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

From: Jyri Sarha
Date: Thu Feb 28 2019 - 09:41:49 EST

On 28/02/2019 16:39, Russell King - ARM Linux admin wrote:
> 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?

Yes, you correctly pointed out that this piece of code does nothing and
is conceptually wrong:

if (params_width(params) > 24)
params->msbits = 24;

Yes. It can be removed.

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