linux-kernel - Re: [PATCH v7 5/9] PCI: Add pci_iomap

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Date:	Thu, 25 Jun 2015 10:52:01 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	"Luis R. Rodriguez" <mcgrof@...e.com>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Toshi Kani <toshi.kani@...com>,
	Andy Lutomirski <luto@...capital.net>,
	Juergen Gross <jgross@...e.com>,
	Tomi Valkeinen <tomi.valkeinen@...com>,
	Arnd Bergmann <arnd@...db.de>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	linux-fbdev <linux-fbdev@...r.kernel.org>,
	Suresh Siddha <sbsiddha@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	Dave Airlie <airlied@...hat.com>,
	Antonino Daplas <adaplas@...il.com>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	venkatesh.pallipadi@...el.com,
	Stefan Bader <stefan.bader@...onical.com>,
	Ville Syrjälä <syrjala@....fi>,
	Mel Gorman <mgorman@...e.de>, Vlastimil Babka <vbabka@...e.cz>,
	Borislav Petkov <bp@...e.de>, Davidlohr Bueso <dbueso@...e.de>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Ville Syrjälä 
	<ville.syrjala@...ux.intel.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Jan Beulich <jbeulich@...e.com>,
	Roger Pau Monné <roger.pau@...rix.com>
Subject: Re: [PATCH v7 5/9] PCI: Add pci_iomap_wc() variants

On Thu, 2015-06-25 at 02:08 +0200, Luis R. Rodriguez wrote:
> 
> OK thanks I'll proceed with these patches then.
> 
> > As for user mappings,
> 
> Which APIs were you considering in this regard BTW?

mmap of the generic /sys/bus/pci/.../resource*

> > maybe the right thing to do is to let us do what we do by
> > default with a quirk that can set a flag in pci_dev to disable that
> > behaviour (maybe on a per BAR basis ?).
> 
> That might mean it could restrict userspace WC to require devices
> to have WC parts on a full PCI BAR. Although this is restrictive
> having reviewed most WC uses in the kernel I'd think this would be
> a fair compromise to make, but again, if things are still murky
> perhaps best we kiss this idea good bye for now and hope for it
> to come in on future buses or ammendments (if that's even possible?).
> 
> > I think the common case is that WC works.
> 
> If WC does not I will note one hack which migh be worth mentioning --
> just for
> the record, this was devised as a shortcoming of a device where they
> failed to
> split things properly and that *without* WC performance suffered quite
> a bit so
> they made one full PCI BAR WC and as a work around this:
> 
> http://lkml.kernel.org/r/20150416041837.GA5712@hykim-PC
> 
> That is for registers that needed it:
> 
> write; wmb;
> 
> Then if they wanted to wait till the NIC has seen the write, they did:
> 
> write; wmb; read;
> 

Right, and as I mentioned, on some archs like powerpc (and possibly
more), writel() and co contains an implicit mb()

>   Luis--


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