Re: hdmi-codec: modifying params in hw_params() callback

[Jyri Sarha]

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.


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