linux-kernel - Re: [PATCH 1/2] asm-generic/io: Pass result on inX() accessor to __io

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Message-ID: <20190213174608.GA29100@fuggles.cambridge.arm.com>
Date:   Wed, 13 Feb 2019 17:46:08 +0000
From:   Will Deacon <will.deacon@....com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     linux-arch <linux-arch@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        andrew.murray@....com, Catalin Marinas <catalin.marinas@....com>,
        linux-riscv@...ts.infradead.org,
        Palmer Dabbelt <palmer@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>
Subject: Re: [PATCH 1/2] asm-generic/io: Pass result on inX() accessor to
 __io_par()

On Tue, Feb 12, 2019 at 12:55:17PM +0100, Arnd Bergmann wrote:
> On Mon, Feb 11, 2019 at 6:45 PM Will Deacon <will.deacon@....com> wrote:
> >
> > The inX() I/O accessors must enforce ordering against subsequent calls
> > to the delay() routines, so that a read-back from a device can be used
> > to postpone a subsequent write to the same device.
> >
> > On some architectures, including arm64, this ordering can only be
> > achieved by creating a dependency on the value returned by the inX()
> > operation, so we need to pass the value we read to the __io_par()
> > macro in this case.
> >
> > Reported-by: Andrew Murray <andrew.murray@....com>
> > Signed-off-by: Will Deacon <will.deacon@....com>
> > ---
> >  include/asm-generic/io.h | 8 ++++----
> 
> For changing the asm-generic file in the arm64 tree,
> 
> Acked-by: Arnd Bergmann <arnd@...db.de>

Thanks, Arnd.

> For all I can see, this should not conflict with the usage of the
> same macros on RISC-V, though it does make add a significant
> difference, so I'd like to see an Ack from the RISC-V folks as
> well (added to Cc), or possibly a change to arch/riscv/include/asm/io.h
> to do a corresponding change.

There's already a comment in that header which says that the accesses are
ordered wrt timer reads, so I don't think anything needs to change there.
For consistency with the macro arguments, I could augment their __io_par to
take the read value as an unused argument, if that's what you mean?

Will