[yocto] Building yocto with hard float support

[Khem Raj]

> On Nov 12, 2015, at 9:03 AM, Mark Hatle <mark.hatle at windriver.com> wrote:
> 
> On 11/12/15 3:17 AM, Toby Gomersall wrote:
>> Hi,
>> 
>> I want to build yocto with hard float support. I understand that I can
>> point the EXTERNAL_TOOLCHAIN variable at the appropriate external hard
>> float compiler (gcc-5-arm-linux-gnueabihf). However, I was wondering if
>> there was anything within the yocto project to do this?
> 
> I'm going to be pedantic for a second.. bear with me.
> 
> ARM has two ABIs, the standard EABI which passes floating point values via
> integer registers and EABIHF that passes floating point values via floating
> point registers.
> 
> EABI allows hardware floating point along with neon or any other core specific
> optimizations.
> 
> EABIHF requires a specific set and size of VFP registers.
> 
> The Yocto Project is capable of building for both ABIs with whatever
> combinations of optimizations you need.
> 
> The ABI and default processor optimization is configured by the 'DEFAULTTUNE'
> parameter.  This is typically defined in your machine configuration file, either
> explicitly or implicitly via the including of the tune files.
> 
> 
> So for example, if you have an ARMv5 CPU you would likely use:
> 
> DEFAULTTUNE="armv5e"
> 
> This would select ARM EABI, no VFP.
> 
> alternatively you could do:
> 
> DEFAULTTUNE="armv5e-vfp"
> 
> This would select ARM EABI w/ VFP floating point.
> 
> OR
> 
> (what I think you are actually trying to ask)
> 
> DEFAULTTUNE="armv5ehf-vfp"
> 
> This would select ARM EABIHF w/ VFP floating point.
> 

all is fine except armv5 here will confuse people thinking that hf ABI works on armv5 as well
would have been better explained with armv7 example.

> 
> So start with your machine and determine which defaulttune (bitbake -e) you have
> selected.  Adjust it to a compatible setting for the ABI and processor
> optimizations that you wish to use.
> 
>> I'm concerned that future updates to an external compiler might affect
>> my build and I would prefer to use a method from within the yocto project.
> 
> Unless you have a specific need, I don't recommend external compilers.  The
> Yocto Project compilers and configurations are well tested and high quality.
> While using an external compiler is support by some vendors, it adds complexity
> that may not be beneficial.  (Sometimes support and other aspects of commercial
> offerings outweigh the complexity..)
> 
>> Thank you,
>> Toby
>> 
>> 
>> 
> 
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