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

[Arnd Bergmann]

On Wed, Mar 22, 2017 at 2:25 PM, David Woodhouse <dwmw2@xxxxxxxxxxxxx> wrote:
> From: David Woodhouse <dwmw@xxxxxxxxxxxx>
>
> 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@xxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx

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