[OE-core] Tune files and knobs to turn

[Khem Raj]

On Tue, Jun 28, 2011 at 12:13 PM, Darren Hart <dvhart at linux.intel.com> wrote:
>
>
> On 06/28/2011 10:38 AM, Koen Kooi wrote:
>>
>> Op 28 jun 2011, om 19:36 heeft Darren Hart het volgende geschreven:
>>
>>>
>>>
>>> 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"
>>
>> Don't confuse softfp calling conventions with softfloat! The above will still emit
>> vfp and neon instructions if your set TARGET_FPU = hard
>
> Ah. So we would need to add something like:
>
> conf/distro/include/angstrom.inc:TARGET_FPU_armv7a ?= "hard"
> conf/distro/include/angstrom.inc:TARGET_FPU_armv7a-vfp ?= "hard"
>
> to something DISTRO specific (poky.conf or similar).
>
> It isn't clear to me why this is a distro policy decision instead of
> part of the tune include or the machine config itself.
>
> Can someone elaborate on why this goes where it does?
>

adding to what Koen said. Its a different incompatible ABI
hardfp binaries will not work on softfp root file system
Since its a ABI changer its best left to distros to choose
what ABI they would prefer but in oe-core we need to make
provisions for that. Which we dont have at the moment.

> Thanks,
>
> --
> Darren Hart
> Intel Open Source Technology Center
> Yocto Project - Linux Kernel
>
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