>       | #2            |  #2   |    | |   |APR/GLINK             |
>>    __ V_______________V_____  |    | |   |______________________|
>>   |USB SND (endpoint.c)     | |    | |              ^
>>   |_________________________| |    | | #8           |
>>               ^               |    | |   ___________V___________
>>               | #3            |    | |->|audio DSP              |
>>    ___________V_____________  |    |    |_______________________|
>>   |XHCI HCD                 |<-    |
>>   |_________________________|      |
>>
>>
>> #1 - USB SND and QC offload:
>> Initialization:
>> - Register platform operations, to receive connect/disconnect events
>>    from USB SND.
>> - QC offload creates a QMI handle, in order to receive QMI requests
>>    from the audio DSP.
>>
>> Runtime:
>> - USB SND passes along "struct snd_usb_audio" in order for QC offload
>>    to reference USB UAC desc parsing/USB SND helper APIs.
>> - USB device disconnection events will result in clearing of the chip
>>    entry.
>>
>> #2 - USB SND and QC offload endpoints:
>> Runtime:
>> - In the non-offloaded path, USB snd will utilize functions exposed by
>>    USB SND endpoint, to help with fetching USB EP references and queuing
>>    URBs.
>> - In the offload path, qc offload will utilize the functions to fetch
>>    USB EP references, so that it can use that information to query the
>>    XHCI HCD.
>> - Likewise, both will clean up endpoints when audio stream is not in use.
>>
>> #3 - XHCI HCD:
>> Initialization:
>> - During XHCI probe timing, when the USB HCD is added to the system, it
>>    will also initialize the secondary event rings.
>>
>> Runtime:
>> - During USB device plug ins/outs, allocates device slot, assigns eps,
>>    and initializes transfer rings.
>>
>> #4 - QC offload and XHCI:
>> Runtime:
>> - QC offload needs to reference the transfer ring and secondary event ring
>>    addresses by executing XHCI offload management APIs.
>> - This happens when audio DSP receives a USB QMI stream request.
>>
>> #5 - ASoC components:
>> Initialization:
>> - The SM8250 platform sound card driver fetches DT node entries defining
>>    the ASoC links. This chain/link has the components involved for a
>>    particular Q6AFE path. (not only USB offload)
>>      - "cpu" - this is the ASoC CPU DAI that handles interaction with the
>>                Q6 DSP's audio protocol. (AFE ports)
>>      - "codec" - the ASoC codec (backend) DAI defined
>> - Registers ASoC platform sound card based on links defined in the DT node.
>>    - Probes DAI components involved, ie Q6USB and Q6AFE
>>
>> Runtime:
>> - Q6AFE has the bulk of the interaction w/ the audio DSP to start an audio
>>    session, such as issuing AFE port start commands (part of the protocol
>>    used to communicate the audio session info)
>> - Q6USB will be there to now check for if format requested is supported by
>>    the device, and maintain offloading status.
>>
>> #6 - Q6USB and SOC-USB:
>> Initialization:
>> - Q6USB will query QC offload for USB device connection states. (through
>>    soc-usb)
>> - Creates a SOC USB entry, that carries information about resources,
>>    such as audio DSP memory information and requested XHCI event ring
>>    index.
>>
>> Runtime:
>> - SOC-USB will receive connect/disconnect events and propagate to Q6USB.
>>    - Q6USB makes devices available for offloading based on these events.
>> - Sets Q6AFE port configurations to select the USB SND card# and PCM#.
>>
>> #7 - SOC-USB and QC offload:
>> Initialization:
>> - Rediscover USB SND devices when the SOC-USB entry is created (if needed)
>>      - For situations where the Q6USB DAI hasn't been probed.
>>
>> Runtime:
>> - Propagate connect/disconnect events.
> 
> Is the SOC-USB module or building blocks intended to be generic or
> Qualcomm agnostic?
> 

This should be generic.

> It's not clear to me how it would handle "audio DSP memory information
> and requested XHCI event ring index."
> 

Each soc-usb entry that is created by the ASoC DPCM backend DAI (q6usb) 
will be able to hold "private data" that, in QC case, is defined as:
struct q6usb_offload

This is passed within the snd_soc_usb_add_port() call:
snd_soc_usb_add_port(component->dev, &data->priv, q6usb_alsa_connection_cb);

So depending on the user, the private data can contain their own struct 
with the information they require.

