linux-kernel - Re: [PATCH v2 06/22] arm64: sys_reg: Define System register encoding

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Date:	Thu, 8 Oct 2015 17:13:27 +0100
From:	"Suzuki K. Poulose" <Suzuki.Poulose@....com>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	mark.rutland@....com, Vladimir.Murzin@....com,
	steve.capper@...aro.org, ard.biesheuvel@...aro.org,
	marc.zyngier@....com, andre.przywara@....com, will.deacon@....com,
	linux-kernel@...r.kernel.org, edward.nevill@...aro.org,
	aph@...hat.com, james.morse@....com, dave.martin@....com,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 06/22] arm64: sys_reg: Define System register encoding

On 08/10/15 15:43, Catalin Marinas wrote:
> On Wed, Oct 07, 2015 at 06:03:34PM +0100, Suzuki K. Poulose wrote:
>> On 07/10/15 17:36, Catalin Marinas wrote:
>>> On Mon, Oct 05, 2015 at 06:01:55PM +0100, Suzuki K. Poulose wrote:
>>>>    * (Ref: ARMv8 ARM, Section: "System instruction class encoding overview",
>>>>    *  C5.2, version:ARM DDI 0487A.f)
>>>>    *	[20-19] : Op0
>>>> @@ -34,15 +34,40 @@
>>>>    *	[15-12] : CRn
>>>>    *	[11-8]  : CRm
>>>>    *	[7-5]   : Op2
>>>> + * Hence we use [ sys_reg() << 5 ] in the mrs/msr instructions.
>>>
>>> Do we really need to have all the ids shifted right by 5? I can't see
>>> where it helps. OTOH, it makes the code more complicated by having to
>>> remember to shift the id left by 5.
>>
>> This is a cosmetic change, to reuse the sys_reg() definitions for both
>> mrs_s/msr_s macros and the CPU ID. The (existing)left shift is only needed
>> for the mrs_s/msr_s, so the existing users don't have to worry about the shift
>> unless they have hard-open-coded values for the register. On the plus
>> side it becomes slightly easier to use the same encoding for CPU id
>> tracking (and manual decoding). If you think this is superfluous, I could
>> change the CPU-id to use right shifted values from sys_reg.
>
> You may be right but I still fail to see whether the shifted values are
> useful to the CPUID code. Are you referring to the 'switch' statements?
>

Yes. Again we can adjust the macros to get the right fields for switch().
Its also about the meaning of sys_reg(). The name kind of implies the system
register encoding defined by ARM ARM. However the current version
defines the encoding as it appears in the mrs/msr instructions, which can
be confusing if we start using the same encoding for representing the sys_reg
elsewhere(in this case CPUID infrastructure). Again, I am not too particular
about this change.

Suzuki


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