[PATCH] ALSA: pcm: comment about relation between msbits hw parameter and [S|U32] formats

[Takashi Sakamoto]

Hi,

On Sun, May 30, 2021, at 16:32, Takashi Iwai wrote:
> On Sat, 29 May 2021 05:33:53 +0200,
> Takashi Sakamoto wrote:
>> 
>> Regarding to handling [U|S][32|24] PCM formats, many userspace
>> application developers and driver developers have confusion, since they
>> require them to understand justification or padding. It easily
>> loses consistency and soundness to operate with many type of devices. In
>> this commit, I attempt to solve the situation by adding comment about
>> relation between [S|U]32 formats and 'msbits' hardware parameter.
>> 
>> The formats are used for 'left-justified' sample format, and the available
>> bit count in most significant bit is delivered to userspace in msbits
>> hardware parameter (struct snd_pcm_hw_params.msbits), which is decided by
>> msbits constraint added by pcm drivers (snd_pcm_hw_constraint_msbits()).
>> 
>> In driver side, the msbits constraint includes two elements; the physical
>> width of format and the available width of the format in most significant
>> bit. The former is used to match SAMPLE_BITS of format. (For my
>> convenience, I ignore wildcard in the usage of the constraint.)
>> 
>> As a result of interaction between ALSA pcm core and ALSA pcm application,
>> when the format in which SAMPLE_BITS equals to physical width of the
>> msbits constaint, the msbits parameter is set by referring to the
>> available width of the constraint. When the msbits parameter is not
>> changed in the above process, ALSA pcm core set it alternatively with
>> SAMPLE_BIT of chosen format.
>> 
>> In userspace application side, the msbits is only available after calling
>> ioctl(2) with SNDRV_PCM_IOCTL_HW_PARAMS request. Even if the hardware
>> parameter structure includes somewhat value of SAMPLE_BITS interval
>> parameter as width of format, all of the width is not always available
>> since msbits can be less than the width.
>> 
>> I note that [S|U24] formats are used for 'right-justified' 24 bit sample
>> formats within 32 bit frame. The first byte in most significant bit
>> should be invalidated. Although the msbits exposed to userspace should be
>> zero as invalid value, actually it is 32 from physical width of format.
>> 
>> Signed-off-by: Takashi Sakamoto <o-takashi at sakamocchi.jp>
>
> Thanks, applied.
>
>
> Takashi

I can not find the patch in your tree. Would I ask you to review again?

If it should be going to be applied, I'd like you to fix my typo in the subject line;

 * "[S|U32]" -> "[S|U]32"

Regards

Takashi Sakamoto