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Re: [Xen-devel] [PATCH v7 5/9] PCI: Add pci_iomap_wc() variants
- To: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
- From: "Luis R. Rodriguez" <mcgrof@xxxxxxxx>
- Date: Fri, 26 Jun 2015 15:41:05 -0700
- Cc: linux-fbdev <linux-fbdev@xxxxxxxxxxxxxxx>, "Michael S. Tsirkin" <mst@xxxxxxxxxx>, "linux-pci@xxxxxxxxxxxxxxx" <linux-pci@xxxxxxxxxxxxxxx>, Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, ville.syrjala@xxxxxxxxxxxxxxx, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Arnd Bergmann <arnd@xxxxxxxx>, Tomi Valkeinen <tomi.valkeinen@xxxxxx>, Daniel Vetter <daniel.vetter@xxxxxxxx>, Dave Airlie <airlied@xxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxx>, Jean-Christophe Plagniol-Villard <plagnioj@xxxxxxxxxxxx>, Casey Leedom <leedom@xxxxxxxxxxx>, Antonino Daplas <adaplas@xxxxxxxxx>, Suresh Siddha <sbsiddha@xxxxxxxxx>, Stefan Bader <stefan.bader@xxxxxxxxxxxxx>, Bjorn Helgaas <bhelgaas@xxxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxxxxx>, Juergen Gross <jgross@xxxxxxxx>, Toshi Kani <toshi.kani@xxxxxx>, "linux-kernel@xxxxxxxxxxxxxxx" <linux-kernel@xxxxxxxxxxxxxxx>, Andy Lutomirski <luto@xxxxxxxxxxxxxx>, David Vrabel <david.vrabel@xxxxxxxxxx>, "venkatesh.pallipadi@xxxxxxxxx" <venkatesh.pallipadi@xxxxxxxxx>, Roger Pau Monnà <roger.pau@xxxxxxxxxx>
- Delivery-date: Fri, 26 Jun 2015 22:41:25 +0000
- List-id: Xen developer discussion <xen-devel.lists.xen.org>
On Jun 26, 2015 3:05 PM, "Benjamin Herrenschmidt" <benh@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Fri, 2015-06-26 at 21:31 +0200, Luis R. Rodriguez wrote:
> > > Yeah either, we need to fix our relaxed implementation (patch
> > > welcome :-)
> >
> > Yikes, so although powerpc has useful heuristics to automatically
> > enable WC the default write ops have been nullifying its effects?
>
> The heuristic is for userspace mapping which don't use the kernel
> accessors. To cut a long story short, this was all done back in the day
> to speed up X.org which doesn't use fancy accessors with barriers to
> access the framebuffer (neither does the kernel fbdev layer).
Great.
>
> > Shouldn't this have been blatantly obvious in performance benchmarks
> > and expectations?
>
> > If not a change should give considerable performance
> > gains... If the WC was nullified on powerpc then the experience and
> > value of the heuristics have less value and even if they are going
> > to be only considered for userspace mmap() it should be taken with
> > a bit grain of salt it seems.
>
> It wasn't nullified for the main user at the time, the fb. And I
> mentioned an IB adapter or two for which the code had been hand tuned.
This still means there could be some affected drivers when used on powerpc, no?
 Luis
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