linux-kernel - Re: Opinion on ordering of writel vs. stores to RAM

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Date:	Sun, 10 Sep 2006 20:02:43 +0200
From:	Michael Buesch <mb@...sch.de>
To:	Linus Torvalds <torvalds@...l.org>
Cc:	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	linux-kernel@...r.kernel.org, akpm@...l.org,
	segher@...nel.crashing.org, davem@...emloft.net,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: Opinion on ordering of writel vs. stores to RAM

On Sunday 10 September 2006 19:49, Linus Torvalds wrote:
> 
> On Sun, 10 Sep 2006, Michael Buesch wrote:
> > > 
> > > That's what IRIX had.  It would let us get rid of mmiowb and avoid doing 
> > > a full sync in writeX, so may be the best option.
> > 
> > Last time I suggested that, people did not want it.
> 
> I would personally _much_ rather have a separate mmiowb() and a regular 
> spinlock, than add a magic new spinlock.

Yeah, as far as I remember it was you who rejected it. ;)
But I second your statement because of the practical issues below.

> Of course, mmiowb() itself is not a great name, and we could/should 
> probably rename it to make it more obvious what the hell it is.
> 
> > There is one little problem in practice with something
> > like spin_unlock_io().
> > 
> > spin_lock_io(&lock);
> > foovalue = new_foovalue;
> > if (device_is_fooing)
> > 	writel(foovalue, REGISTER);
> > spin_unlock_io(&lock);
> > 
> > That would be an unneccessary sync in case device is not fooing.
> > In contrast to the explicit version:
> > 
> > spin_lock(&lock);
> > foovalue = new_foovalue;
> > if (device_is_fooing) {
> > 	writel(foovalue, REGISTER);
> > 	mmiowb();
> > }
> > spin_unlock(&lock);
> 
> I think this is even more important when the actual IO is done somewhere 
> totally different from the locking. It's really confusing if you have a 
> "spin_unlock_io()" just because some routine you called wanted it.
> 
> But more importantly, I don't want to have "spin_unlock_io[_xyzzy]()", 
> where "xyzzy()" is all the irq/irqrestore/bh variations. It's just not 
> worth it. We already have enough variations on spinlocks, but at least 
> right now they are all in the "same category", ie it's all about what the 
> context of the _locking_ is, and at least the lock matches the unlock, and 
> there are no separate rules.
> 
> 			Linus
> 

-- 
Greetings Michael.
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