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

[Wesley Cheng]

Hi Takashi,

On 7/25/2023 2:27 AM, Takashi Iwai wrote:
> On Tue, 25 Jul 2023 11:15:11 +0200,
> 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?
>
> That should be protected by the register_mutex.  But there can be
> other races (see below :)
>
> > 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.
>
> Yes, we need to be more careful about binding.
>
> For example, in the current patch set, I see no way to prevent
> unloading snd-usb-audio-qmi module, and it allows user to cut off the
> stuff during operation, which may break things while the kernel is
> running the code of the unloaded module.  You need to have a proper
> module refcount management for avoiding such a scenario.  Most of
> drivers don't need it because ALSA core part already takes care of
> it.  But in this case, it requires a manual adjustment.

Sorry for the delayed response...this was routed to another folder, and 
missed it.

I see, that is a valid situation, so I think the best way to address it 
is to do something like the following:

static void __exit qc_usb_audio_offload_exit(void)
{
...
	snd_usb_unregister_platform_ops();
	for (idx = 0; idx < SNDRV_CARDS; idx++)
		qc_usb_audio_offload_disconnect(uadev[idx].chip);

We'll first unregister the platform ops, so that we get no further 
connect/disconnect CBs, and then we'll forcefully ensure that all 
devices are removed/cleaned.  Part of the USB offload disconnect 
sequence will also forcefully stop the offload path on the external DSP 
by issuing a disconnect indication QMI message.

Then we can safely clean up the rest of the resources.  We do have 
refcounting in place for several of the other structures, but I think in 
the module exit case, we need to ensure the offload path is stopped fully.

Thanks
Wesley Cheng