linux-kernel - Re: [PATCH 2/3] ARM: vfp: fix VFPv3 hwcap detection on non-ARM vfp implementations

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Date:	Fri, 19 Sep 2014 11:24:01 -0700
From:	Stephen Boyd <sboyd@...eaurora.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	linux-arm-kernel@...ts.infradead.org,
	linux-arm-msm@...r.kernel.org, Will Deacon <will.deacon@....com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] ARM: vfp: fix VFPv3 hwcap detection on non-ARM vfp
 implementations

On 09/18/14 15:46, Russell King - ARM Linux wrote:
> On Thu, Sep 18, 2014 at 02:43:11PM -0700, Stephen Boyd wrote:
>> diff --git a/arch/arm/include/asm/vfp.h b/arch/arm/include/asm/vfp.h
>> index f4ab34fd4f72..76d3f6907cce 100644
>> --- a/arch/arm/include/asm/vfp.h
>> +++ b/arch/arm/include/asm/vfp.h
>> @@ -21,7 +21,7 @@
>>  #define FPSID_FORMAT_MASK	(0x3  << FPSID_FORMAT_BIT)
>>  #define FPSID_NODOUBLE		(1<<20)
>>  #define FPSID_ARCH_BIT		(16)
>> -#define FPSID_ARCH_MASK		(0xF  << FPSID_ARCH_BIT)
>> +#define FPSID_ARCH_MASK		(0x7F  << FPSID_ARCH_BIT)
> This is incorrect.  On VFPv2, the architecture field is four bits long.
> As you can see from the above, bit 20 indicates that there are no
> double operations provided, and the next two bits indicate the FSTMX/
> FLDMX format.
>
> I know that you're changing this to conform with the ARM ARM, but we
> have to consider that before VFP was subsumed into the ARM ARM, this
> register had the format described as per this file, and these other
> bits may be set for an ARM part.  Including these bits in the mask
> means that we will mis-identify these older parts as VFPv3.
>
> Welcome to the lack of standardisation!
>

Thank you for the warm welcome! I looked at the TRMs for ARM11 and ARM9.
I can't find anywhere where VFPv2 is supported and these bits are set.

Bits 22-16 of FPSID:

ARM1136r1p5:     0x01
ARM1136r1p3:     0x01
ARM1176:         0x01
ARM11MPCorer2p0: 0x01
ARM11MPCorer1p0: 0x01
ARM1156:         0x01
ARM9:            0x01


Do you, or anyone else, know of other implementations? I *hope* that
this same exercise was done by the VFP architects before they
re-purposed bits but who knows. If nobody is actually setting these
higher bits then is there any problem widening the mask (besides it
being slightly confusing)?

-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
hosted by The Linux Foundation

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