[alsa-devel] [PATCH RFC 2/3] ASoC: hdmi-codec: add support for bclk_ratio

[Russell King - ARM Linux admin]

On Wed, Feb 27, 2019 at 07:48:33PM +0200, Jyri Sarha wrote:
> On 27/02/2019 13:47, Russell King - ARM Linux admin wrote:
> > On Mon, Feb 25, 2019 at 10:58:34PM +0200, Jyri Sarha wrote:
> >> I think there are other options too. The obvious one would be passing
> >> the blck_ratio callback trough HDMI-codec to HDMI-bridge-driver, but
> >> yes, I do not like that either.
> >>
> >> The other would be changing the implicit bclk_ratio to 2 x sample-width,
> >> and accepting blck_ratios of 36 and 40 in tda998x, and set CTS_N_M = 3
> >> and CTS_N_K = 2 for them too.
> > To put a bit of further information out there...
> > 
> > I really don't like that idea - what if we have a transmitter that
> > really does use a bclk_ratio of 36 or 40?  That would mess up the
> > CTS calculation in the TDA998x.
> > 
> > The equation I've come up to fit what we know for TDA998x is:
> > 
> > 	k = m * bclk_ratio / 128
> > 
> > where k and m are parameters that we values we select for TDA998x.
> > This reflects the entire clock regeneration system including the sink.
> > The possible values of k are 1, 2, 3, 4, or 8.  m are 1, 2, 4, or 8.
> > I also have this equation which defines the fs regenerated at the sink:
> > 
> > 	fso = bclk_ratio * fsi * m / (128 * k)
> > 
> > If we did have a ratio of 36 or 40, the first equation above fails
> > since k is not one of the possible integers.  If we round down and use
> > e.g. 2 for the 36fs case, then we'd actually end up with a sample
> > frequency on the sink of 1.125x faster than it should be, and the sink
> > would suffer starvation of audio samples.  If we rounded up to 3, then
> > the sample frequency will be 3/4 of it's true value, so the sink will
> > overflow and would have to discard audio samples.
> > 
> > We know this happens, because that's the root of the problem at hand:
> > wrongly setting the m and k values for the bclk_ratio being supplied
> > to the TDA998x causes audio to be corrupted when reproduced by the
> > sink.
> > 
> > Given that, we do need some way to validate the bclk_ratio when it is
> > set, and not during hw_params which would (a) lead to ALSA devices
> > failing when userspace is negotiating the parameters and (b) gives no
> > way in the kernel for set_bclk_ratio() to discover whether a particular
> > ratio is supported by the codec.
> > 
> > So, I think there's three possible ways around this:
> > 1. adding a set_bclk_ratio() method to hdmi_codec_ops
> > 2. calling hw_params() when our set_bclk_ratio() method is called
> >    (what if the rest of the format isn't set or incompatible, which
> >    may cause the hw_params() method to fail?)
> > 3. adding a list of acceptable bclk_ratio values to hdmi_codec_pdata
> 
> Or leaving the bclk_ratio field zero in struct hdmi_codec_params if
> set_bclk_ratio() is not called and leaving the bridge driver to decide
> what to do in such a situation.
> 
> And in tda998x_audio_hw_params() doing something like this:
> 
> if (audio.bclk_ratio == 0)
> 	audio.bclk_ratio = DIV_ROUND_UP(params->sample_width, 8) * 8;
> 
> But then again I think it would be quite sane option to set bclk_ration
> in hdmi-codec to 2*sample_width and simply refuse the ratios of 36 or 40
> in tda998x.

... which then leads to breakage of userspace if 18 or 20 bit formats
were attempted, making the bridge driver's capabilities undiscoverable.

Returning an error from hw_params causes hard failures.

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