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

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Message-ID: <20180129102558.jcnusie6y6wwooan@yury-thinkpad>
Date:   Mon, 29 Jan 2018 13:25:58 +0300
From:   Yury Norov <ynorov@...iumnetworks.com>
To:     Will Deacon <will.deacon@....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 06:11:49PM +0000, Will Deacon wrote:
> 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).

OK. Understand that. So we'll drop this RFC and implement those accessors 
in driver.

Thank you and all for the review. Maybe later I'll submit 128-bit unification
patch that was discussed here.

Yury