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

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Date:   Tue, 19 Feb 2019 12:31:50 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Cc:     Will Deacon <will.deacon@....com>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        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>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
        Gregory CLEMENT <gregory.clement@...e-electrons.com>,
        Russell King - ARM Linux <linux@...linux.org.uk>
Subject: Re: [RFC PATCH] docs/memory-barriers.txt: Rewrite "KERNEL I/O BARRIER
 EFFECTS" section

On Tue, Feb 19, 2019 at 11:27 AM Thomas Petazzoni
<thomas.petazzoni@...tlin.com> wrote:
> On Mon, 18 Feb 2019 21:37:25 +0100
> Arnd Bergmann <arnd@...db.de> wrote:
>
> > Ah, it seems we actually do that on 32-bit ARM, at least on one platform,
> > see 6a02734d420f ("ARM: mvebu: map PCI I/O regions strongly ordered")
> > and prior commits.
>
> Yes, some Marvell Armada 32-bit platforms have an errata that require
> the PCI MEM and PCI I/O regions to be mapped strongly ordered.
>
> BTW, this requirement prevents us from using the pci_remap_iospace()
> API from drivers/pci, because it assumes page attributes of
> pgprot_device(PAGE_KERNEL). That's why we're still using the
> ARM-specific pci_ioremap_io() function.

Ok.

> > > I would still prefer to document the weaker semantics as the portable
> > > interface, unless there are portable drivers relying on this today (which
> > > would imply that it's widely supported by other architectures).
> >
> > I don't know of any portable driver that actually relies on it, but
> > that's mainly because there are very few portable drivers that
> > use inb()/outb() in the first place. How many of those require
> > the non-posted behavior I don't know
> >
> > Adding Thomas, Gregory and Russell to Cc, as they were involved
> > in the discussion that led to the 32-bit change, maybe they are
> > aware of a specific example.
>
> I'm just arriving in the middle of this thread, and I'm not sure to
> understand what is the question. If the question is whether PCI I/O is
> really used in practice, then I've never seen it be used with Marvell
> platforms (but I'm also not aware of all PCIe devices people are
> using). I personally have a hacked-up version of the e1000e driver
> that intentionally does some PCI I/O accesses, that I use as a way to
> validate that PCI I/O support is minimally working, but that's it.

The main question is whether we know of a portable (not specific
to a single architecture) driver for PCIe (or PCI, but probably not ISA)
that uses outb/outw/outl in a way that relies on the on-posted
behavior of those, compared to posted writeb/writew/writel that are
only required to complete before another I/O operation on the same
device but whose completion is not ordered with regard to the rest of
the system.

I think an example of this would be a driver using outb() to disable
an interrupt, and then relying on the the interrupt no longer happening
after the outb().

     Arnd