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

[Takashi Iwai <tiwai@suse.de>]

On Tue, 27 Oct 2020 11:15:58 +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@kernel.org>
> Signed-off-by: Maxime Ripard <maxime@cerno.tech>

Reviewed-by: Takashi Iwai <tiwai@suse.de>


thanks,

Takashi

> ---
>  drivers/gpu/drm/vc4/vc4_hdmi.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/vc4/vc4_hdmi.c b/drivers/gpu/drm/vc4/vc4_hdmi.c
> index 74da7c00ecd0..ec3ba3ecd32a 100644
> --- a/drivers/gpu/drm/vc4/vc4_hdmi.c
> +++ b/drivers/gpu/drm/vc4/vc4_hdmi.c
> @@ -1077,6 +1077,7 @@ static int vc4_hdmi_audio_hw_params(struct snd_pcm_substream *substream,
>  				    struct snd_soc_dai *dai)
>  {
>  	struct vc4_hdmi *vc4_hdmi = dai_to_hdmi(dai);
> +	struct drm_encoder *encoder = &vc4_hdmi->encoder.base.base;
>  	struct device *dev = &vc4_hdmi->pdev->dev;
>  	u32 audio_packet_config, channel_mask;
>  	u32 channel_map;
> @@ -1136,6 +1137,8 @@ static int vc4_hdmi_audio_hw_params(struct snd_pcm_substream *substream,
>  	HDMI_WRITE(HDMI_AUDIO_PACKET_CONFIG, audio_packet_config);
>  	vc4_hdmi_set_n_cts(vc4_hdmi);
>  
> +	vc4_hdmi_set_audio_infoframe(encoder);
> +
>  	return 0;
>  }
>  
> @@ -1143,11 +1146,9 @@ static int vc4_hdmi_audio_trigger(struct snd_pcm_substream *substream, int cmd,
>  				  struct snd_soc_dai *dai)
>  {
>  	struct vc4_hdmi *vc4_hdmi = dai_to_hdmi(dai);
> -	struct drm_encoder *encoder = &vc4_hdmi->encoder.base.base;
>  
>  	switch (cmd) {
>  	case SNDRV_PCM_TRIGGER_START:
> -		vc4_hdmi_set_audio_infoframe(encoder);
>  		vc4_hdmi->audio.streaming = true;
>  
>  		if (vc4_hdmi->variant->phy_rng_enable)
> -- 
> 2.26.2
>