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 11:27:47 +0100
From:   Thomas Petazzoni <thomas.petazzoni@...tlin.com>
To:     Arnd Bergmann <arnd@...db.de>
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

Hello,

On Mon, 18 Feb 2019 21:37:25 +0100
Arnd Bergmann <arnd@...db.de> wrote:

> > > I would say we should strengthen the behavior of outX() where possible.
> > > I don't know if arm64 actually has a way of doing that, my understanding
> > > earlier was that the AXI bus was already posted, so there is not much
> > > you can do here to define __io_paw() in a way that will prevent posted
> > > writes.  
> >
> > If we could map I/O space using different page table attributes (probably by
> > hacking pci_remap_iospace() ?) then we could disable the
> > early-write-acknowledge hint and implement __io_paw() as a completion
> > barrier, although it would be at the mercy of the system as to whether or
> > not that requires a response from the RC.  
> 
> 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.

> > 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.

Best regards,

Thomas
-- 
Thomas Petazzoni, CTO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com