[PATCH v4 31/32] sound: usb: card: Allow for rediscovery of connected USB SND devices

[Pierre-Louis Bossart]

On 8/15/23 20:38, Wesley Cheng wrote:
> Hi Pierre,
> 
> On 7/25/2023 2:15 AM, Pierre-Louis Bossart wrote:
>>
>>
>> On 7/25/23 04:34, Wesley Cheng wrote:
>>> In case of notifying SND platform drivers of connection events, some of
>>> these use cases, such as offloading, require an ASoC USB backend
>>> device to
>>> be initialized before the events can be handled.  If the USB backend
>>> device
>>> has not yet been probed, this leads to missing initial USB audio device
>>> connection events.
>>>
>>> Expose an API that traverses the usb_chip array for connected
>>> devices, and
>>> to call the respective connection callback registered to the SND
>>> platform
>>> driver.
>>>
>>> Signed-off-by: Wesley Cheng <quic_wcheng at quicinc.com>
>>> ---
>>>   sound/usb/card.c | 19 +++++++++++++++++++
>>>   sound/usb/card.h |  2 ++
>>>   2 files changed, 21 insertions(+)
>>>
>>> diff --git a/sound/usb/card.c b/sound/usb/card.c
>>> index 365f6d978608..27a89aaa0bf3 100644
>>> --- a/sound/usb/card.c
>>> +++ b/sound/usb/card.c
>>> @@ -170,6 +170,25 @@ struct snd_usb_stream
>>> *snd_usb_find_suppported_substream(int card_idx,
>>>   }
>>>   EXPORT_SYMBOL_GPL(snd_usb_find_suppported_substream);
>>>   +/*
>>> + * in case the platform driver was not ready at the time of USB SND
>>> + * device connect, expose an API to discover all connected USB devices
>>> + * so it can populate any dependent resources/structures.
>>> + */
>>> +void snd_usb_rediscover_devices(void)
>>> +{
>>> +    int i;
>>> +
>>> +    mutex_lock(&register_mutex);
>>> +    for (i = 0; i < SNDRV_CARDS; i++) {
>>> +        if (usb_chip[i])
>>> +            if (platform_ops && platform_ops->connect_cb)
>>> +                platform_ops->connect_cb(usb_chip[i]);
>>
>> what happens if the USB device is removed while the platform device adds
>> a port?
>>
>> This sounds super-racy to me. It's the same set of problems we're having
>> between audio and display/DRM, I would be surprised if this function
>> dealt with all corner cases of insertion/removal, bind/unbind.
>>
> 
> The chip array entries are all populated and removed while under the
> register_mutex, so going over your race condition, we should see:
> 
> 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() we won't notify of any
> connection for that particular chip index.

You really want to capture this locking model as part of the commit
messages or code, it'll help reviewers figure things out.