linux-kernel - Re: [PATCH RFC 0/3] API for 128-bit IO access

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Date:   Fri, 26 Jan 2018 18:11:49 +0000
From:   Will Deacon <will.deacon@....com>
To:     Yury Norov <ynorov@...iumnetworks.com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrew Pinski <Andrew.Pinski@...ium.com>,
        Arnd Bergmann <arnd@...db.de>,
        Catalin Marinas <catalin.marinas@....com>,
        "David S . Miller" <davem@...emloft.net>,
        Geethasowjanya Akula <Geethasowjanya.Akula@...ium.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Laura Abbott <labbott@...hat.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Sunil Goutham <Sunil.Goutham@...ium.com>
Subject: Re: [PATCH RFC 0/3] API for 128-bit IO access

On Fri, Jan 26, 2018 at 12:05:42PM +0300, Yury Norov wrote:
> On Wed, Jan 24, 2018 at 10:22:13AM +0000, Will Deacon wrote:
> > On Wed, Jan 24, 2018 at 12:05:16PM +0300, Yury Norov wrote:
> > > This series adds API for 128-bit memory IO access and enables it for ARM64.
> > > The original motivation for 128-bit API came from new Cavium network device
> > > driver. The hardware requires 128-bit access to make things work. See
> > > description in patch 3 for details.
> > > 
> > > Also, starting from ARMv8.4, stp and ldp instructions become atomic, and
> > > API for 128-bit access would be helpful in core arm64 code.
> > 
> > Only for normal, cacheable memory, so they're not suitable for IO accesses
> > as you're proposing here.
> 
> Hi Will,
> 
> Thanks for clarification.
> 
> Could you elaborate, do you find 128-bit read/write API useless, or
> you just correct my comment?
> 
> I think that ordered uniform 128-bit access API would be helpful, even
> if not atomic.

Sorry, but I strongly disagree here. Having an IO accessor that isn't
guaranteed to be atomic is a recipe for disaster if it's not called out
explicitly. You're much better off implementing something along the lines
of <linux/io-128-nonatomic-hi-lo.h> using 2x64-bit accessors like we already
have for the 2x32-bit case.

However, that doesn't solve your problem and is somewhat of a distraction.
I'd suggest that in your case, where you have a device that relies on
128-bit atomic access that is assumedly tightly integrated into your SoC,
then the driver just codes it's own local implementation of the accessor,
given that there isn't a way to guarantee the atomicity architecturally
(and even within your SoC it might not be atomic to all endpoints).

Will