> In addition, it seems to be the "bridge" or means of communication
> between qc_audio_offload and q6usb, is this not based on custom events
> or triggers?
> 

Ideally, no, it shouldn't be based on custom events.  Intention for the 
connect_cb() that is defined is just to receive USB device discovery 
events from USB SND.  From the qc_audio_offload, we call 
snd_soc_usb_connect() within our platform op that we register to USB SND.

//Platform connect_cb() - called from USB SND probe (device connected)
static void qc_usb_audio_offload_probe(struct snd_usb_audio *chip)
{
...
snd_soc_usb_connect(usb_get_usb_backend(udev), chip->card->number,
				chip->index, chip->pcm_devs);

In the QC situation, we used this to build a list of active devices 
connected.

> Along the same lines, this SOC-USB entity interfaces with APR/GLINK
> which doesn't speak to me so it must be a QCOM interface?
> 

Sorry for not labeling those in the diagram, but yes, those are QC 
specific interfaces.  You can just think of it as a type of IPC transport.

> I am trying to see if this design could be used for other architectures,
> and the QCOM-specific and generic parts are not obvious.
> 
>> #8 - audio DSP and QC offload:
>> Runtime:
>> - Handle QMI requests coming from audio DSP.  These requests come AFTER
>>    the Q6AFE port is opened by the Q6AFE DAI(#6)
>> - Returns memory information about resources allocated by XHCI.
>> - Enables audio playback when this QMI transaction is completed.
>>
>>>>
>>>>>> 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@quicinc.com/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
>>>
>>> I must admit I've never seen kcontrols being used to identify what the
>>> card is, and in this case it's a pretend-card that's just an improved
>>> version of another. It might be easier to use something else, such as
>>> the component strings.
>>
>> Its not exactly a pretend card, right?  This is part of the overall
>> platform sound card we have in the system.  At the moment, I'm only
>> testing by adding the USB audio routing, but there can be several ASoC
>> links defined in the overall platform card.
> 
> Sorry, I misunderstood the proposal. I thought there would be one card
> for "generic USB Audio", and another one for "DSP-offloaded USB Audio".
> I assumed, probably mistakenly, that all local audio endpoints
> (speaker,mics) would be exposed as a separate card.
> 

Ah got it.  No, that isn't the case here.

> It looks like it's more "generic USB Audio" and "DSP Audio", with the
> USB offload being exposed as a PCM device of the latter.
> 
> Did I get this right? In this case, presumably there can be some sort of

Yep that's correct!

> UCM file for the "DSP Audio card" that contains the configuration or
> knows which kcontrols to look for. But my point about detection hold.
> You could perfectly well have a 'Jack control' that tells userspace when
> a device is connected. That way there's no guess work, it's similar to
> HDMI for Intel: the device is exposed but only valid when the jack
> control is set.
> 

Hmm, ok.  Let me see if I can switch up some things.  Maybe replace the 
current snd_soc_dapm_enable_pin() calls in the q6usb connection_cb and 
replace that with a snd jack report. (the snd jack implementation 
already takes care of updating the pin if needed)

Thanks
Wesley Cheng

>> The Q6AFE CPU DAI has multiple audio AFE "ports" they can handle.  USB
>> is only one of those.
>>
>>>>
>>>>> 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.
>>>
>>> Can I ask if this solution has been used with a complete userspace stack
>>> already? I could see how this might be used with a relatively fixed
>>
>> Its been used only with the Android HAL.
> 
> ok, that aligns with what I was thinking.
> 
>>
>>> Android HAL, where the platform and routing are relatively controlled. I
>>> don't see how a more generic audio server would deal with the discovery
>>> and routing.
>>>
>>
>> This is why your input is helpful, since it provides another use case
>> that wasn't considered.  I think in our previous discussions the tagging
>> possibility was a good idea, and was hoping that it could help.  Could
>> tag all USB SND cards to the platform sound card as well, and if the
>> power saving path is chosen, it would issue the playback on the platform
>> sound card. (if not in use)
>>
>> In this case, the offload path wouldn't be the default routing, and only
>> enabled for power optimized path.
>>
>> Thanks
>> Wesley Cheng

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