Re: [PATCH v7 5/9] PCI: Add pci_iomap_wc() variants

From: "Luis R. Rodriguez" <mcgrof@suse.com>
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: linux-fbdev <linux-fbdev@vger.kernel.org>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	"linux-pci@vger.kernel.org" <linux-pci@vger.kernel.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Jan Beulich <jbeulich@suse.com>,
	ville.syrjala@linux.intel.com,
	"xen-devel@lists.xensource.com" <xen-devel@lists.xensource.com>,
	Arnd Bergmann <arnd@arndb.de>,
	Tomi Valkeinen <tomi.valkeinen@ti.com>,
	Daniel Vetter <daniel.vetter@ffwll.ch>,
	Dave Airlie <airlied@redhat.com>, Ingo Molnar <mingo@elte.hu>,
	Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>,
	Casey Leedom <leedom@chelsio.com>,
	Antonino Daplas <adaplas@gmail.com>,
	Suresh Siddha <sbsiddha@gmail.com>,
	Stefan Bader <stefan.bader@canonical.com>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Juergen Gross <jgross@suse.com>, Toshi Kani <toshi.kani@hp.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vg>
Subject: Re: [PATCH v7 5/9] PCI: Add pci_iomap_wc() variants
Date: Fri, 26 Jun 2015 15:41:05 -0700	[thread overview]
Message-ID: <CAB=NE6XwczF6FYXV=RHyK=7ikVk_tGp+h9ouWPyORNENq4+=Kw@mail.gmail.com> (raw)
In-Reply-To: <1435356258.26815.13.camel@kernel.crashing.org>

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On Jun 26, 2015 3:05 PM, "Benjamin Herrenschmidt" <benh@kernel.crashing.org>
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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