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Date: Thu, 22 Jun 2017 21:36:55 +0100
From: Alan Cox <gnomes@lxorguk.ukuu.org.uk>
Subject: Re: [PATCH 3/7] asm-generic/io.h: make ioread64 and iowrite64
 universally available
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To: Logan Gunthorpe <logang@deltatee.com>
Cc: linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, linux-ntb@googlegroups.com, linux-alpha@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-crypto@vger.kernel.org, dri-devel@lists.freedesktop.org, Arnd Bergmann <arnd@arndb.de>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Stephen Bates <sbates@raithlin.com>
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On Thu, 22 Jun 2017 14:24:58 -0600
Logan Gunthorpe <logang@deltatee.com> wrote:

> On 6/22/2017 2:14 PM, Alan Cox wrote:
> > If a platform doesn't support 64bit I/O operations from the CPU then you
> > either need to use some kind of platform/architecture specific interface
> > if present or accept you don't have one.  
> 
> Yes, I understand that.
> 
> The thing is that every user that's currently using it right now is 
> patching in their own version that splits it on non-64bit systems.
> 
> > It's not safe to split it. Possibly for some use cases you could add an
> > ioread64_maysplit()  
> 
> I'm open to doing something like that.

I think that makes sense for the platforms with that problem. I'm not
sure there are many that can't do it for mmio at least. 486SX can't do it
and I guess some ARM32 but I think almost everyone else can including
most 32bit x86.

What's more of a problem is a lot of platforms can do 64bit MMIO via
ioread/write64 but not 64bit port I/O, and it's not clear how you
represent that via an ioread/write API that abstracts it away.

Alan