From: Arnd Bergmann <arnd@arndb.de>
To: Casey Leedom <leedom@chelsio.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        "Luis R. Rodriguez" <mcgrof@suse.com>,
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Subject: Re: [PATCH v7 5/9] PCI: Add pci_iomap_wc() variants
Date: Thu, 25 Jun 2015 22:44:07 +0200
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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
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