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 12:56:09 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Dave Martin <Dave.Martin@....com>,
        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 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.