From: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
To: Matt Flax <flatmax@flatmax.org>, alsa-devel@alsa-project.org
Subject: Re: ASoc: soc_core.c stream direction from snd_soc_dai
Date: Thu, 12 Mar 2020 17:55:36 -0500 [thread overview]
Message-ID: <0d9a9459-9cd7-d384-b1ff-72860895ad13@linux.intel.com> (raw)
In-Reply-To: <5598a2fc-9b49-ad5e-2265-fbc0593ead1b@flatmax.org>
On 3/11/20 5:54 PM, Matt Flax wrote:
> Hi there,
>
> A large number of audio codecs allow different formats for playback and
> capture. This becomes very useful when there are different latencies
> between playback and capture hardware data lines. For example digital
> isolation chips typically have a 1 bit delay in propagation as the bit
> clock rate gets faster for higher sample rates. By setting the capture
> and playback formats to differ by one or two bit clock cycles, the delay
> problem is solved.
>
> There doesn't seem to be a simple way to detect stream direction in the
> codec driver's set_fmt function.
>
> The snd_soc_runtime_set_dai_fmt function :
>
> https://github.com/torvalds/linux/blob/master/sound/soc/soc-core.c#L1480
>
> calls the snd_soc_dai_set_fmt function :
>
> https://github.com/torvalds/linux/blob/master/sound/soc/soc-dai.c#L101
>
> which calls the set_fmt function :
>
> https://github.com/torvalds/linux/blob/master/include/sound/soc-dai.h#L189
>
>
> The snd_soc_dai_ops set_fmt function is defined as :
>
> int (*set_fmt)(struct snd_soc_dai *dai, unsigned int fmt);
>
>
> Is there a simple way to find the stream direction from a snd_soc_dai ?
>
> If the stream direction can be detected then the playback and capture
> formats can be set independently for the codec.
>
> It there a different way to set the playback and capture formats for the
> codec independently at runtime, depending on the sample rate ?
FWIW I remember a discussion in the past on how to deal with interfaces
that may have different clocks sources for capture and playback
(typically with the 6-pin version of I2S/TDM), and the answer was: use
two dais, with one dealing with capture and the other with playback.
I would bet this applies for the format as well. If you use a DAI that
can do both directions, then indeed there's no obvious way to specify
that formats or clock ownership could be different between the two
directions.
It would probably make sense anyway to have a representation with two
dais, e.g. the codec capture dai receives data from somewhere and the
codec playback dai forwards it to another destination.
My 2 cents
-Pierre
next prev parent reply other threads:[~2020-03-12 22:56 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-11 22:54 ASoc: soc_core.c stream direction from snd_soc_dai Matt Flax
2020-03-12 22:55 ` Pierre-Louis Bossart [this message]
2020-03-12 23:19 ` Matt Flax
2020-03-13 3:56 ` Matt Flax
2020-03-13 9:59 ` Lars-Peter Clausen
2021-02-20 9:29 ` Shengjiu Wang
2021-02-23 13:57 ` Mark Brown
2021-02-26 5:58 ` Shengjiu Wang
2021-02-26 17:06 ` Mark Brown
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