[alsa-devel] [RFC PATCH] ASoC: pcm512x: Implement the set_bclk_ratio interface

[Peter Rosin]

On 2019-01-15 12:11, Dimitris Papavasiliou wrote:
> On Tue, Jan 15, 2019 at 10:06 AM Peter Rosin <peda at axentia.se> wrote:
>> ...and when double-checking that, this dai op is only ever called from
>> snd_soc_dai_set_bclk_ratio, and that function is in turn not called from
>> anywhere AFAICT. What's the point of this dai op anyway? Or, what am I
>> missing?
> 
> Well, my use for this, would be to allow the machine driver to call
> snd_soc_dai_set_bclk_ratio, in order to explicitly set the frame
> width.  This would allow it to use 32-bit frames for 24-bit data and
> avoid the fractional dividers that would otherwise result.  Right now,
> the machine driver (which lives in the Raspberry Pi kernel tree) uses
> a workaround for this, that relies on a patch made to the CODEC driver
> to half-support TDM slots, which hasn't be pushed upstream, and
> wouldn't be accepted if it had, since it's essentially a hack.  You
> can find more details here:
> 
> http://mailman.alsa-project.org/pipermail/alsa-devel/2018-December/143507.html
> 
> What the intended use for the snd_soc_dai_set_bclk_ratio is, I'm not
> so clear about, hence this RFC.

I did some digging. The dai op was added in 3.13 (without users), then one
user appeared in 3.19 (sound/soc/intel/boards/bytcr_rt5640.c) and another
(presumably copy-pasted) in 4.5 (sound/soc/intel/boards/bytcr_rt5651.c).
Both these users are suspect since neither the rt5640 nor the rt5651 codec
have ever sported a set_bclk_ratio dai op, and presumably the boards have
the named codecs on them. But what do I know...

The user added in 3.19 was removed for 4.9 and the other for 4.17, so
that's why there are no upstream users at the moment.

Cheers,
Peter