[RFC PATCH 1/2] ASoC: simple-card: add support for bclk_ratio

[Sven Van Asbroeck]

On Tue, Feb 26, 2019 at 4:11 AM Russell King - ARM Linux admin
<linux at armlinux.org.uk> wrote:
>
> This code actually requires a lot more thought - while it may solve
> Sven's issue, it isn't generic.

Wholeheartedly agree. My patches are marked RFC in the hope that they will be
picked apart by folks much more in touch with the "big audio picture" than
I am. It's great to learn from this process.

> So far, I have this table put
> together from various sources of information:
>
>                         bclk_ratio
> sample width    current mcasp   fslssi  kirkwood
> 16              32      32      64      64
> 24              48      48      64      64
> 32              64      64      64      64
>
> Let's also consider whether it should depend on the number of channels.
> I2S uses a WS/LRCLK signal to differentiate each channel and to demark
> where the MSB bit is.
>
> If userspace negotiates one channel, what happens - if WS becomes
> static, then there is no signal indicating where the MSB bit is in
> the stream, so there is no way for a receiver to synchronise.  So,
> it is highly unlikely that bclk_ratio = channels * sample_width.
>
> If the signal continues toggling, what does it do for the inactive
> channel - is that just one BCLK period long or does it assume there
> is still the second channel?  If the former, it means we could end
> up with bclk_ratios of 17, 25, 33 which imho is unlikely, and
> would mess up TDA998x's CTS measurement.

Good point. i2s with a single channel makes no sense.

>
> What about setups where we have multiple I2S data signals (to support
> multi-channel audio) - if we have four channels transmitted on two
> data lines (as would be required by the TDA998x), that doesn't mean
> BCLK becomes sample_width * 4.
>
> So, I don't think the number of channels comes into the bclk_ratio
> calculation at all.  Given the above code, it effectively means we'd
> always specify channels = 2 to snd_soc_calc_frame_size().

I notice that hdmi-codec.c supports up to 8 channels in hdmi multi-channel
playback mode. If we had a _theoretical_ hdmi xmitter with 8chan support,
would the bclk_ratio not be 8 x slot_size - or 8 x 32 if using an fsl_ssi
in master mode?

This will of course never happen with the tda998x, because
        .max_i2s_channels = 2,
but we are thinking about a generic solution here.

>
> Given that it is normal to talk about I2S being clocked at "64fs",
> "32fs" etc, wouldn't it just be much neater to specify _this_ value
> in DT, rather than half that value?
>
> --
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