[alsa-devel] snd_device_register / snd_device_disconnect

[Takashi Iwai]

At Tue, 15 May 2012 08:07:41 +1000,
Barratis wrote:
> 
> Hi Takashi,
> 
> I have a virtual card with pcm which can temporarily go offline, I
> wanted a way to prevent the user seeing them until they come back
> online without having to destroy/recreate the pcm everytime. Since
> there was a disconnect call separate from the free, I thought this was
> meant to be possible.
> 
> I guess I'll simply destroy/recreate my pcms then.

Unregistering the device is a heavy task, if you think how the device
registration is propagated (sysfs, udev, alsa-lib, apps...)  So,
freeing in the driver is just a small piece of the task in the global
view.

In many cases, the driver simply refuses the open (e.g. -EAGAIN) when
such a temporary restriction is applied.  But if you really want to
hide them, freeing and recreating PCM is the way to go, IMO.


Takashi

> 
> Thanks for your answer.
> 
> 
> On Tue, May 15, 2012 at 1:03 AM, Takashi Iwai <tiwai at suse.de> wrote:
> > At Mon, 14 May 2012 08:23:47 +1000,
> > Barratis wrote:
> >>
> >> Hi All,
> >>
> >> I would have a question regarding the behavior of snd_device_register.
> >>
> >> Currently, the register operation will only be carried out if the
> >> state is SNDRV_DEV_BUILD. This effectively prevents device which have
> >> been disconnected via snd_device_disconnect (and thus having the state
> >> SNDRV_DEV_DISCONNECTED) to be registered again.
> >>
> >> Is this a wanted behavior? If so, I can't seem to find a reason for
> >> this as per my testing and code reading registering a device in
> >> DISCONNECTED sate works just fine.
> >
> > The disconnected device is disconnected.  Why it must be registered
> > again?  In other words, the disconnected device should be freed
> > eventually later.  The disconnected state is an intermediate state,
> > thus it's never a state ready for any use.
> >
> >
> > Takashi
> 
> 
> 
> -- 
> Barratis
> Any society that would give up a little liberty to gain a little
> security will deserve neither and lose both. (Benjamin Franklin)
>