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
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