linux-kernel - Re: [RFC PATCH] docs/memory-barriers.txt: Rewrite "KERNEL I/O BARRIER EFFECTS" section

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Date:   Tue, 12 Feb 2019 15:01:13 +1100
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Will Deacon <will.deacon@....com>
Cc:     linux-arch <linux-arch@...r.kernel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Arnd Bergmann <arnd@...db.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Daniel Lustig <dlustig@...dia.com>,
        David Howells <dhowells@...hat.com>,
        Alan Stern <stern@...land.harvard.edu>
Subject: Re: [RFC PATCH] docs/memory-barriers.txt: Rewrite "KERNEL I/O
 BARRIER EFFECTS" section

On Mon, 2019-02-11 at 14:34 -0800, Linus Torvalds wrote:
> On Mon, Feb 11, 2019 at 9:30 AM Will Deacon <will.deacon@....com> wrote:
> > +
> > +     1. All readX() and writeX() accesses to the same peripheral are ordered
> > +        with respect to each other. For example, this ensures that MMIO register
> > +       writes by the CPU to a particular device will arrive in program order.
> 
> Hmm. I'd like more people look at strengthening this one wrt across
> CPUs and locking.
> 
> Right now we document mmiowb(), but that "documentation" is really
> just a fairy tale. Very *very* few drivers actually do mmiowb() on
> their own.
> 
> IOW, we should seriously just consider making the rule be that locking
> will order mmio too. Because that's practically the rule anyway.
> 
> Powerpc already does it. IO within a locked region will serialize with the lock.

Yup. It's a bit ugly but I felt back then that getting drivers to use
mmiowb() properly was going to be a losing battle.

Cheers,
Ben.