[Buildroot] [PATCH 10/16] arch/arm: add support for hard-float on Cortex-M4

[Arnout Vandecappelle <arnout@mind.be>]

On 03/16/16 22:43, Thomas Petazzoni wrote:
> Cortex-M4 cores can optionally have a FPv4-SP floating point unit
> (which is different than the VFPv4). This commit adds the necessary
> Config.in options to allow the user to enable the use of this FPU, and
> to build an EABIhf toolchain for Cortex-M4.
>
> Note that for now the kernel does not have CONFIG_VFP support for
> ARMv7-M, so in practice, this VFP support cannot really be used for
> the moment.

  As in, when you select this option, then your target will fail dramatically 
because it's hardfloat so even a simple printf will crash violently if a 
reschedule happens in the middle?

  If so, I'm not so sure we really want this option...

>
> Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
> ---
>   arch/Config.in.arm | 23 +++++++++++++++++++++--
>   1 file changed, 21 insertions(+), 2 deletions(-)
>
> diff --git a/arch/Config.in.arm b/arch/Config.in.arm
> index ee612f5..ad3c725 100644
> --- a/arch/Config.in.arm
> +++ b/arch/Config.in.arm
> @@ -31,6 +31,12 @@ config BR2_ARM_CPU_HAS_VFPV4
>   	bool
>   	select BR2_ARM_CPU_HAS_VFPV3
>
> +config BR2_ARM_CPU_MAYBE_HAS_FPV4_SP

  Since the gcc option has -d16, shouldn't we have that as well?

> +	bool
> +
> +config BR2_ARM_CPU_HAS_FPV4_SP
> +	bool
> +
>   config BR2_ARM_CPU_HAS_ARM
>   	bool
>
> @@ -177,6 +183,7 @@ config BR2_cortex_m3
>   config BR2_cortex_m4
>   	bool "cortex-M4"
>   	select BR2_ARM_CPU_HAS_THUMB2
> +	select BR2_ARM_CPU_MAYBE_HAS_FPV4_SP
>   	select BR2_ARM_CPU_ARMV7M
>   config BR2_fa526
>   	bool "fa526/626"
> @@ -218,10 +225,11 @@ config BR2_ARM_ENABLE_NEON
>
>   config BR2_ARM_ENABLE_VFP
>   	bool "Enable VFP extension support"

  As I understand it (but I could be wrong), FPv4 is not again different from 
VFP, i.e. it's again a different ABI (like softfloat and VFP are different 
ABIs). At least, you get different flags in the ELF files. So I would tend to 
really make a separate option for FPv4, to avoid all confusion. That is, 
assuming that I'm right about it being a different ABI.

  Note that in practice it doesn't make much of a difference becaus you never 
have both VFP and FPv4 on the same processor. Well, at least for the time being 
- who knows what kind of ugliness ARM will still invent :-)

  Regards,
  Arnout

> -	depends on BR2_ARM_CPU_MAYBE_HAS_VFPV2
> +	depends on BR2_ARM_CPU_MAYBE_HAS_VFPV2 || BR2_ARM_CPU_MAYBE_HAS_FPV4_SP
>   	select BR2_ARM_CPU_HAS_VFPV4 if BR2_ARM_CPU_MAYBE_HAS_VFPV4
>   	select BR2_ARM_CPU_HAS_VFPV3 if BR2_ARM_CPU_MAYBE_HAS_VFPV3
>   	select BR2_ARM_CPU_HAS_VFPV2 if BR2_ARM_CPU_MAYBE_HAS_VFPV2
> +	select BR2_ARM_CPU_HAS_FPV4_SP if BR2_ARM_CPU_MAYBE_HAS_FPV4_SP
>   	help
>   	  For some CPU cores, the VFP extension is optional. Select
>   	  this option if you are certain your particular
> @@ -264,7 +272,7 @@ config BR2_ARM_EABI
>
>   config BR2_ARM_EABIHF
>   	bool "EABIhf"
> -	depends on BR2_ARM_CPU_HAS_VFPV2
> +	depends on BR2_ARM_CPU_HAS_VFPV2 || BR2_ARM_CPU_HAS_FPV4_SP
>   	help
>   	  The EABIhf is an extension of EABI which supports the 'hard'
>   	  floating point model. This model uses the floating point
> @@ -285,6 +293,7 @@ endchoice
>   choice
>   	prompt "Floating point strategy"
>   	depends on BR2_ARM_EABI || BR2_ARM_EABIHF
> +	default BR2_ARM_FPU_FPV4_SP if BR2_ARM_CPU_HAS_FPV4_SP
>   	default BR2_ARM_FPU_VFPV4D16 if BR2_ARM_CPU_HAS_VFPV4
>   	default BR2_ARM_FPU_VFPV3D16 if BR2_ARM_CPU_HAS_VFPV3
>   	default BR2_ARM_FPU_VFPV2 if BR2_ARM_CPU_HAS_VFPV2
> @@ -374,6 +383,15 @@ config BR2_ARM_FPU_VFPV4D16
>   	  cores, including the earlier Cortex-A{8, 9}, you should
>   	  instead select VFPv3-D16.
>
> +config BR2_ARM_FPU_FPV4_SP
> +	bool "FPv4-SP-D16"
> +	depends on BR2_ARM_CPU_HAS_FPV4_SP
> +	help
> +	  This option allows to use the FPv4-SP floating point unit,
> +	  as available in some ARMv7-M processors (Cortex-M4). This
> +	  option requires a FPv4-SP unit that has 16 double-precision
> +	  registers.
> +
>   config BR2_ARM_FPU_NEON
>   	bool "NEON"
>   	depends on BR2_ARM_CPU_HAS_NEON
> @@ -475,6 +493,7 @@ config BR2_GCC_TARGET_FPU
>   	default "vfpv3-d16" 	if BR2_ARM_FPU_VFPV3D16
>   	default "vfpv4" 	if BR2_ARM_FPU_VFPV4
>   	default "vfpv4-d16" 	if BR2_ARM_FPU_VFPV4D16
> +	default "fpv4-sp-d16"	if BR2_ARM_FPU_FPV4_SP
>   	default "neon" 		if BR2_ARM_FPU_NEON
>   	default "neon-vfpv4" 	if BR2_ARM_FPU_NEON_VFPV4
>
>


-- 
Arnout Vandecappelle                          arnout at mind be
Senior Embedded Software Architect            +32-16-286500
Essensium/Mind                                http://www.mind.be
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