[OE-core] Tune files and knobs to turn

[Khem Raj]

On Tue, Jun 28, 2011 at 10:36 AM, Darren Hart <dvhart at linux.intel.com> wrote:
>
>
> On 06/24/2011 04:54 AM, Koen Kooi wrote:
>> Hi,
>>
>> We discussed tune files a bit during last nights TSC meeting and Khem had
>> expressed the need before, so I'd like to get this discussion started by using
>> armv7a as an example.
>>
>> For armv7a capable cores we have the following hardware features:
>>
>> * armv7a instruction set
>> * thumb1 instruction set
>> * thumb2 instruction set
>> * VFP coprocessor
>> * optional NEON coprocessor
>>
>> For the ABI we can choose the following:
>>
>> * softtp without hw support (e.g. no VFP instructions emitted, slow)
>> * softfp with hw support (e.g. VFP and/or NEON instructions emitted, fast)
>> * hardfp, emits VFP and/or NEON instructions, slightly faster than softfp/hw,
>>   incompatible with everything else
>>
>> And the extra knobs:
>>
>> * pure thumb1, no arm instructions (limited use)
>> * thumb1/arm interworking
>> * pure thumb2,  no arm instructions
>> * thumb2 interworking (not sure if that's actually usefull, thumb2 has complete coverage)
>>
>> In OE .dev we have the following vars:
>>
>> TARGET_FPU: switches between hw float and sw float, no reflection in package arch
>> ARM_FP_ABI: switches between softfp and hardfp, will create 'armv7a' or
>>             'armv7a-hardfp' as package arch
>> ARM_INSTRUCTION_SET: switches between arm and thumb1, no reflection in package arch
>> THUMB_INTERWORK: turns on interworking, no reflection in package arch
>>
>> (side note, oe-core/distroless and meta-yocto/poky don't turn set TARGET_FPU
>>  for armv7a and will generate slow code, angstrom does turn it on)
>
>
> oe-core tune-cortexa8.inc doesn't make use of these variables (unlike
> meta-texasinstruments) and does make use of the neon coprocessor, but
> still uses the softfp float-api:
>
> TARGET_CC_ARCH = "-march=armv7-a -mtune=cortex-a8 -mfpu=neon
> -mfloat-abi=softfp -fno-tree-vectorize"
>
> Seems like the oe-core tune files need to be synced up with vendor layers?
>

Well for enabling hardfp its a fundamental decision and I guess using softfloat
in oe-core is probably best choice and the floating point parameter passing ABI
I am taking about we still use -mfpu=neon so gcc will still try to utilize it
but -fno-tree-vectorize is going to subdue the use of neon intrs since gcc
is disallowed to vectorize

> --
> Darren
>
>
>>
>> Khem and I would like to start building armv7a (and armv6) in pure thumb2 mode
>> but we want to have the variables to turn those knobs make sense and be
>> consistent. RP has expressed his desire to sort this all out before merging
>> multilib. I'm sure x86/mips/ppc/etc have a similar need, so let's get this
>> discussion started.
>>
>> regards,
>>
>> Koen
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>
> --
> Darren Hart
> Intel Open Source Technology Center
> Yocto Project - Linux Kernel
>
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