linux-kernel - Re: [PATCH 9/9] ASoC: cs35l41: Use devm_pm_runtime

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Message-ID: <20230906163725.GN103419@ediswmail.ad.cirrus.com>
Date:   Wed, 6 Sep 2023 16:37:25 +0000
From:   Charles Keepax <ckeepax@...nsource.cirrus.com>
To:     Cristian Ciocaltea <cristian.ciocaltea@...labora.com>
CC:     James Schulman <james.schulman@...rus.com>,
        David Rhodes <david.rhodes@...rus.com>,
        Richard Fitzgerald <rf@...nsource.cirrus.com>,
        Jaroslav Kysela <perex@...ex.cz>,
        Takashi Iwai <tiwai@...e.com>,
        "Liam Girdwood" <lgirdwood@...il.com>,
        Mark Brown <broonie@...nel.org>,
        "Stefan Binding" <sbinding@...nsource.cirrus.com>,
        <alsa-devel@...a-project.org>, <patches@...nsource.cirrus.com>,
        <linux-kernel@...r.kernel.org>, <kernel@...labora.com>
Subject: Re: [PATCH 9/9] ASoC: cs35l41: Use devm_pm_runtime_enable()

On Tue, Sep 05, 2023 at 10:15:46PM +0300, Cristian Ciocaltea wrote:
> On 9/5/23 12:45, Charles Keepax wrote:
> > On Sun, Sep 03, 2023 at 12:06:21AM +0300, Cristian Ciocaltea wrote:
> >> Simplify runtime PM during probe by converting pm_runtime_enable() to
> >> the managed version.
> >>
> >> Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@...labora.com>
> >> ---
> >> @@ -1376,7 +1379,6 @@ void cs35l41_remove(struct cs35l41_private *cs35l41)
> >>  	cancel_work_sync(&cs35l41->mdsync_up_work);
> >>  
> >>  	pm_runtime_get_sync(cs35l41->dev);
> >> -	pm_runtime_disable(cs35l41->dev);
> >>  
> >>  	regmap_write(cs35l41->regmap, CS35L41_IRQ1_MASK1, 0xFFFFFFFF);
> >>  	if (cs35l41->hw_cfg.bst_type == CS35L41_SHD_BOOST_PASS ||
> > 
> > Are we sure this is safe? The remove handler appears to be
> > written to disable pm_runtime at the start presumably to stop the
> > resume/suspend handler running during the remove callback.
> > Whereas after this change the pm_runtime isn't disabled until
> > after the remove callback has run. Does this open a window were
> > we could get an erroneous pm_runtime suspend after the
> > pm_runtime_put_noidle?
> 
> I've just made a test adding a 6s sleep before returning from the remove 
> handler: 
> 
> [14444.894316] cs35l41 spi-VLV1776:00: Runtime resume
> [14444.894469] cs35l41 spi-VLV1776:00: sleep 6s before return of cs35l41_remove()
> [14448.338994] cs35l41 spi-VLV1776:00: Runtime suspend
> [14451.079649] cs35l41 spi-VLV1776:00: return from cs35l41_remove()
> [14451.080129] cs35l41 spi-VLV1776:00: Runtime resume
> [14451.080165] cs35l41 spi-VLV1776:00: ASoC: Unregistered DAI 'cs35l41-pcm'
> [14451.080181] cs35l41 spi-VLV1776:00: Runtime suspend
> [14451.813639] acp5x_i2s_playcap acp5x_i2s_playcap.0: ASoC: Unregistered DAI 'acp5x_i2s_playcap.0'
> 
> As expected, suspend triggered, but a resume was issued later, before DAI
> got unregistered.
> 
> I didn't notice any issues while repeating the test several times, hence 
> I wonder what would be the reason to prevent getting suspend/resume events 
> at this point?

The enter/exit hibernate code might run, which at the very
least might result in a bunch of unexpected and failing bus
traffic. Having a bit of a poke through the code, I guess the
most dangerous thing would if you actually got as far as an
extra runtime resume. This might cause cs35l41_init_boost
to run which would undo the work done by the call to
cs35l41_safe_reset in remove, which could leave the boost in a
dangerous state when we enable reset/power down the supplies,
which I think was not considered good. But its just likely
simpler/cleaner if we don't have to think about all the
possible implications of such things by just not allowing
it to happen.

Thanks,
Charles