netdev - Re: RFC on writel and writel

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Date:   Wed, 28 Mar 2018 09:11:10 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Will Deacon <will.deacon@....com>,
        Sinan Kaya <okaya@...eaurora.org>,
        Jason Gunthorpe <jgg@...pe.ca>,
        David Laight <David.Laight@...lab.com>,
        Oliver <oohall@...il.com>,
        "open list:LINUX FOR POWERPC (32-BIT AND 64-BIT)" 
        <linuxppc-dev@...ts.ozlabs.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: RFC on writel and writel_relaxed

On Wed, Mar 28, 2018 at 8:56 AM, Benjamin Herrenschmidt
<benh@...nel.crashing.org> wrote:
> On Wed, 2018-03-28 at 06:53 +0000, Linus Torvalds wrote:
>> On Tue, Mar 27, 2018, 20:43 Benjamin Herrenschmidt <benh@...nel.crashing.org> wrote:
>> That's why in/out were *so* slow, and why nobody uses them any more
>> (well, the address size limitations and the lack of any remapping of
>> the address obviously also are a reason).
>
> All true indeed, though a lot of other archs never quite made them
> fully synchronous, which was another can of worms ... oh well.

Many architectures have no way of providing PCI compliant semantics
for outb, as their instruction set and/or bus interconnect lacks a
method of waiting for completion of an outb.

In practice, it doesn't seem to matter for any of the devices one would
encounter these days: very few use I/O space, and those that do don't
actually rely on the strict ordering. Some architectures (in particular
s390, but I remember seeing the same thing elsewhere) explicitly
disallow I/O space access on PCI because of this. On ARM, the typical
PCI implementations have other problems that are worse than this
one, so most drivers are fine with the almost-working semantics.

        Arnd