ALSA: pcm: comment about relation between msbits hw parameter and [S|U]32 formats

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|U]24 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.

[ corrected typos -- tiwai ]
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20210529033353.21641-1-o-takashi@sakamocchi.jpSigned-off-by: Takashi Iwai <tiwai@suse.de>

include/sound/pcm.h

@@ -147,6 +147,9 @@ struct snd_pcm_ops {
 #define SNDRV_PCM_FMTBIT_S24_BE		_SNDRV_PCM_FMTBIT(S24_BE)
 #define SNDRV_PCM_FMTBIT_U24_LE		_SNDRV_PCM_FMTBIT(U24_LE)
 #define SNDRV_PCM_FMTBIT_U24_BE		_SNDRV_PCM_FMTBIT(U24_BE)
+// For S32/U32 formats, 'msbits' hardware parameter is often used to deliver information about the
+// available bit count in most significant bit. It's for the case of so-called 'left-justified' or
+// `right-padding` sample which has less width than 32 bit.
 #define SNDRV_PCM_FMTBIT_S32_LE		_SNDRV_PCM_FMTBIT(S32_LE)
 #define SNDRV_PCM_FMTBIT_S32_BE		_SNDRV_PCM_FMTBIT(S32_BE)
 #define SNDRV_PCM_FMTBIT_U32_LE		_SNDRV_PCM_FMTBIT(U32_LE)

include/uapi/sound/asound.h

@@ -202,6 +202,9 @@ typedef int __bitwise snd_pcm_format_t;
 #define	SNDRV_PCM_FORMAT_S24_BE	((__force snd_pcm_format_t) 7) /* low three bytes */
 #define	SNDRV_PCM_FORMAT_U24_LE	((__force snd_pcm_format_t) 8) /* low three bytes */
 #define	SNDRV_PCM_FORMAT_U24_BE	((__force snd_pcm_format_t) 9) /* low three bytes */
+// For S32/U32 formats, 'msbits' hardware parameter is often used to deliver information about the
+// available bit count in most significant bit. It's for the case of so-called 'left-justified' or
+// `right-padding` sample which has less width than 32 bit.
 #define	SNDRV_PCM_FORMAT_S32_LE	((__force snd_pcm_format_t) 10)
 #define	SNDRV_PCM_FORMAT_S32_BE	((__force snd_pcm_format_t) 11)
 #define	SNDRV_PCM_FORMAT_U32_LE	((__force snd_pcm_format_t) 12)