Re: [PATCH v3] arm: Enable FPU for M-profile

I have sent an updated version as part of a new series (now 3 patches):
https://sourceware.org/pipermail/newlib/2026/022515.html

On 6/22/26 15:40, Christophe Lyon wrote:
> 
> 
> On 6/22/26 13:08, Richard Earnshaw wrote:
>> On 19/06/2026 14:04, Christophe Lyon wrote:
>>> Before using the FPU, we must grant access to it.  This helps when
>>> running the GCC testsuite on a simluator where FPU access is not
>>> granted by default.
>>>
>>> This patch grants full access, we may want to consider granting only
>>> unpriviledged access later.
>>>
>>> 2026-05-29  Christophe Lyon  <[email protected]>
>>>
>>>     * libgloss/arm/cpu-init/rdimon-aem.S: Enable full FPU access on
>>>     M-profile.
>>>     * libgloss/arm/crt0.S: Call _rdimon_hw_init_hook for M profile
>>>     too.
>>> ---
>>>   libgloss/arm/cpu-init/rdimon-aem.S | 28 +++++++++++++++++++++++++---
>>>   libgloss/arm/crt0.S                |  4 ++--
>>>   2 files changed, 27 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/libgloss/arm/cpu-init/rdimon-aem.S b/libgloss/arm/cpu- 
>>> init/rdimon-aem.S
>>> index c960924d4..c8640632a 100644
>>> --- a/libgloss/arm/cpu-init/rdimon-aem.S
>>> +++ b/libgloss/arm/cpu-init/rdimon-aem.S
>>> @@ -25,9 +25,10 @@
>>>   /* This file gives a basic initialisation of a Cortex-A series 
>>> core.  It is
>>>      the bare minimum required to get Cortex-A core running with a 
>>> semihosting
>>> -   interface.  For M-profile we only define a hardfault handler, 
>>> required to
>>> -   prevent the simulator from entering into an infinite loop.  The 
>>> comments
>>> -   below apply to A-profile only.
>>> +   interface.  For M-profile we define a hardfault handler, required to
>>> +   prevent the simulator from entering into an infinite loop and we 
>>> enable
>>> +   the FPU when __ARM_FP is set.
>>> +   The comments below apply to A-profile only.
>>
>> Isn't this comment part of your other patch series?
> 
> The other patch adds the hardfault handler. This patch adds the code to 
> enable the PFU, so it updates the comment accordingly.
> 
>>
>>>      It sets up a basic 1:1 phsyical address to virtual address mapping;
>>>      turns the MMU on; enables branch prediction; activates any 
>>> integrated
>>> @@ -542,6 +543,27 @@ page_tables:
>>>        PT7(0x1c0e)
>>>   #elif  (__ARM_ARCH_PROFILE == 'M')
>>> +    .syntax    unified
>>> +    .thumb
>>> +
>>> +    @ CPU Initialisation
>>> +    .globl    _rdimon_hw_init_hook
>>> +    .type    _rdimon_hw_init_hook, %function
>>> +
>>> +_rdimon_hw_init_hook:
>>> +    /* Enable FPU for M-profile only .  */
>>> +#if __ARM_FP
>>> +    /* Read CPACR */
>>> +    ldr    r0, =0xE000ED88
>>> +    ldr    r1, [R0]
>>> +    /* Enable full access to CP10 and CP11.  */
>>> +    orr    r1, r1, #(0xFF << 20)
>>> +    str    r1, [R0]
>>> +    dsb
>>> +    isb
>>
>> What about the NS version of this register (@0xE002ED88) when starting 
>> in secure state?  Even if we don't want to set it here, I think we 
>> should note that in a comment and explain why.
> 
> I didn't think of it.... TBH I have been using this code in a different 
> context for a very long time, I wrote it before NS/S existed ;-)
> 
> So keep the above code unchanged and add something like
> #if __ARM_ARCH >= 800
> ldr    r0, =0xE020ED88
> str    r1, [R0]
> #endif
> 
> Not sure how to test it, though.
> 
>>
>>> +#endif
>>> +    bx    lr
>>> +
>>>       /* On M-profile, call abort in case of hardfault: this causes 
>>> the simulator to
>>>          abort execution instead of going into an infinite loop.  */
>>>       .section .hardfault_handler_addr, "a"
>>> diff --git a/libgloss/arm/crt0.S b/libgloss/arm/crt0.S
>>> index b9e768007..d355405e3 100644
>>> --- a/libgloss/arm/crt0.S
>>> +++ b/libgloss/arm/crt0.S
>>> @@ -253,9 +253,9 @@
>>>       /* __ARM_ARCH_PROFILE is defined from GCC 4.8 onwards, however 
>>> __ARM_ARCH_7A
>>>       has been defined since 4.2 onwards, which is when v7-a support 
>>> was added
>>>       and hence 'A' profile support was added in the compiler.  Allow 
>>> for this
>>> -    file to be built with older compilers.  We only call this for A 
>>> profile
>>> +    file to be built with older compilers.  We only call this for A 
>>> and M profile
>>>       cores.  */
>>> -#if defined (__ARM_ARCH_7A__) || (__ARM_ARCH_PROFILE == 'A')
>>> +#if defined (__ARM_ARCH_7A__) || (__ARM_ARCH_PROFILE == 'A') || 
>>> (__ARM_ARCH_PROFILE == 'M')
>>
>> We should include arm-acle-compat.h and drop the __ARM_ARCH_7A__ test 
>> now.
> 
> Ack
> 
> Thanks,
> 
> Christophe
> 
>>
>>>   /*  The init hook does not use the stack and is called before the 
>>> stack has been set up.  */
>>>   #ifdef ARM_RDI_MONITOR
>>>       bl    _rdimon_hw_init_hook
>>
>> R.
>