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

[Wesley Cheng]

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@xxxxxxxxxxx>
> > ---
> >   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.

Thanks
Wesley Cheng