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

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Message-ID: <20180514112036.GI7753@e103592.cambridge.arm.com>
Date:   Mon, 14 May 2018 12:20:37 +0100
From:   Dave Martin <Dave.Martin@....com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     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 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! */

Cheers
---Dave