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

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Date:	Thu, 14 Apr 2016 17:46:08 +0100
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,
	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 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.

Suzuki