Re: [alsa-devel] [PATCH] ALSA: hda: Use standard waitqueue for RIRB wakeup

[Takashi Iwai <tiwai@suse.de>]

On Fri, 13 Dec 2019 14:34:13 +0100,
Takashi Iwai wrote:
> 
> On Fri, 13 Dec 2019 14:05:31 +0100,
> Kai Vehmanen wrote:
> > 
> > Hey,
> > 
> > On Tue, 10 Dec 2019, Takashi Iwai wrote:
> > 
> > > This patch attempts to improve the situation by introducing the
> > > standard waitqueue in the RIRB waiter side instead of polling.  The
> > 
> > this patch was part of the testing as well, so looks good. One minor
> > nit only:
> > 
> > > @@ -216,6 +216,9 @@ void snd_hdac_bus_update_rirb(struct hdac_bus *bus)
> > >  		else if (bus->rirb.cmds[addr]) {
> > >  			bus->rirb.res[addr] = res;
> > >  			bus->rirb.cmds[addr]--;
> > > +			if (!bus->rirb.cmds[addr] &&
> > > +			    waitqueue_active(&bus->rirb_wq))
> > > +				wake_up(&bus->rirb_wq);
> > 
> > Checkpath would like to have a comment here:
> > 
> > WARNING: waitqueue_active without comment
> > #77: FILE: sound/hda/hdac_controller.c:220:
> > +                           waitqueue_active(&bus->rirb_wq))
> 
> Yeah, that was known to me, too.  Actually it's misleading; what
> actually matters is the memory barrier or other synchronization there,
> and this should work as is because of the current code path.
> (Besides, majority of existing waitqueue_active() have no comments at
> all :)
> 
> And, now I found wa_has_sleeper() as a better replacement of
> waitqueue_active() that explicitly cares about synchronization.
> So I'll change with this in a later patch (after unification to
> hda-core).

On the second thought, the extra barrier should be utterly
superfluous, so I'll go for commenting about the spinlock instead...


Takashi
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