From mboxrd@z Thu Jan 1 00:00:00 1970 From: robin.murphy@arm.com (Robin Murphy) Date: Mon, 14 May 2018 12:56:09 +0100 Subject: [PATCH 02/18] arm64: move SCTLR_EL{1,2} assertions to In-Reply-To: <20180514112036.GI7753@e103592.cambridge.arm.com> References: <20180514094640.27569-1-mark.rutland@arm.com> <20180514094640.27569-3-mark.rutland@arm.com> <20180514100053.GX7753@e103592.cambridge.arm.com> <20180514100858.4xjp5d3axbyy74ap@lakrids.cambridge.arm.com> <20180514112036.GI7753@e103592.cambridge.arm.com> Message-ID: <8d66d231-d8a2-07c1-6c5b-24c9474c851f@arm.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org 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." Robin.