linux-kernel - Re: [PATCH 1/3] mpt2sas: remove the use of writeq, since writeq is not atomic

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Date:	Thu, 19 May 2011 20:15:00 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Cc:	Roland Dreier <roland@...nel.org>, Milton Miller <miltonm@....com>,
	Hitoshi Mitake <h.mitake@...il.com>,
	Sam Ravnborg <sam@...nborg.org>,
	Ingo Molnar <mingo@...hat.com>,
	"Desai, Kashyap" <Kashyap.Desai@....com>,
	"Prakash, Sathya" <Sathya.Prakash@....com>,
	James Bottomley <James.Bottomley@...senpartnership.com>,
	Matthew Wilcox <matthew@....cx>,
	linux scsi dev <linux-scsi@...r.kernel.org>,
	"paulus@...ba.org" <paulus@...ba.org>,
	linux powerpc dev <linuxppc-dev@...ts.ozlabs.org>,
	linux pci <linux-pci@...r.kernel.org>,
	linux kernel <linux-kernel@...r.kernel.org>,
	linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [PATCH 1/3] mpt2sas: remove the use of writeq, since writeq is
 not atomic


* Benjamin Herrenschmidt <benh@...nel.crashing.org> wrote:

> On Wed, 2011-05-18 at 21:16 -0700, Roland Dreier wrote:
> > On Wed, May 18, 2011 at 11:31 AM, Milton Miller <miltonm@....com> wrote:
> > > So the real question should be why is x86-32 supplying a broken writeq
> > > instead of letting drivers work out what to do it when needed?
> > 
> > Sounds a lot like what I was asking a couple of years ago :)
> > http://lkml.org/lkml/2009/4/19/164
> > 
> > But Ingo insisted that non-atomic writeq would be fine:
> > http://lkml.org/lkml/2009/4/19/167
> 
> Yuck... Ingo, I think that was very wrong.
> 
> Those are for MMIO, which must almost ALWAYS know precisely what the
> resulting access size is going to be. It's not even about atomicity
> between multiple CPUs. I have seen plenty of HW for which a 64-bit
> access to a register is -not- equivalent to two 32-bit ones. In fact, in
> some case, you can get the side effects twice ... or none at all.
> 
> The only case where you can be lax is when you explicitely know that
> there is no side effects -and- the HW cope with different access sizes.
> This is not the general case and drivers need at the very least a way to
> know what the behaviour will be.

Ok, that's pretty convincing.

Unless hpa or tglx disagrees with reverting this, could any of you send a patch 
with a proper changelog etc. that applies cleanly to v2.6.39?

Thanks,

	Ingo
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