[alsa-devel] Power management state flow in ALSA

[Takashi Iwai]

At Wed, 30 May 2007 15:36:33 +0530,
Nobin Mathew wrote:
> 
> In suspend () the application is dead (freezed state) before ALSA
> driver suspend() is called, so in this there is no way application
> will get to know the SUSPENDED state of driver.
> 
> 
> In resume () ALSA driver resume () (changes the state of driver) is
> called first and then applications are activated.
> 
> So how the application will get to know the SUSPENDED state of driver
> through syscall.No syscall () from ALSA apps(freezed) is happening
> during the SUSPENDED duration of ALSA driver.

Your app shall issue syscalls sooner or later, otherwise you'll have
no I/O :)

The concept of the (PCM) resume in ALSA is a passive way.  The driver
does _NOT_ resume streams by itself.  It waits until the app requests
to resume.  This is designed so because usually the hardware cannot be
recovered in 100% identical state as before, and often the app needs
to reset something for the proper restart.

So, when resume callback is executed and the whole kernel PM thing is
finished, the user-process restarts again.  Then it issues syscalls,
and gets to know to know that the stream is in the suspended state.
Now it calls alsa-lib snd_pcm_resume() function which issues RESUME
ioctl to restart.


Takashi

> 
> 
> 
> On 5/30/07, Takashi Iwai <tiwai at suse.de> wrote:
> > At Wed, 30 May 2007 11:52:31 +0530,
> > Nobin Mathew wrote:
> > >
> > > I am having a doubt regarding ALSA power management.
> > >
> > > My understanding of APM suspend() is like this.
> > >
> > > Freeze the ALSA apps
> > >
> > > Call ALSA driver suspend ()
> > >
> > > in the ALSA suspend() function it saves the current state of substream
> > > and changes the state of substream to SUSPENDED.
> > >
> > > My understanding of APM resume() is like this
> > >
> > > Call ALSA driver resume ()
> > >
> > > Activate the ALSA apps
> > >
> > > In ALSA resume function it restores the saved state of substream.
> > >
> > >
> > > So my question is when ALSA app will get to know the SUSPENDED state
> > > of substream.???
> >
> > When issuing any syscalls.  Then you'll get ESTRPIPE error, which
> > indicitaes the stream is in the SUSPEND state.
> >
> >
> > Takashi
> >
>