Re: Tune files and knobs to turn

[Darren Hart <dvhart@linux.intel.com>]

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?

Thanks,

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