linux-kernel - Re: [PATCH v9 34/34] ASoC: usb: Rediscover USB SND devices on USB port add

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Date:   Tue, 24 Oct 2023 08:35:06 -0500
From:   Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
To:     Wesley Cheng <quic_wcheng@...cinc.com>, mathias.nyman@...el.com,
        gregkh@...uxfoundation.org, lgirdwood@...il.com,
        broonie@...nel.org, perex@...ex.cz, tiwai@...e.com,
        agross@...nel.org, andersson@...nel.org, konrad.dybcio@...aro.org,
        robh+dt@...nel.org, krzysztof.kozlowski+dt@...aro.org,
        conor+dt@...nel.org, srinivas.kandagatla@...aro.org,
        bgoswami@...cinc.com, Thinh.Nguyen@...opsys.com
Cc:     linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        alsa-devel@...a-project.org, linux-arm-msm@...r.kernel.org,
        devicetree@...r.kernel.org
Subject: Re: [PATCH v9 34/34] ASoC: usb: Rediscover USB SND devices on USB
 port add



On 10/23/23 16:54, Wesley Cheng wrote:
> Hi Pierre,
> 
> On 10/17/2023 4:11 PM, Pierre-Louis Bossart wrote:
>>
>>
>> On 10/17/23 15:01, Wesley Cheng wrote:
>>> In case the USB backend device has not been initialized/probed, USB SND
>>> device connections can still occur.  When the USB backend is eventually
>>> made available, previous USB SND device connections are not
>>> communicated to
>>> the USB backend.  Call snd_usb_rediscover_devices() to generate the
>>> connect
>>> callbacks for all USB SND devices connected.  This will allow for the
>>> USB
>>> backend to be updated with the current set of devices available.
>>>
>>> The chip array entries are all populated and removed while under the
>>> register_mutex, so going over potential race conditions:
>>>
>>> Thread#1:
>>>    q6usb_component_probe()
>>>      --> snd_soc_usb_add_port()
>>>        --> snd_usb_rediscover_devices()
>>>          --> mutex_lock(register_mutex)
>>>
>>> Thread#2
>>>    --> usb_audio_disconnect()
>>>      --> mutex_lock(register_mutex)
>>>
>>> So either thread#1 or thread#2 will complete first.  If
>>>
>>> Thread#1 completes before thread#2:
>>>    SOC USB will notify DPCM backend of the device connection.  Shortly
>>>    after, once thread#2 runs, we will get a disconnect event for the
>>>    connected device.
>>>
>>> Thread#2 completes before thread#1:
>>>    Then during snd_usb_rediscover_devices() it won't notify of any
>>>    connection for that particular chip index.
>> Looks like you are assuming the regular USB audio stuff is probed first?
>>
>> What if it's not the case? Have you tested with a manual 'blacklist' and
>> "modprobe" sequence long after all the DSP stuff is initialized?
>>
>> It really reminds me of audio+display issues, and the same opens apply
>> IMHO.
> 
> Not necessarily...if the USB audio driver is not probed, then that is
> the same scenario as when there is no USB audio capable device plugged
> in, while the offload path is waiting for the connect event. I think
> this is the standard scenario.
> 
> In the situation where the platform sound card hasn't probed yet and USB
> audio devices are being identified, then that is basically the scenario
> that would be more of an issue, since its USB SND that notifies of the
> connection state (at the time of connect/disconnect).

Not following if this scenario is covered?

> I've tried with building these drivers as modules and probing them at
> different times/sequences, and I haven't seen an issue so far.

The scenario I have in mind is this:

the platform driver is on the deny list, the USB driver detects a
device. When the platform driver probes at a later time (with a manual
modprobe to make delays really long), how would the notification be handled?

Between audio and display, we use the 'drm_audio_component' layer to
model these sort of run-time binding between independent driver stacks.
It's not used here but we need a moral equivalent, don't we?

It would really help if you documented a bit more the dependencies or
timing assumptions, to make sure we have a stable solution to build on.