linux-kernel - Re: [PATCH 2/5] arm64: cpufeature: Track unsigned fields

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Date:	Fri, 20 Nov 2015 13:37:00 +0000
From:	"Suzuki K. Poulose" <Suzuki.Poulose@....com>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	AKASHI Takahiro <takahiro.akashi@...aro.org>,
	linux-arm-kernel@...ts.infradead.org, mark.rutland@....com,
	will.deacon@....com, linux-kernel@...r.kernel.org,
	ard.biesheuvel@...aro.org
Subject: Re: [PATCH 2/5] arm64: cpufeature: Track unsigned fields

On 19/11/15 18:45, Catalin Marinas wrote:
> On Thu, Nov 19, 2015 at 10:03:13AM +0000, Suzuki K. Poulose wrote:
>> On 19/11/15 04:57, AKASHI Takahiro wrote:


> a) A precise value (number of breakpoint registers) or a value from
>     which you derive some precise value. You mentioned these above
>
> b) Fields defining the presence of a feature (1, 2, 3). These would
>     always be positive since the absence of such feature would mean a
>     value of 0
>
> c) Fields defining the absence of a feature by setting 0xf. These are
>     usually fields that were initial RAZ and turned to -1. I don't expect
>     such field be greater than 0, nor smaller than -1.
>
> So I think we can treat (a) and (b) as unsigned permanently.

Agreed.

> We could treat (c) as unsigned as well with a value of 0xf though I'm not sure
> how it matches your LOWER/HIGHER_SAFE definitions.

I think we should treat (c) as signed, as we never know what could change,
given that meaning of (0xf - implies unsupported) < meaning of (0 - supported).
Treating them unsigned could break the LOWER/HIGHER_SAFE definitions and makes
the safe value selection ugly.

Cheers
Suzuki

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