linux-kernel - Re: [PATCH] drm/vc4: hdmi: Avoid sleeping in atomic context

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Message-ID: <20201027213447.3fhnkqqik3pbhw5d@gilmour.lan>
Date:   Tue, 27 Oct 2020 22:34:47 +0100
From:   Maxime Ripard <maxime@...no.tech>
To:     Liam Girdwood <lgirdwood@...il.com>,
        Mark Brown <broonie@...nel.org>,
        Jaroslav Kysela <perex@...ex.cz>, Takashi Iwai <tiwai@...e.com>
Cc:     alsa-devel@...a-project.org, linux-kernel@...r.kernel.org,
        Dave Stevenson <dave.stevenson@...pberrypi.com>,
        Tim Gover <tim.gover@...pberrypi.com>,
        Phil Elwell <phil@...pberrypi.com>,
        Dom Cobley <dom@...pberrypi.com>,
        dri-devel@...ts.freedesktop.org,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Thomas Zimmermann <tzimmermann@...e.de>
Subject: Re: [PATCH] drm/vc4: hdmi: Avoid sleeping in atomic context

On Tue, Oct 27, 2020 at 11:15:58AM +0100, Maxime Ripard wrote:
> When running the trigger hook, ALSA by default will take a spinlock, and
> thus will run the trigger hook in atomic context.
> 
> However, our HDMI driver will send the infoframes as part of the trigger
> hook, and part of that process is to wait for a bit to be cleared for up to
> 100ms. To be nicer to the system, that wait has some usleep_range that
> interact poorly with the atomic context.
> 
> There's several ways we can fix this, but the more obvious one is to make
> ALSA take a mutex instead by setting the nonatomic flag on the DAI link.
> That doesn't work though, since now the cyclic callback installed by the
> dmaengine helpers in ALSA will take a mutex, while that callback is run by
> dmaengine's virt-chan code in a tasklet where sleeping is not allowed
> either.
> 
> Given the delay we need to poll the bit for, changing the usleep_range for
> a udelay and keep running it from a context where interrupts are disabled
> is not really a good option either.
> 
> However, we can move the infoframe setup code in the hw_params hook, like
> is usually done in other HDMI controllers, that isn't protected by a
> spinlock and thus where we can sleep. Infoframes will be sent on a regular
> basis anyway, and since hw_params is where the audio parameters that end up
> in the infoframes are setup, this also makes a bit more sense.
> 
> Fixes: bb7d78568814 ("drm/vc4: Add HDMI audio support")
> Suggested-by: Mark Brown <broonie@...nel.org>
> Signed-off-by: Maxime Ripard <maxime@...no.tech>

Applied to drm-misc-fixes

Maxime

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