linux-kernel - Re: [PATCH 5/9] arm64: cpufeature: Define helpers for sys

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Date:   Wed, 30 Nov 2016 14:16:16 +0000
From:   Suzuki K Poulose <Suzuki.Poulose@....com>
To:     Will Deacon <will.deacon@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        catalin.marinas@....com, dave.martin@....com, aph@...hat.com,
        ryan.arnold@...aro.org, sid@...erved-bit.com,
        adhemerval.zanella@...aro.org, mark.rutland@....com,
        marc.zyngier@....com
Subject: Re: [PATCH 5/9] arm64: cpufeature: Define helpers for sys_reg id

On 30/11/16 11:04, Will Deacon wrote:
> On Thu, Nov 24, 2016 at 01:40:05PM +0000, Suzuki K Poulose wrote:
>> Define helper macros to extract op0, op1, CRn, CRm & op2
>> for a given sys_reg id.
>>
>> Cc: Catalin Marinas <catalin.marinas@....com>
>> Cc: Mark Rutland <mark.rutland@....com>
>> Cc: Will Deacon <will.deacon@....com>
>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
>> ---
>>  arch/arm64/include/asm/sysreg.h | 21 ++++++++++++++++++++-
>>  1 file changed, 20 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
>> index 6c80b36..488b939 100644
>> --- a/arch/arm64/include/asm/sysreg.h
>> +++ b/arch/arm64/include/asm/sysreg.h
>> @@ -34,8 +34,27 @@
>>   *	[11-8]  : CRm
>>   *	[7-5]   : Op2
>>   */
>> +#define Op0_shift	19
>> +#define Op0_mask	0x3
>> +#define Op1_shift	16
>> +#define Op1_mask	0x7
>> +#define CRn_shift	12
>> +#define CRn_mask	0xf
>> +#define CRm_shift	8
>> +#define CRm_mask	0xf
>> +#define Op2_shift	5
>> +#define Op2_mask	0x7
>> +
>>  #define sys_reg(op0, op1, crn, crm, op2) \
>> -	((((op0)&3)<<19)|((op1)<<16)|((crn)<<12)|((crm)<<8)|((op2)<<5))
>> +	((((op0) & Op0_mask) << Op0_shift) | \
>> +	 ((op1) << Op1_shift) | ((crn) << CRn_shift) | \
>> +	 ((crm) << CRm_shift) | ((op2) << Op2_shift))
>
> You're preserving the current behaviour here, but why do we care so much
> about masking op0 but then not bother masking any of the other fields?

I don't remember why it was there. But I do remember that there was some confusion
about using only the last bit (Op0 & 1), the other bit reserved as 1 in the mrs/msr
instructions. I think we changed it explicitly to use the 2 bits from the sys reg Op0,
which brought in support for using the mrs_s for things like PSTATE. We could take
out that mask and depend on the user to do the right thing, just like we do for
the rest of the fields.

Suzuki

>
> Will
>