[alsa-devel] hdmi-codec: modifying params in hw_params() callback

[Russell King - ARM Linux admin]

On Thu, Feb 28, 2019 at 04:35:29PM +0200, Jyri Sarha wrote:
> On 28/02/2019 15:10, Russell King - ARM Linux admin wrote:
> > On Thu, Feb 28, 2019 at 02:54:29PM +0200, Jyri Sarha wrote:
> >> On 28/02/2019 14:15, Russell King - ARM Linux admin wrote:
> >>> Hi all,
> >>>
> >>> While looking at hdmi-codec issues, I spotted this code:
> >>>
> >>> static int hdmi_codec_hw_params(struct snd_pcm_substream *substream,
> >>>                                 struct snd_pcm_hw_params *params,
> >>>                                 struct snd_soc_dai *dai)
> >>> {
> >>> ...
> >>>         if (params_width(params) > 24)
> >>>                 params->msbits = 24;
> >>>
> >>> params->msbits is a parameter that is negotiated and refined by
> >>> libasound, and is part of the ALSA constraint system.  The "Writing an
> >>> ALSA driver" documentation says about the hw_params callback:
> >>>
> >>> "This is called when the hardware parameter (``hw_params``) is set up
> >>>  by the application, that is, once when the buffer size, the period
> >>>  size, the format, etc. are defined for the pcm substream."
> >>>
> >>> which suggests we should only be reading the parameters, not writing
> >>> to them.
> >>>
> >>> What's more is that the msbits is a parameter that is refined with
> >>> userspace, so surely the above should be a declared constraint?
> >>>
> >>
> >> Certainly not a constraint. While HDMI can only pass up to 24-bit per
> >> sample audio the most (or at least the two I have worked with) encoders
> >> can take 32-bit wide (and probably wider) samples trough i2s. So the
> >> system can play 32-bit samples, just the 4 LSB is ignored.
> > 
> > You have not explained why "not a constraint".  From the explanation
> > that Mark gave me on IRC, msbits is the number of bits that the codec
> > respects, whereas the sample bits is the number of bits in the sample.
> > If sample bits - msbits != 0, then that is the number of bits ignored.
> > Mark basically described it as "feel free to send me more than 24 bits
> > but I'm ignoring the LSBs".
> > 
> > If the msbits is limited to 24, then userspace gets to know about that
> > limitation when negotiating with ALSA if it is a constraint.  If it
> > isn't a constraint, userspace has no way to know.
> > 
> >> Actually there is very little difference between the case of playing
> >> 32-bit stereo audio with matching i2s bclk ratio, and playing 24-bit
> >> stereo audio with 64-bit bclk ratio. Especially when the 4 LSB is anyway
> >> ignored like in HDMI audio case.
> > 
> > You mean 8 LSB throughout the above two paragraphs.
> > 
> >>> Digging a bit deeper, ASoC passes a private copy of the params to each
> >>> codec - a copy is made, then fixups for TDM slots are applied, followed
> >>> by any topology fixups by the DAI link (be_hw_params_fixup) before the
> >>> codec driver's hw_params() callback is made.  Afterwards, ASoC reads
> >>> back the rate, channels and physical (memory) width and stores them
> >>> in the codec's DAI structure.  The msbits are not read.
> >>>
> >>> hdmi-codec also seems to do nothing with the msbits parameter other
> >>> than the above code.
> >>>
> >>> So, all in all, it seems that the above code limiting msbits is
> >>> redundant - nothing will read this modified value.  Can we kill it?
> >>>
> >>
> >> It certainly looks that way, so yes. In any case struct
> >> snd_soc_dai_driver .playback.sig_bits = 24 should be all that is needed.
> > 
> > This sets a constraint to limit msbits to 24 for any sample width
> > in core ASoC code, doing exactly what you said "certainly not" to
> > above.
> > 
> > It also means that the quoted code in hdmi-codec's hw_params() is
> > redundant for another reason.
> > 
> > See the description of:
> > snd_pcm_hw_constraint_msbits() in sound/core/pcm_lib.c,
> > soc_pcm_set_msb() and soc_pcm_apply_msb() in sound/soc/soc-pcm.c.
> > 
> > Can you please clarify, because there seems to be some confusion.
> > 
> 
> If I understand right, the only confusion here is around the term
> "constraint". I did not follow that you meant in strictly alsa-term. Yes
> we should set msbits pcm_lib constraint to 24 (as hdmi-codec aready
> does), because HDMI interface can not deliver more than 24 bits per
> sample, and any extra LSB bit will be ignored.

Right, so we can remove the above code then?

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