linux-kernel - Re: [PATCH v4 2/2] ASoC: fsl_ssi: add 20-bit sample format for AC'97 and use it for capture

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Date:   Wed, 29 Nov 2017 23:23:58 -0800
From:   Nicolin Chen <nicoleotsuka@...il.com>
To:     "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
Cc:     Timur Tabi <timur@...i.org>, Xiubo Li <Xiubo.Lee@...il.com>,
        Jaroslav Kysela <perex@...ex.cz>,
        Takashi Iwai <tiwai@...e.com>,
        Fabio Estevam <fabio.estevam@....com>,
        Liam Girdwood <lgirdwood@...il.com>,
        Mark Brown <broonie@...nel.org>, alsa-devel@...a-project.org,
        linuxppc-dev@...ts.ozlabs.org,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Takashi Sakamoto <o-takashi@...amocchi.jp>
Subject: Re: [PATCH v4 2/2] ASoC: fsl_ssi: add 20-bit sample format for AC'97
 and use it for capture

Hi Maciej,

On Mon, Nov 27, 2017 at 11:34:44PM +0100, Maciej S. Szmigiero wrote:
> There is no problem in using different bit widths in playback and capture
> in AC'97 mode so allow this, too.

> @@ -1557,11 +1558,12 @@ static int fsl_ssi_probe(struct platform_device *pdev)
>  
>  	/* Are the RX and the TX clocks locked? */
>  	if (!of_find_property(np, "fsl,ssi-asynchronous", NULL)) {
> -		if (!fsl_ssi_is_ac97(ssi_private))
> +		if (!fsl_ssi_is_ac97(ssi_private)) {
>  			ssi_private->cpu_dai_drv.symmetric_rates = 1;
> +			ssi_private->cpu_dai_drv.symmetric_samplebits = 1;
> +		}
>  
>  		ssi_private->cpu_dai_drv.symmetric_channels = 1;
> -		ssi_private->cpu_dai_drv.symmetric_samplebits = 1;
>  	}

I was actually wondering how the AC97 works in the synchronous mode
while being able to handle different bit widths. Then I found that
the drivers does corresponding configurations for synchronous mode
only if symmetric_rates is set -- which is unset for AC97 cases. So
in fact AC97 case (symmetric_rates unset) is probably being treated
as asynchronous mode by the driver -- it'd be better if you confirm
this for me.

And I am not so sure about the physical pin connections in an AC97
situation, but I started to think that, instead of having a change
above, AC97 cases might be supposed to have "fsl,ssi-asynchronous"
property in DT since it's working when the driver sets both TX and
RX control registers (i.e. asynchronous mode), not like synchronous
mode that only sets TX's registers.

Thanks
Nicolin