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@xxxxxxxxxxxxx> Thanks, applied. Takashi
