Thanks a lot dear Takashi, helpful as the habit 2017-03-12 2:24 GMT+01:00 Takashi Sakamoto <o-takashi@xxxxxxxxxxxxx>: > Hi, > > On Mar 10 2017 01:23, Chemsi Mehdi wrote: > >> I am working with an alsa codec driver where the driver is calling >> err = snd_pcm_hw_constraint_mask64(substream->runtime, >> SNDRV_PCM_HW_PARAM_FORMAT, formats); >> my driver is already setting format before that so I wonder why this >> function is called? >> Looks like it is needed to add audio constraints to substreams at >> open/startup time, but why ? >> what can be the issue if I do not call it ? >> > > It's for user space applications. > > In protocol of ALSA PCM interface between kernel/userspace, parameters of > PCM substream are decided via interaction between drivers and applications. > Typically, applications keep memory object in user space for 'struct > snd_pcm_hw_params' and set their preferable parameters to it, then call > ioctl(2) system call for it with SNDRV_PCM_IOCTL_HW_PARAMS or > SNDRV_PCM_IOCTL_HW_REFINE. In kernel mode, the memory object is copied to > kernel space and handled by ALSA PCM core. The constrains and rules > registered by drivers are applied to it, then the result is copied to user > space. Finally, applications can get available parameters of PCM substream. > > Without this interaction, even if your driver can handle your devices > correctly, applications cannot use them with proper parameters of PCM > substream; e.g. wrong PCM samples per frame, wrong sampling rate, and so on. > > > Regards > > # I'm another Takashi. > > Takashi Sakamoto > _______________________________________________ Alsa-devel mailing list Alsa-devel@xxxxxxxxxxxxxxxx http://mailman.alsa-project.org/mailman/listinfo/alsa-devel
