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

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Message-ID: <575EF01D.4020203@arm.com>
Date:	Mon, 13 Jun 2016 18:40:45 +0100
From:	Suzuki K Poulose <Suzuki.Poulose@....com>
To:	Mark Rutland <mark.rutland@....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 13/06/16 18:26, Mark Rutland wrote:
> 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.
>   */

Sorry about that, will fix it.