From: Suzuki.Poulose@arm•com (Suzuki K. Poulose)
To: linux-arm-kernel@lists•infradead.org
Subject: [PATCH] arm64: extract a field correctly in cpuid_feature_extract_field()
Date: Tue, 17 Nov 2015 09:27:19 +0000 [thread overview]
Message-ID: <564AF2F7.9030106@arm.com> (raw)
In-Reply-To: <CAKv+Gu9=zfocXVOsqHQHGu+afZisE_nZWCQ-bN5Fe_X=NacYHg@mail.gmail.com>
On 17/11/15 07:15, Ard Biesheuvel wrote:
> On 17 November 2015 at 06:05, AKASHI Takahiro
> <takahiro.akashi@linaro•org> wrote:
>> Basically, cpuid_feature_extract_field() does shift-left and then
>> shift-right to extract a specific field in an operand. But
>> a shift-left'ed value is casted to 's64' and so a succeeding shift-right
>> operation results in creating a sign-extended (and bogus) value.
>>
>
> This is intentional. This function was created specifically for
> extracting CPU feature fields, which are signed 4-bit quantities,
> where positive values represent incremental functionality, and
> negative values are reserved. This is poorly documented in the ARM ARM
> though.
Right. Akash's fix could break other pieces (like FP/ASIMD support in IDAA64PFR0
where, 0xf => function not implemented).
>
> Using this function for extracting 4-bit unsigned values is a mistake.
>
I will take a look at this one.
Thanks for pointing it out.
Suzuki
next prev parent reply other threads:[~2015-11-17 9:27 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-11-17 5:05 [PATCH] arm64: extract a field correctly in cpuid_feature_extract_field() AKASHI Takahiro
2015-11-17 7:15 ` Ard Biesheuvel
2015-11-17 9:27 ` Suzuki K. Poulose [this message]
2015-11-17 10:39 ` Suzuki K. Poulose
2015-11-18 7:04 ` AKASHI Takahiro
2015-11-18 7:26 ` Ard Biesheuvel
2015-11-18 8:08 ` AKASHI Takahiro
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