linux-kernel - Re: [PATCH V3 03/16] arm64/cpufeature: Make doublelock a signed feature in ID

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Date:   Tue, 5 May 2020 12:10:45 +0100
From:   Will Deacon <will@...nel.org>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V3 03/16] arm64/cpufeature: Make doublelock a signed
 feature in ID_AA64DFR0

On Sat, May 02, 2020 at 07:03:52PM +0530, Anshuman Khandual wrote:
> Double lock feature can have the following possible values.
> 
> 0b0000 - Double lock implemented
> 0b1111 - Double lock not implemented
> 
> But in case of a conflict the safe value should be 0b1111. Hence this must
> be a signed feature instead. Also change FTR_EXACT to FTR_LOWER_SAFE.
> 
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Will Deacon <will@...nel.org>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: Suzuki K Poulose <suzuki.poulose@....com>
> Cc: linux-arm-kernel@...ts.infradead.org
> Cc: linux-kernel@...r.kernel.org
> 
> Suggested-by: Suzuki K Poulose <suzuki.poulose@....com>
> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
> ---
>  arch/arm64/kernel/cpufeature.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 51386dade423..cba43e4a5c79 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -338,7 +338,7 @@ static const struct arm64_ftr_bits ftr_id_mmfr0[] = {
>  };
>  
>  static const struct arm64_ftr_bits ftr_id_aa64dfr0[] = {
> -	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_EXACT, 36, 28, 0),
> +	S_ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, 36, 28, 0),

Wait, isn't this buggered today? Shouldn't that 28 be a 4? I think we really
need to:

	1. Make it impossible to describe overlapping fields, incomplete
	   registers etc (ideally at build-time)

	2. Have a macro that for 4-bit fields so you don't have to type '4'
	   all the time

Suzuki, any ideas how we can make this a bit more robust?

Will