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authorTakashi Sakamoto <o-takashi@sakamocchi.jp>2021-05-29 06:33:53 +0300
committerTakashi Iwai <tiwai@suse.de>2021-12-13 10:18:43 +0300
commitfb6723daf89083a0d2290f3a0abc777e40766c84 (patch)
treec14a7aa9b39ac70df1f8086b2106c6e649148b01 /include/sound/pcm.h
parentb73dad806533cad55df41a9c0349969b56d4ff7f (diff)
downloadlinux-fb6723daf89083a0d2290f3a0abc777e40766c84.tar.xz
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.jp Signed-off-by: Takashi Iwai <tiwai@suse.de>
Diffstat (limited to 'include/sound/pcm.h')
-rw-r--r--include/sound/pcm.h3
1 files changed, 3 insertions, 0 deletions
diff --git a/include/sound/pcm.h b/include/sound/pcm.h
index 33451f8ff755..9b187d86e1bd 100644
--- a/include/sound/pcm.h
+++ b/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)