Re: [PATCH] soundwire: bus_type: Avoid lockdep assert in sdw_drv_probe()

[Richard Fitzgerald]

On 29/11/2022 15:44, Pierre-Louis Bossart wrote:
> > > > Don't hold sdw_dev_lock while calling the peripheral driver
> > > > probe() and remove() callbacks.
> > > >
> > > > Holding sdw_dev_lock around the probe() and remove() calls
> > > > causes a theoretical mutex inversion which lockdep will
> > > > assert on. The peripheral driver probe will probably register
> > > > a soundcard, which will take ALSA and ASoC locks. During
> > >
> > > It's extremely unlikely that a peripheral driver would register a sound
> > > card, this is what machine drivers do.
> > >
> > > Which leads me to the question: is this a real problem?
> >
> > Yes, try turning on lockdep checking and you will get an assert.
> > During probe the existing code takes sdw_dev_lock and then calls the
> > codec driver probe, so you will get a mutex sequence like:
> >
> > sdw_dev_lock -> controls_rw_sem -> pcm_mutex
> >
> > but in normal operation the ALSA/ASoC code will take its mutexes first
> > and call runtime_resume which then takes the sdw_dev_lock, so you get
> >
> > pcm_mutex -> sdw_dev_lock
> >
> > and lockdep will assert on that opposite ordering.
> > The full assert is at the end of this email.
>
> Humm, you lost me with the reference to runtime_resume. I don't fully
> understand how it's possible to invoke pm_runtime during probe.
> pm_runtime should only enabled during the codec update_status() which
> can only be done once the probe completes.
>
> I am fine with the changes that you are suggesting, the introduction of
> the sdw_dev_lock was probably too conservative and it'd be fine to only
> protect what is required.
>
> However we do have lockdep enabled

I wonder whether this is because the Cirrus devices use full DP prepare,
so there will be a DP prepare interrupt during the attempt to prepare
the dailink. The lockdep assert was when sdw_update_slave_status() tried
to take sdw_dev_lock.

If the Realtek codecs only use Soundwire interrupts for jack detect you
probably won't see a sdw_dev_lock inside a pcm_mutex.