Re: [Buildroot] [PATCH v2] configs/kontron_bl_imx8mm_defconfig: new defconfig

[Thomas Petazzoni <thomas.petazzoni@bootlin.com>]

On Mon, 17 Jan 2022 20:58:52 +0100
Giulio Benetti <giulio.benetti@benettiengineering.com> wrote:

> > diff --git a/configs/kontron_bl_imx8mm_defconfig b/configs/kontron_bl_imx8mm_defconfig
> > new file mode 100644
> > index 0000000000..5b5648cc14
> > --- /dev/null
> > +++ b/configs/kontron_bl_imx8mm_defconfig
> > @@ -0,0 +1,59 @@
> > +# Architecture
> > +BR2_aarch64=y
> > +BR2_ARM_FPU_VFPV3=y  
> 
> i.MX8MM supports VFPv4-D16, so I would substitute this ^^^ with:
> BR2_ARM_FPU_VFPV4D16
> 
> This is to achieve the maximum performance.

Nope, that's not really how it works. VFPv3 is better than VFPv3-D16.
Indeed VFPv3 means that the the FPU has 32 double precision registers,
while VFPv3-D16 means that it has "only" 16 double precision registers.

So, if the i.MX8MM has only the VFPv3-D16, then indeed it should be
chosen, because code compiled with VFPv3 may not work, as it might use
too many double precision registers.

On the other hand, if the i.MX8MM has the full VFPv3, then
BR2_ARM_FPU_VFPV3=y should be used.

That being said, the gcc man page only documents vfpv3, vfpv3-d16-fp16,
vfpv3-fp16 as extension for armv7-a. Interesting, would need to look
into this.

Best regards,

Thomas
-- 
Thomas Petazzoni, co-owner and CEO, Bootlin
Embedded Linux and Kernel engineering and training
https://bootlin.com
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