[alsa-devel] [PATCH] ALSA: hda - Don't power up when not powered down.

[Takashi Iwai]

At Thu, 21 Jun 2012 23:57:43 -0700,
Dylan Reid wrote:
> 
> On Thu, Jun 21, 2012 at 10:59 PM, Takashi Iwai <tiwai at suse.de> wrote:
> > At Thu, 21 Jun 2012 21:51:22 -0700,
> > Dylan Reid wrote:
> >>
> >> After cancel_delayed_work_sync returns, the power down work either never
> >> started (power_on == 1) or finished (power_on == 0).  In the former case
> >> there is no need to power up again.
> >>
> >> Signed-off-by: Dylan Reid <dgreid at chromium.org>
> >
> > There is still a small window between cancel_delayed_work_sync() and
> > the spinlock, so the power down might happen between them, in theory.
> > (And due to the obvious reason, cancel_delayed_work_sync() can't be in
> >  spinlock :)
> 
> I'm probably missing something here, but I think three scenarios could
> happen during that window:
> 
> 1) snd_hda_suspend or snd_hda_codec_reset, which will leave power_on =
> 0, and when power_up gets the spin lock it will turn back on.  I don't
> think that's a problem, power_count is still > 0 so it should be on.
> 
> 2) snd_hda_power_down is called and power_count goes to zero while
> power_on == 0.  Might be a problem, this would not schedule new power
> down work.
>   Is this expected?  One thread calling power_up moving the count from
> 0 to 1 and another thread (that must not have called power_up) calling
> power_down?
>   This hole could be plugged by checking if power_count is still >0
> after getting the spin lock. does that sound right?
> 
> 3)  snd_hda_power_down is called and schedules new work, I think this
> is OK because when hda_power_work runs it will check power_count and
> do the right thing.

OK, on the second look, the check looks feasible.


Thanks!

Takashi