linux-kernel - Re: [PATCH v3 3/7] arm64: Add helpers for detecting AArch32 support at EL0

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Message-ID: <20160414164727.GK4584@arm.com>
Date:	Thu, 14 Apr 2016 17:47:27 +0100
From:	Will Deacon <will.deacon@....com>
To:	Suzuki K Poulose <Suzuki.Poulose@....com>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	mark.rutland@....com, marc.zyngier@....com,
	ynorov@...iumnetworks.com
Subject: Re: [PATCH v3 3/7] arm64: Add helpers for detecting AArch32 support
 at EL0

On Thu, Apr 14, 2016 at 05:46:08PM +0100, Suzuki K Poulose wrote:
> On 14/04/16 17:39, Will Deacon wrote:
> >On Thu, Mar 31, 2016 at 06:27:31PM +0100, Suzuki K Poulose wrote:
> >>Adds a helper to extract the support for AArch32 at EL0
> >>
> >>Tested-by: Yury Norov <ynorov@...iumnetworks.com>
> >>Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
> 
> >>+static inline bool id_aa64pfr0_32bit_el0(u64 pfr0)
> >>+{
> >>+	u32 val = cpuid_feature_extract_unsigned_field(pfr0, ID_AA64PFR0_EL0_SHIFT);
> >>+
> >>+	return val == ID_AA64PFR0_EL0_32BIT_64BIT;
> >
> >Should this be >=? What are the rules for this register?
> 
> This feature value is kind of "FTR_EXACT" where, we don't know what the relationship
> of the values are. Here is the list of possible values :
> 
> 0001	EL0 can be executed in AArch64 state only.
> 0010	EL0 can be executed in either AArch64 or AArch32 state.
> 
> All the other values are reserved. So I believe "==" is better check.

Yeah, and thinking about it some more, that makes sense. Thanks.

Will