Re: [PATCH v3 00/28] Introduce QC USB SND audio offloading support

From: Wesley Cheng <quic_wcheng@quicinc.com>
To: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>,
	<srinivas.kandagatla@linaro.org>, <mathias.nyman@intel.com>,
	<perex@perex.cz>, <broonie@kernel.org>, <lgirdwood@gmail.com>,
	<krzysztof.kozlowski+dt@linaro.org>, <agross@kernel.org>,
	<Thinh.Nguyen@synopsys.com>, <bgoswami@quicinc.com>,
	<andersson@kernel.org>, <robh+dt@kernel.org>,
	<gregkh@linuxfoundation.org>, <tiwai@suse.com>
Cc: <linux-kernel@vger.kernel.org>, <linux-arm-msm@vger.kernel.org>,
	<alsa-devel@alsa-project.org>, <devicetree@vger.kernel.org>,
	<linux-usb@vger.kernel.org>, <quic_jackp@quicinc.com>,
	<quic_plai@quicinc.com>
Subject: Re: [PATCH v3 00/28] Introduce QC USB SND audio offloading support
Date: Wed, 15 Mar 2023 12:42:08 -0700	[thread overview]
Message-ID: <3d332f71-cccd-651a-88b1-9e33a81592e5@quicinc.com> (raw)
In-Reply-To: <6b811766-cd2a-54c0-d090-640812686a45@linux.intel.com>

Hi Pierre,

On 3/15/2023 7:30 AM, Pierre-Louis Bossart wrote:
> Hi Wesley,
> 
>> Sorry made a mistake on the diagram.  There is no connection from
>> SOC-USB to the APR/GLINK.  The Q6USB driver will be the one that is
>> going to configure some of the Q6AFE ports along withe the Q6AFE DAI
>> driver.
>>
>> |            ASoC
>> ----------------------------------
>> |  _________________________
>> | |sm8250 platform card     |
>> | |_________________________|
>> |         |           |
>> |      ___V____   ____V____
>> |     |Q6USB   | |Q6AFE    |  #5
>> |     |"codec" | |"cpu"    |
>> |     |________| |_________|
>> |         ^  ^        ^
>> |      #6 |  |________|
>> |      ___V____     |
>> |     |SOC-USB |    |
>> #7    |        |    |
>> ----->|________|    |
>> ---                 |
>> | |                 |
>> | |    _____________V________
>> | |   |APR/GLINK             |
>> | |   |______________________|
>> | |              ^
>> | | #8           |
>> | |   ___________V___________
>> | |->|audio DSP              |
>> |    |_______________________|
>> |
>> |
>>
>>>>
> 
> Makes sense now, thank you for the clarification.
> 
> I'll have to dig more in this 'soc-usb' block, it's clearly a key
> component that will have to maintain a consistent state across two
> different parts of the stack and deal with probe/remove/shutdown.
> 
>>> My initial thought was to add a 'DSP offload' PCM to the USB card, you
>>> added a "USB offload" PCM to the DSP card. Nice logical swap!
>>>
>>> Your proposal might be easier in practice since there's typically a
>>> vendor-specific configuration file (UCM or custom) file for the DSP,
>>> where USB information can be added.
>>>
>>> It's more problematic to change a generic USB card as we know it today
>>> and bolt vendor-specific DSP information on top.
>>>
>>> The only open I have with your option is that there are still two
>>> control paths to e.g. set the volume. It would be so much easier for
>>> userspace if there was a single volume control no matter what path is
>>> used for data, or make sure the kcontrols are 'mirrored' somehow. If we
>>> found a way to address this issue that would be ideal.
>>>
>>
>> Got it.  Let me look to see if that is something we can address/add.  I
>> think the current implementation is that USB SND will expose some mixer
>> controls based on the UAC descriptor parsing.  Then when they want to
>> change the volume (for example) it will result in a USB SETUP transaction.
>>
>> So USB SND will eventually be the entity controlling these changes.
> 
> That's probably ok then, am I getting this right that the the DSP card
> would not expose any USB-related kcontrols then, i.e. the ONLY path to
> change volumes, etc.,  would through the regular USB card kcontrols?
> 
> That would limit the changes in the platform sound card to the addition
> of a PCM USB device.

Yes, that's correct.  There won't be any exposed USB volume controls 
from the DSP card.

Thanks
Wesley Cheng