[OE-core] Tune files and knobs to turn

[Koen Kooi]

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

regards,

Koen