linux-kernel - Re: [PATCH 01/20] asm-generic/mmiowb: Add generic implementation of mmiowb() tracking

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Date:   Sun, 03 Mar 2019 13:34:36 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andrea Parri <andrea.parri@...rulasolutions.com>,
        Arnd Bergmann <arnd@...db.de>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Rich Felker <dalias@...c.org>,
        David Howells <dhowells@...hat.com>,
        Daniel Lustig <dlustig@...dia.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        "Maciej W. Rozycki" <macro@...ux-mips.org>,
        Ingo Molnar <mingo@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Palmer Dabbelt <palmer@...ive.com>,
        Paul Burton <paul.burton@...s.com>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Tony Luck <tony.luck@...el.com>,
        Will Deacon <will.deacon@....com>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>
Subject: Re: [PATCH 01/20] asm-generic/mmiowb: Add generic implementation of
 mmiowb() tracking

Linus Torvalds's on March 3, 2019 12:18 pm:
> On Sat, Mar 2, 2019 at 5:43 PM Nicholas Piggin <npiggin@...il.com> wrote:
>>
>> Is there a reason to call this "mmiowb"? We already have wmb that
>> orders cacheable stores vs mmio stores don't we?
> 
> Sadly no it doesn't. Not on ia64, and people tried to make that the
> new rule because of the platform breakage on what some people thought
> would be a major platform.

Let me try this again, because I was babbling a train of thought 
continuing from my past mails on the subject.

  Kill mmiowb with fire.

It was added for a niche platform that hasn't been produced for 10
years for a CPU ISA that is no longer being developed. Let's make mb/wmb
great again (aka actually possible for normal people to understand).

If something comes along again that reorders mmios from different CPUs 
in the IO controller like the Altix did, they implement wmb the slow and 
correct way. They can add a new faster primitive for the few devices 
they care about in the couple of perf critical places that matter.

It doesn't have to be done all at once with this series, obviously this 
is a big improvement on its own. But why perpetuate the nomenclature
and concept for new code added now? 

Thanks,
Nick