linux-kernel - Re: [PATCH 02/18] arm64: move SCTLR_EL{1,2} assertions to <asm/sysreg.h>

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Date:   Mon, 14 May 2018 13:06:10 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Dave Martin <Dave.Martin@....com>, marc.zyngier@....com,
        catalin.marinas@....com, will.deacon@....com,
        linux-kernel@...r.kernel.org, linux@...inikbrodowski.net,
        james.morse@....com, viro@...iv.linux.org.uk,
        linux-fsdevel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 02/18] arm64: move SCTLR_EL{1,2} assertions to
 <asm/sysreg.h>

On Mon, May 14, 2018 at 12:56:09PM +0100, Robin Murphy wrote:
> On 14/05/18 12:20, Dave Martin wrote:
> > On Mon, May 14, 2018 at 11:08:59AM +0100, Mark Rutland wrote:
> > > On Mon, May 14, 2018 at 11:00:53AM +0100, Dave Martin wrote:
> > > > On Mon, May 14, 2018 at 10:46:24AM +0100, Mark Rutland wrote:
> > > > > -/* Check all the bits are accounted for */
> > > > > -#define SCTLR_EL2_BUILD_BUG_ON_MISSING_BITS	BUILD_BUG_ON((SCTLR_EL2_SET ^ SCTLR_EL2_CLEAR) != ~0)
> > > > > -
> > > > > +#if (SCTLR_EL2_SET ^ SCTLR_EL2_CLEAR) != 0xffffffff
> > > > > +#error "Inconsistent SCTLR_EL2 set/clear bits"
> > > > > +#endif
> > > > 
> > > > Can we have a comment on the != 0xffffffff versus != ~0 here?
> > > > 
> > > > The subtle differences in evaluation semantics between #if and
> > > > other contexts here may well trip people up during maintenance...
> > > 
> > > Do you have any suggestion as to the wording?
> > > 
> > > I'm happy to add a comment, but I don't really know what to say.
> > 
> > 
> > How about the following?
> > 
> > /* Watch out for #if evaluation rules: ~0 is not ~(int)0! */
> 
> Or, more formally, perhaps something even less vague like "Note that in
> preprocessor arithmetic these constants are effectively of type intmax_t,
> which is 64-bit, thus ~0 is not what we want."

I'll drop something in the commit message to that effect, rather than a
comment.

A comment will end up terse and vague or large and bloatsome (and
redundant as we have this pattern twice).

Anyone attempting to "clean" this up will find things break, and they can
look at the git log to find out why it is the way it is...

Thanks,
Mark.