From mboxrd@z Thu Jan  1 00:00:00 1970
From: Arnd Bergmann <arnd@arndb.de>
Date: Thu, 25 Jun 2015 20:44:07 +0000
Subject: Re: [PATCH v7 5/9] PCI: Add pci_iomap_wc() variants
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To: Casey Leedom <leedom@chelsio.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>, "Luis R. Rodriguez" <mcgrof@suse.com>, "Michael S. Tsirkin" <mst@redhat.com>, Bjorn Helgaas <bhelgaas@google.com>, Toshi Kani <toshi.kani@hp.com>, Andy Lutomirski <luto@amacapital.net>, Juergen Gross <jgross@suse.com>, Tomi Valkeinen <tomi.valkeinen@ti.com>, "linux-pci@vger.kernel.org" <linux-pci@vger.kernel.org>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, "xen-devel@lists.xensource.com" <xen-devel@lists.xensource.com>, linux-fbdev <linux-fbdev@vger.kernel.org>, Suresh Siddha <sbsiddha@gmail.com>, Ingo Molnar <mingo@elte.hu>, Thomas Gleixner <tglx@linutronix.de>, Daniel Vetter <daniel.vetter@ffwll.ch>, Dave Airlie <airlied@redhat.com>, Antonino Daplas <adaplas@gmail.com>, Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>, Dave Hansen <dave.hansen@linux.intel.com>, "venkatesh.pallipadi@intel.com" <venkatesh.pallipadi@intel.com>, Stefan Bader <stefan.bader@canonical.com>, Ville =?ISO-8859-1?Q?Syrj=E4l=E4?= <syrjala@sci.fi>, Mel Gorman <mgorman@suse.de>, Vlastimil Babka <vbabka@suse.cz>, Borislav Petkov <bp@suse.de>, Davidlohr Bueso <dbueso@suse.de>, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>, Ville =?ISO-8859-1?Q?Syrj=E4l=E4?= <ville.syrjala@linux.intel.com>, David Vrabel <david.vrabel@citrix.com>, Jan Beulich <jbeulich@suse.com>, Roger Pau =?ISO-8859-1?Q?Monn=E9?= <roger.pau@citrix.com>

On Thursday 25 June 2015 15:01:56 Casey Leedom wrote:
> 
>   Is there a reference I can read on this so I can understand
> when and where we can use the __raw_*() APIs?  Can these
> Raw Read/Write operations be reordered with respect to
> each other or are the use of the various flavors of SYNC
> instructions just to maintain order between Cached Memory
> Accesses and I/O Instructions?

The interpretation is not consistent across architectures.

My best description would be that the __raw_*() accessors should
only be used for accessing RAM areas that are known to have no
side-effects and can be read in any size (8-bit to 64-bit wide),
any alignment, and do not have a specific endianess.

If you are dealing with MMIO registers that have a fixed endianess
and size, the correct accessor would be readl_relaxed(), which
is like readl() but lacks the barriers on certain architectures.

	Arnd