linux-kernel - Re: [PATCH] soundwire: bus_type: Avoid lockdep assert in sdw_drv

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Date:   Thu, 1 Dec 2022 14:32:26 +0000
From:   Richard Fitzgerald <rf@...nsource.cirrus.com>
To:     Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>,
        <vkoul@...nel.org>, <yung-chuan.liao@...ux.intel.com>,
        <sanyog.r.kale@...el.com>
CC:     <patches@...nsource.cirrus.com>, <alsa-devel@...a-project.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] soundwire: bus_type: Avoid lockdep assert in
 sdw_drv_probe()

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.