linux-kernel - Re: [PATCHv2] ASoC: fsl: imx-ssi: omit ssi counter to avoid harm in unbalanced situation

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Date:	29 Nov 2013 14:49:25 +0000
From:	"Oskar Schirmer" <oskar@...ra.com>
To:	"Liam Girdwood" <lgirdwood@...il.com>,
	" Fabio Estevam" <fabio.estevam@...escale.com>,
	" Mark Brown" <broonie@...nel.org>
Cc:	"Sascha Hauer" <s.hauer@...gutronix.de>,
	alsa-devel@...a-project.org, linux-kernel@...r.kernel.org,
	stable@...nel.org, "	Andrew Morton" <akpm@...ux-foundation.org>
Subject: Re: [PATCHv2] ASoC: fsl: imx-ssi: omit ssi counter to avoid harm in unbalanced situation

Someone around who would like to add his acked-by?

thanks,
  Oskar

On Sat, Nov 16, 2013 at 07:52:25 +0000, Oskar Schirmer wrote:
> Unbalanced calls to imx_ssi_trigger() may result in endless
> SSI activity and thus provoke eternal sound. While on the first glance,
> the switch statement looks pretty symmetric, the SUSPEND/RESUME
> pair is not: the suspend case comes along snd_pcm_suspend_all(),
> which for fsl/imx-pcm-fiq is called only at snd_soc_suspend(),
> but the resume case originates straight from the SNDRV_PCM_IOCTL_RESUME.
> This way userland may provoke an unbalanced resume, which might cause
> the ssi->enabled counter to increase and never return to zero again,
> so eventually SSI_SCR_SSIEN is never disabled.
> 
> As the information on whether to enable the SSI or not is contained
> in the two bits for TE/RE, we save all the software mirroring of
> hardware state here and simply use the hardware register itself
> to keep the state of whether someone is currently playing or capturing.
> 
> This is essentially the same stuff as in sound/soc/fsl/imx-pcm-fiq.c
> which I send a patch for three days ago. Astonishing enough this
> highly fragile scheme is used twice in parallel to serve the very
> same control function, synchronously: Once out of sync you are lost
> until reboot.
> 
> Note, that these fixes wont prevent state machine distortion on alsa
> level to cut sound or the like. It just makes sure we have a chance
> to synchronise again later on.
> 
> Signed-off-by: Oskar Schirmer <oskar@...ra.com>
> ---
> v2: drop all software flag handling in favour of hardware registers
> 
>  sound/soc/fsl/imx-ssi.c |    5 ++---
>  sound/soc/fsl/imx-ssi.h |    1 -
>  2 files changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/sound/soc/fsl/imx-ssi.c b/sound/soc/fsl/imx-ssi.c
> index f5f248c..2eb2691 100644
> --- a/sound/soc/fsl/imx-ssi.c
> +++ b/sound/soc/fsl/imx-ssi.c
> @@ -304,8 +304,7 @@ static int imx_ssi_trigger(struct snd_pcm_substream *substream, int cmd,
>  			scr |= SSI_SCR_RE;
>  		sier |= sier_bits;
>  
> -		if (++ssi->enabled == 1)
> -			scr |= SSI_SCR_SSIEN;
> +		scr |= SSI_SCR_SSIEN;
>  
>  		break;
>  
> @@ -318,7 +317,7 @@ static int imx_ssi_trigger(struct snd_pcm_substream *substream, int cmd,
>  			scr &= ~SSI_SCR_RE;
>  		sier &= ~sier_bits;
>  
> -		if (--ssi->enabled == 0)
> +		if (!(scr & (SSI_SCR_TE | SSI_SCR_RE)))
>  			scr &= ~SSI_SCR_SSIEN;
>  
>  		break;
> diff --git a/sound/soc/fsl/imx-ssi.h b/sound/soc/fsl/imx-ssi.h
> index 560c40f..be65623 100644
> --- a/sound/soc/fsl/imx-ssi.h
> +++ b/sound/soc/fsl/imx-ssi.h
> @@ -213,7 +213,6 @@ struct imx_ssi {
>  
>  	int fiq_init;
>  	int dma_init;
> -	int enabled;
>  };
>  
>  #endif /* _IMX_SSI_H */
> -- 
> 1.7.9.5
> 
> 
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