linux-kernel - Re: [PATCH 03/17] pci: Only allow WC mmap on prefetchable resources

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Message-ID: <1490375074.11996.29.camel@infradead.org>
Date:   Fri, 24 Mar 2017 17:04:34 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     linux-pci <linux-pci@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 03/17] pci: Only allow WC mmap on prefetchable resources

On Fri, 2017-03-24 at 17:05 +0100, Arnd Bergmann wrote:
> On Wed, Mar 22, 2017 at 2:25 PM, David Woodhouse <dwmw2@...radead.org> wrote:
> > 
> > From: David Woodhouse <dwmw@...zon.co.uk>
> > 
> > The /proc/bus/pci mmap interface allows the user to specify whether they
> > want WC or not. Don't let them do so on non-prefetchable BARs.
> > 
> > Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
> > Cc: stable@...r.kernel.org
> Can you explain this a little more? I understand that we usually want to use
> WC mappings on prefetchable BARs, and uncached non-WC on
> non-prefetchable, but why do we have to force an error here?
> 
> Couldn't there be some hardware that allows write-combining access
> but no prefetches?

Note that this is only for the procfs interface. The sysfs interface
already imposes the same restriction by just *not* creating the
'resourceX_wc' attr when the resource isn't prefetchable. So this just
brings procfs into line.
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