[OE-core] Tune files and knobs to turn

[Koen Kooi]

Op 28 jun 2011, om 21:13 heeft Darren Hart het volgende geschreven:

> 
> 
> 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).

That won't work because FEED_ARCH/BASE_PACKAGE_ARCH isn't in overrides for oe-core/poky

> 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?

It isn't inherently hardware specific and might need to link to evil sourceless binaries from $evil_vendor and you want to be able to have your DISTRO control that knob. There are other usecases as well, but the evil vendor one is the most common.