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

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Date:   Fri, 24 Mar 2017 17:05:48 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     David Woodhouse <dwmw2@...radead.org>
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 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?

       Arnd