linux-kernel - Re: mmotm 2008-11-13-17-22 uploaded (pc-speaker)

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Date:	Wed, 26 Nov 2008 14:40:05 +0100
From:	Takashi Iwai <tiwai@...e.de>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Randy Dunlap <randy.dunlap@...cle.com>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: mmotm 2008-11-13-17-22 uploaded (pc-speaker)

At Wed, 26 Nov 2008 11:52:12 +0100,
I wrote:
> 
> At Fri, 14 Nov 2008 17:04:46 +0100,
> I wrote:
> > 
> > At Fri, 14 Nov 2008 09:36:10 +0100,
> > I wrote:
> > > 
> > > At Fri, 14 Nov 2008 09:30:01 +0100,
> > > Peter Zijlstra wrote:
> > > > 
> > > > On Fri, 2008-11-14 at 09:17 +0100, Takashi Iwai wrote:
> > > > > At Fri, 14 Nov 2008 09:03:14 +0100,
> > > > > Peter Zijlstra wrote:
> > > > > > 
> > > > > > On Fri, 2008-11-14 at 07:47 +0100, Takashi Iwai wrote:
> > > > > > > At Fri, 14 Nov 2008 07:36:54 +0100,
> > > > > > > I wrote:
> > > > > > > > 
> > > > > > > > At Thu, 13 Nov 2008 21:18:42 -0800,
> > > > > > > > Randy Dunlap wrote:
> > > > > > > > > 
> > > > > > > > > 
> > > > > > > > > mmotm-2008-1113-1722/sound/drivers/pcsp/pcsp.c: In function 'snd_card_pcsp_probe':
> > > > > > > > > mmotm-2008-1113-1722/sound/drivers/pcsp/pcsp.c:99: error: 'HRTIMER_CB_IRQSAFE' undeclared (first use in this function)
> > > > > > > > > 
> > > > > > > > > # CONFIG_SND_HRTIMER is not set
> > > > > > > > 
> > > > > > > > snd-pcsp and CONFIG_SND_HRTIMER are independent.
> > > > > > > > The snd-pcsp driver code isn't changed over weeks, thus it must be the
> > > > > > > > change in hrtimer side.
> > > > > > > 
> > > > > > > It's turned out to be the recent commint in the upstream:
> > > > > > > 
> > > > > > >   commit 621a0d5207c18012cb39932f2d9830a11a6cb03d
> > > > > > >   Author: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> > > > > > >   Date:   Wed Nov 12 09:36:35 2008 +0100
> > > > > > > 
> > > > > > >     hrtimer: clean up unused callback modes
> > > > > > > 
> > > > > > >     Impact: cleanup
> > > > > > > 
> > > > > > >     git grep HRTIMER_CB_IRQSAFE revealed half the callback modes are actually
> > > > > > >     unused.
> > > > > > > 
> > > > > > >     Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> > > > > > >     Signed-off-by: Ingo Molnar <mingo@...e.hu>
> > > > > > > 
> > > > > > > This hits on snd-pcsp driver on linux-next, since it was switched to
> > > > > > > use this dropped flag.  Now we get a build error.
> > > > > > > 
> > > > > > > Can this commit be reverted?
> > > > > > 
> > > > > > I think we determined the silly pc speaker driver should be using the
> > > > > > SOFTIRQ timer, why was this changed back again?
> > > > > 
> > > > > It uses a tasklet inside now.
> > > > > The background story is:   pcsp driver does register bit flips at each
> > > > > hrtimer callback.  This should be done as accurate as possible for the
> > > > > sound quality (heh, who matters?).  The register flip itself doesn't
> > > > > take time and no lock problem.  Thus, IRQSAFE is more appropriate just
> > > > > for this task.
> > > > > 
> > > > > The reason we used the softirq mode is the call of the ALSA core
> > > > > update part.  This is eventually called after the given samples have
> > > > > been processed.  And, this could cause a spin deadlock if called
> > > > > directly from hrtimer callback.
> > > > > 
> > > > > In the latest code, the call of ALSA PCM core is off-loaded via
> > > > > tasklet for avoiding both spin deadlock and too long hrtimer
> > > > > handling.
> > > > 
> > > > Aside from the fact that I think tasklets should die a horrible death
> > > > too,
> > > 
> > > Oh, if you'll kill them, please provide something compatible...
> > > 
> > > > could you, for now, try to use HRTIMER_CB_IRQSAFE_UNLOCKED ?
> > > 
> > > OK, I'll check it later.
> > 
> > Looks running, so far.  I fixed it on for-next branch.
> 
> Just checking again, and found out that this is broken.
> 
> The behavior of HRTIMER_CB_IRQSAFE_UNLOCKED isn't compatible with
> HRTIMER_CB_IRQSAFE in the pretty fundamental manner for restarting.
> So, just replacing with IRQSAFE_UNLOCKED doesn't work...

Meanwhile, I fixed the pcsp driver side so that it works now with
HRTIMER_CB_IRQSAFE_UNLOCKED mode.

The problem was that pcsp driver expected the callback function gets
called immediately when started with zero expire time.  It worked with
HRTIMER_CB_SOFT and _IRQSAFE, but not with others.


Takashi
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