linux-kernel - Re: [PATCH v4] arm64: cpuinfo: Expose MIDR_EL1 and REVIDR

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Message-ID: <20160613172616.GC17128@leverpostej>
Date:	Mon, 13 Jun 2016 18:26:16 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	Suzuki K Poulose <suzuki.poulose@....com>
Cc:	catalin.marinas@....com, will.deacon@....com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	steve.capper@....com, linux@....linux.org.uk,
	Steve Capper <steve.capper@...aro.org>
Subject: Re: [PATCH v4] arm64: cpuinfo: Expose MIDR_EL1 and REVIDR_EL1 to
 sysfs

On Mon, Jun 13, 2016 at 06:08:09PM +0100, Suzuki K Poulose wrote:
> +/*
> + * Both MIDR_EL1 and REVIDR_EL1 are 32bit registers. However, per C5.1.1,
> + * "Principles of the System instruction class encoding" in ARM DDI 0487A.i,
> + * when a system register is escribed as 32-bit, this only means that the
> + * upper 32 bits are RES0, not that they will never be made use of. To avoid
> + * changing the ABI for the future, the values are exported as 64bit values.
> + */

I see this is a direct copy+paste of my earlier message, typo and all.

I'd prefer something like the below:

/*
 * The ARM ARM uses the phrase "32-bit register" to describe a register
 * whose upper 32 bits are RES0 (per C5.1.1, ARM DDI 0487A.i), however
 * no statement is made as to whether the upper 32 bits will or will not
 * be made use of in future, and between ARM DDI 0487A.c and ARM DDI
 * 0487A.d CLIDR_EL1 was expanded from 32-bit to 64-bit.
 *
 * Thus, while both MIDR_EL1 and REVIDR_EL1 are described as 32-bit
 * registers, we expose them both as 64 bit values to cater for possible
 * future expansion without an ABI break.
 */

Thanks,
Mark.