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

[Arnout Vandecappelle]

On 03/17/16 09:16, Thomas Petazzoni wrote:
> Hello,
>
> On Thu, 17 Mar 2016 00:45:19 +0100, Arnout Vandecappelle wrote:
>> 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?
>
> My tests show that a VFP-using userspace indeed doesn't start. I've
> mailed the STM32 kernel maintainer to see what the plans are in terms
> of FPU support.
>
>>    If so, I'm not so sure we really want this option...
>
> Agreed. I saw that the STM32F4 had a FPU, so I wanted to support that,
> and it's only when I started testing on HW that I realized that the
> kernel CONFIG_VFP option could not be enabled for this platform.
>
> Depending on the answer from the STM32 kernel maintainer, I will either
> drop or keep (disabled in some form?) this floating point support.

  It's also an option to just keep it lingering in patchwork for a while.

>
>>>    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 :-)
>
> The BR2_ARM_ENABLE_VFP option is *not* about choosing the ABI. The
> BR2_ARM_ENABLE_VFP option is only here for cores that have an
> *optional* FPU so that the user can say whether his specific SoC has
> chosen to include the FPU or not.

  Oops my bad, I was confusing it with the BR2_ARM_FPU_* options.

  Regards,
  Arnout

>
> If you for example have an hypothetical Cortex-M5 core that has a
> mandatory FPU, then this BR2_ARM_ENABLE_VFP option would not appear, as
> we know all Cortex-M5 based SoCs have FPU.
>
> Thomas
>


-- 
Arnout Vandecappelle                          arnout at mind be
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