Hi folks,
I would like to get the FPU working on the STM32F4xx chip that I am using, but I am stuck, and I believe the problem has to do with the version of ThreadX that is bundled with the SDK.
I found this old thread, but it seemed like there was no resolution: How to use HW FPU on STM32F4xx?
I have had partial success by adding a USE_FPU_CM4F flag to the build system, and then changing the chip-specific flags in wiced_toolchain_ARM_GNU.mk:
# Chip specific flags for GCC
ifeq ($(HOST_ARCH),ARM_CM4)
# flag added to support building for CM4 micros with FPU; define in platform makefile
ifeq ($(USE_FPU_CM4F),1)
__FPU_PRESENT := 1
__FPU_USED := 1
CPU_CFLAGS := -mthumb -mcpu=cortex-m4 -mfloat-abi=softfp
CPU_CXXFLAGS := -mthumb -mcpu=cortex-m4 -mfloat-abi=softfp
CPU_ASMFLAGS := -mcpu=cortex-m4 -mfloat-abi=softfp -mfpu=fpv4-sp-d16
CPU_LDFLAGS := -mthumb -mcpu=cortex-m4 -Wl,-A,thumb
else
CPU_CFLAGS := -mthumb -mcpu=cortex-m4
CPU_CXXFLAGS := -mthumb -mcpu=cortex-m4
CPU_ASMFLAGS := -mcpu=cortex-m4 -mfpu=softvfp
CPU_LDFLAGS := -mthumb -mcpu=cortex-m4 -Wl,-A,thumb
endif #USE_FPU_CM4F
endif #ARM_CM4
Then, in my platform makefile, I define USE_FPU_CM4F . The idea was that I wouldn't have to define a whole new CM4F variant in the build system, just tweak the CM4 one a little bit. Based on what I read about GCC flags, this should work, since soft-fp will still be link-compatible with existing libraries built without FPU support. And the choice of -mfpu was just from other STM32F4xx examples I found.
Then, in system_stm32f4xx.c, at the top of SystemInit(), FPU access is enabled:
/* FPU settings ------------------------------------------------------------*/
#if (__FPU_PRESENT == 1) && (__FPU_USED == 1)
SCB->CPACR |= ((3UL << 10*2)|(3UL << 11*2)); /* set CP10 and CP11 Full Access */
#endif
So, then the compiler does indeed generate FPU instructions, and they even appear to run OK.
BUT: the system hits a hard fault after running for not very long.
Using ETM on the target, I am seeing a crash that appears to originate in tx_thread_create, when the PC is popped, it gets a bad address, spirals out of control, and eventually ends up at a hard fault. See attached screenshot. I could provide a longer trace history privately, if that would help.
I contacted ExpressLogic to get some insight into this, and they indicate that for FPU usage, there must be a call to void tx_thread_fpu_enable(void) to set up correct FPU context saves.
Unfortunately, looking at the tx_port.h file provided in the WICED SDK, it seems like there is a single file that covers both CM3 and CM4, and there's no mention of this function anywhere. Looking at the headers from the objdump of ThreadX.ARM , there's no mention of the tx_thread_fpu_enable() function anywhere.
And the ThreadX User Guide doesn't mention it, either.
So ... any chance that there is a solution for this?
It seems like it is going to take an upgrade to ThreadX. Or possibly a rebuild of the existing archive(s), with the FPU enables functions not being dropped.
