linux-kernel - Re: [PATCH V2] arm64/cpuinfo: Move HWCAP name arrays alongside their bit definitions

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Date:   Thu, 14 May 2020 07:14:58 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Dave Martin <Dave.Martin@....com>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Mark Rutland <mark.rutland@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        linux-kernel@...r.kernel.org, Mark Brown <broonie@...nel.org>,
        Will Deacon <will@...nel.org>, Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [PATCH V2] arm64/cpuinfo: Move HWCAP name arrays alongside their
 bit definitions



On 05/13/2020 08:34 PM, Dave Martin wrote:
> On Thu, May 07, 2020 at 06:59:10PM +0530, Anshuman Khandual wrote:
>> All HWCAP name arrays (i.e hwcap_str, compat_hwcap_str, compat_hwcap2_str)
>> that are scanned for /proc/cpuinfo output are detached from their bit fild
>> definitions making it difficult to corelate. This is also bit problematic
>> because during /proc/cpuinfo dump these arrays get traversed sequentially
>> assuming that they reflect and match HWCAP bit sequence, to test various
>> features for a given CPU.
>>
>> This moves all HWCAP name arrays near their bit definitions. But first it
>> defines all missing COMPAT_HWCAP_XXX that are present in the name string.
>>
>> Cc: Catalin Marinas <catalin.marinas@....com>
>> Cc: Will Deacon <will@...nel.org>
>> Cc: Mark Brown <broonie@...nel.org>
>> Cc: Ard Biesheuvel <ardb@...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
>>
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
>> Acked-by: Mark Rutland <mark.rutland@....com>
>> ---
>> This applies on 5.7-rc4
>>
>> Changes in V2:
>>
>> - Defined COMPAT_KERNEL_HWCAP[2] and updated the name arrays per Mark
>> - Updated the commit message as required
>>
>> Changes in V1: (https://patchwork.kernel.org/patch/11532945/)
>>
>>  arch/arm64/include/asm/hwcap.h | 101 +++++++++++++++++++++++++++++++++
>>  arch/arm64/kernel/cpuinfo.c    |  90 -----------------------------
>>  2 files changed, 101 insertions(+), 90 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/hwcap.h b/arch/arm64/include/asm/hwcap.h
>> index 0f00265248b5..589ac02e1ddd 100644
>> --- a/arch/arm64/include/asm/hwcap.h
>> +++ b/arch/arm64/include/asm/hwcap.h
>> @@ -8,18 +8,27 @@
>>  #include <uapi/asm/hwcap.h>
>>  #include <asm/cpufeature.h>
>>  
>> +#define COMPAT_HWCAP_SWP	(1 << 0)
>>  #define COMPAT_HWCAP_HALF	(1 << 1)
>>  #define COMPAT_HWCAP_THUMB	(1 << 2)
>> +#define COMPAT_HWCAP_26BIT	(1 << 3)
>>  #define COMPAT_HWCAP_FAST_MULT	(1 << 4)
>> +#define COMPAT_HWCAP_FPA	(1 << 5)
>>  #define COMPAT_HWCAP_VFP	(1 << 6)
>>  #define COMPAT_HWCAP_EDSP	(1 << 7)
>> +#define COMPAT_HWCAP_JAVA	(1 << 8)
>> +#define COMPAT_HWCAP_IWMMXT	(1 << 9)
>> +#define COMPAT_HWCAP_CRUNCH	(1 << 10)
>> +#define COMPAT_HWCAP_THUMBEE	(1 << 11)
>>  #define COMPAT_HWCAP_NEON	(1 << 12)
>>  #define COMPAT_HWCAP_VFPv3	(1 << 13)
>> +#define COMPAT_HWCAP_VFPV3D16	(1 << 14)
>>  #define COMPAT_HWCAP_TLS	(1 << 15)
>>  #define COMPAT_HWCAP_VFPv4	(1 << 16)
>>  #define COMPAT_HWCAP_IDIVA	(1 << 17)
>>  #define COMPAT_HWCAP_IDIVT	(1 << 18)
>>  #define COMPAT_HWCAP_IDIV	(COMPAT_HWCAP_IDIVA|COMPAT_HWCAP_IDIVT)
>> +#define COMPAT_HWCAP_VFPD32	(1 << 19)
>>  #define COMPAT_HWCAP_LPAE	(1 << 20)
>>  #define COMPAT_HWCAP_EVTSTRM	(1 << 21)
> 
> With the possible exception of SWP (does the swp emulation allow us to
> report this as supported?), I think all these weren't mentioned because
> they aren't included in ARMv8 and so can never be reported.
> 
> If we find ourselves reporting them, there's a bug somewhere.
> 
> So, can we just default all obsolete string entries to NULL?
> 
> When generating the cpuinfo strings we could WARN and just emit an empty
> string for that hwcap.

All these above will be a change in the existing user visible behavior on
the system and this patch never intended to create one. Hence, I will just
defer this to maintainers on whether we should change existing /proc/cpuinfo
output (including non-practically-possible ones on ARMv8) or even treat swap
emulation as SWP.

But IMHO, in case we decide to go for a /proc/cpuinfo change (which I would
be happy implement), it should be applied over and above this patch which
just preserved existing semantics and behavior.

- Anshuman