linux-kernel - Re: [PATCH 3/7] asm-generic/io.h: make ioread64 and iowrite64 universally available

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Date:   Thu, 22 Jun 2017 21:36:55 +0100
From:   Alan Cox <gnomes@...rguk.ukuu.org.uk>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-ntb@...glegroups.com, linux-alpha@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-crypto@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Stephen Bates <sbates@...thlin.com>
Subject: Re: [PATCH 3/7] asm-generic/io.h: make ioread64 and iowrite64
 universally available

On Thu, 22 Jun 2017 14:24:58 -0600
Logan Gunthorpe <logang@...tatee.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