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

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Message-ID: <56155066.9000000@arm.com>
Date:	Wed, 7 Oct 2015 18:03:34 +0100
From:	"Suzuki K. Poulose" <Suzuki.Poulose@....com>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	linux-arm-kernel@...ts.infradead.org, mark.rutland@....com,
	Vladimir.Murzin@....com, steve.capper@...aro.org,
	ard.biesheuvel@...aro.org, marc.zyngier@....com,
	will.deacon@....com, linux-kernel@...r.kernel.org,
	edward.nevill@...aro.org, aph@...hat.com, james.morse@....com,
	andre.przywara@....com, dave.martin@....com
Subject: Re: [PATCH v2 06/22] arm64: sys_reg: Define System register encoding

On 07/10/15 17:36, Catalin Marinas wrote:
> On Mon, Oct 05, 2015 at 06:01:55PM +0100, Suzuki K. Poulose wrote:
>> --- a/arch/arm64/include/asm/sysreg.h
>> +++ b/arch/arm64/include/asm/sysreg.h
>> @@ -22,11 +22,11 @@
>>
>>   #include <asm/opcodes.h>
>>
>> -#define SCTLR_EL1_CP15BEN	(0x1 << 5)
>> -#define SCTLR_EL1_SED		(0x1 << 8)
>> -
>>   /*
>> - * ARMv8 ARM reserves the following encoding for system registers:
>> + * sys_reg: Defines the ARMv8 ARM encoding for the System register.
>> + *
>> + * ARMv8 ARM reserves the following encoding for system registers in the
>> + * instructions accessing them.
>
> Nitpick: the sentence should end with a colon.

OK

>
>>    * (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.

>
>> + *
>
> Nitpick: no need for another line.
>

OK

Suzuki

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