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

[Christophe Lyon]

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  <christophe.lyon@arm.com>
>>
>>     * 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.