linux-kernel - Re: [PATCH 06/10] MIPS: pm-cps: Use MIPS standard lightweight ordering barrier

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Date:   Wed, 31 Aug 2016 16:28:21 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Matt Redfearn <matt.redfearn@...tec.com>
Cc:     Ralf Baechle <ralf@...ux-mips.org>, linux-mips@...ux-mips.org,
        Adam Buchbinder <adam.buchbinder@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        linux-kernel@...r.kernel.org,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Markos Chandras <markos.chandras@...tec.com>,
        Paul Burton <paul.burton@...tec.com>
Subject: Re: [PATCH 06/10] MIPS: pm-cps: Use MIPS standard lightweight
 ordering barrier

On Wed, Aug 31, 2016 at 02:36:26PM +0100, Matt Redfearn wrote:
> 
> The code previously had 0x10 as a magic number, this patch just replaces
> that with a #defined name. The value is documented in the MIPS64 instruction
> set manual, https://imgtec.com/?do-download=4302, table 6.5.
> 
> This sync type has been standard since MIPSr2. That document also states
> that "If an implementation does not use one of these non-zero values to
> define a different synchronization behavior, then that non-zero value of
> stype must act the same as stype zero completion barrier." As such,
> stype_ordering can always be set to this sync type rather than setting it
> only for certain CPUs.

Right. We all had a bunch of fun trying to decode that manual a while
back, and IIRC were left with a bunch of questions on what it all meant
in 3+ CPU scenarios.

In anycase, not sure why I was Cc'ed to this patch, but in general I
have low confidence in barrier patches that lack lots of detail. And the
code in question has woefully inadequate comments:

                /* Ordering barrier */
                uasm_i_sync(&p, stype_ordering);

Order what against what and why? Is my first question. A comment really
should explain.

In any case, you've removed the only (runtime) assignment to the
variable, it can become 'const'.