[meta-xilinx] [RFC][Question] Enable VFP?

[Nathan Rossi]

On Fri, Jul 10, 2015 at 5:09 PM, Bian, Naimeng <biannm at cn.fujitsu.com> wrote:
> Hi all,
>
> As I know, VFP is an optional unit for ARMv7, but do all of Zynq Soc support VFP,
> if so, IMO we should set DEFAULTTUNE to 'cortexa9thf-neon' in conf/machine/include/tune-zynq.

Hi Bian,

So this topic I believe has come up before. There are some specifics
that need highlighting so that the concepts are fully understood as
well as the reasons behind the choice for the DEFAULTTUNE used in
meta-xilinx.

Firstly let me clear up your understanding of the Yocto/OE tune
features and the setup of toolchain settings in relation to VFP. The
cortexa9 tunes in OE are a derivative of the armv7a tunes, with the
additional of the "cortexa9" feature. By default the "armv7a" tune
includes the "arm", "armv7a" and "vfp" tunes (see here
https://github.com/openembedded/openembedded-core/blob/master/meta/conf/machine/include/arm/arch-armv7a.inc#L17).
Also of important note is that the "hf" refers to the
"callconvention-hard" tune. So based on the DEFAULTTUNE set in
meta-xilinx which is "cortexa9-neon" the 'vfp' and 'neon' tune
features are enabled. This means that the default config for Zynq
machines in meta-xilinx will emit code that uses the VFP.

What you are suggesting is to use the "cortexa9thf-neon" tune as the
default. This would by additionally enable 'callconvention-hard' as
well as 'thumb'. If you are unfamiliar with the "hard-float" vs
"soft-fp" vs "soft-float" for arm here is a quick summary:
* hard-float -> The calling ABI will pass float variables across calls
in the VFP registers. This removes the need to copy back data into the
general purpose registers (ABI not compatible with soft-fp)
* soft-fp -> (not to be confused with soft-float) The calling ABI will
pass float variables via the general purpose registers, which may or
may not require copying from the VFP registers. This allows for
compatibility with the existing ARM abi.
* soft-float -> complete software float implementation, calling ABI is
same as soft-fp

The reason for using soft-fp by default for Zynq machines/tune is that
Xilinx started out using soft-fp since the beginning for Zynq, all the
Xilinx tooling relies on soft-fp (e.g. FSBL, XSDK, etc). So it made
sense to default to this in meta-xilinx to ensure users would not hit
issues when moving to Yocto/OE or if they were porting older ARM
applications/etc.

Yocto/OE is very flexible in this manner, if you need the performance
gain that hard-float can provide (roughly 10-15% in best case) and you
are prepared to break the compatibility with the soft-fp ABI then you
can switch the DEFAULTTUNE for you machine (or just in your
local.conf) to "cortexa9hf-neon" or "cortexa9thf-neon" (for thumb
support).

Regards,
Nathan

>
> Thanks
>  Bian
> --
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