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

[Takashi Iwai]

On Sun, 12 Dec 2021 12:30:30 +0100,
Takashi Sakamoto wrote:
> 
> 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?

Hrm, it seems that the commit was lost by some reason.  Sorry!

> 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"

There was another similar typo in the patch description and I
corrected both.  Now applied to for-next, commit
fb6723daf89083a0d2290f3a0abc777e40766c84.


thanks,

Takashi