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rtos/FreeRTOS: support ARMv8-M (Cortex-M33) stacking #151

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rtos/FreeRTOS: support ARMv8-M (Cortex-M33) stacking #151

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4ba5ef4
rtos: add ARMv8-M (Cortex-M33/M23) FreeRTOS register stackings
anujdeshpande May 21, 2026
bd4179d
rtos/FreeRTOS: select ARMv8-M stacking from EXC_RETURN
anujdeshpande May 21, 2026
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34 changes: 33 additions & 1 deletion src/rtos/freertos.c
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Original file line number Diff line number Diff line change
Expand Up @@ -39,6 +39,8 @@ struct freertos_params {
const struct rtos_register_stacking *stacking_info_cm3;
const struct rtos_register_stacking *stacking_info_cm4f;
const struct rtos_register_stacking *stacking_info_cm4f_fpu;
const struct rtos_register_stacking *stacking_info_cm33;
const struct rtos_register_stacking *stacking_info_cm33_fpu;
};

static const struct freertos_params freertos_params_list[] = {
Expand All @@ -55,6 +57,8 @@ static const struct freertos_params freertos_params_list[] = {
&rtos_standard_cortex_m3_stacking, /* stacking_info */
&rtos_standard_cortex_m4f_stacking,
&rtos_standard_cortex_m4f_fpu_stacking,
&rtos_standard_cortex_m33_stacking,
&rtos_standard_cortex_m33_fpu_stacking,
},
{
"hla_target", /* target_name */
Expand All @@ -69,6 +73,8 @@ static const struct freertos_params freertos_params_list[] = {
&rtos_standard_cortex_m3_stacking, /* stacking_info */
&rtos_standard_cortex_m4f_stacking,
&rtos_standard_cortex_m4f_fpu_stacking,
&rtos_standard_cortex_m33_stacking,
&rtos_standard_cortex_m33_fpu_stacking,
},
};

Expand Down Expand Up @@ -417,9 +423,35 @@ static int freertos_get_thread_reg_list(struct rtos *rtos, int64_t thread_id,
thread_id + param->thread_stack_offset,
stack_ptr);

struct armv7m_common *armv7m_target = target_to_armv7m(rtos->target);

/* ARMv8-M FreeRTOS ports (Cortex-M33/M23, e.g. RP2350_ARM_NTZ) always push
* two extra software-saved words (PSPLIM, then EXC_RETURN) ahead of R4-R11,
* so both the EXC_RETURN position and all register offsets differ from the
* classic Cortex-M3/M4F layouts. The software-saved EXC_RETURN (at offset
* 0x04) is the authoritative indicator of whether the extended FPU frame
* was stacked, so use it directly here. Note we deliberately do NOT gate
* this on the armv7m FPU detection below (fp_feature / CPACR): that probe
* does not reliably report the FPU on ARMv8-M cores, and getting it wrong
* would silently misread every FPU-frame task with the basic layout. */
if (is_armv7m(armv7m_target) && armv7m_target->arm.arch == ARM_ARCH_V8M) {
uint32_t exc_return = 0;
retval = target_read_u32(rtos->target, stack_ptr + 0x04, &exc_return);
if (retval != ERROR_OK) {
LOG_ERROR("Error reading EXC_RETURN from FreeRTOS thread stack");
return retval;
}
/* EXC_RETURN bit 4 == 0 means the extended (FPU) frame is in use. */
const struct rtos_register_stacking *stacking;
if ((exc_return & 0x10) == 0)
stacking = param->stacking_info_cm33_fpu;
else
stacking = param->stacking_info_cm33;
return rtos_generic_stack_read(rtos->target, stacking, stack_ptr, reg_list, num_regs);
}

/* Check for armv7m with *enabled* FPU, i.e. a Cortex-M4F */
int cm4_fpu_enabled = 0;
struct armv7m_common *armv7m_target = target_to_armv7m(rtos->target);
if (is_armv7m(armv7m_target)) {
if ((armv7m_target->fp_feature == FPV4_SP) || (armv7m_target->fp_feature == FPV5_SP) ||
(armv7m_target->fp_feature == FPV5_DP)) {
Expand Down
89 changes: 89 additions & 0 deletions src/rtos/rtos_standard_stackings.c
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Original file line number Diff line number Diff line change
Expand Up @@ -74,6 +74,61 @@ static const struct stack_register_offset rtos_standard_cortex_m4f_fpu_stack_off
};


/*
* ARMv8-M (Cortex-M33/M23) FreeRTOS ports (e.g. RP2350_ARM_NTZ) push an extra
* software-saved frame that the classic Cortex-M3/M4F layouts do not:
*
* [low addr = saved pxTopOfStack]
* 0x00 PSPLIM
* 0x04 EXC_RETURN (software-saved LR)
* 0x08 R4 .. 0x24 R11
* (0x28 S16..S31, only when the task used the FPU - extended frame)
* <hardware exception frame: R0..R3, R12, LR, PC, xPSR (+ S0..S15, FPSCR)>
*
* Compared to rtos_standard_cortex_m4f*, every register is shifted by the two
* leading PSPLIM/EXC_RETURN words and R4-R11 follow EXC_RETURN rather than
* preceding it. Using the M4F tables here yields a garbage call stack.
*/
static const struct stack_register_offset rtos_standard_cortex_m33_stack_offsets[] = {
{ ARMV7M_R0, 0x28, 32 }, /* r0 */
{ ARMV7M_R1, 0x2c, 32 }, /* r1 */
{ ARMV7M_R2, 0x30, 32 }, /* r2 */
{ ARMV7M_R3, 0x34, 32 }, /* r3 */
{ ARMV7M_R4, 0x08, 32 }, /* r4 */
{ ARMV7M_R5, 0x0c, 32 }, /* r5 */
{ ARMV7M_R6, 0x10, 32 }, /* r6 */
{ ARMV7M_R7, 0x14, 32 }, /* r7 */
{ ARMV7M_R8, 0x18, 32 }, /* r8 */
{ ARMV7M_R9, 0x1c, 32 }, /* r9 */
{ ARMV7M_R10, 0x20, 32 }, /* r10 */
{ ARMV7M_R11, 0x24, 32 }, /* r11 */
{ ARMV7M_R12, 0x38, 32 }, /* r12 */
{ ARMV7M_R13, -2, 32 }, /* sp */
{ ARMV7M_R14, 0x3c, 32 }, /* lr */
{ ARMV7M_PC, 0x40, 32 }, /* pc */
{ ARMV7M_XPSR, 0x44, 32 }, /* xPSR */
};

static const struct stack_register_offset rtos_standard_cortex_m33_fpu_stack_offsets[] = {
{ ARMV7M_R0, 0x68, 32 }, /* r0 */
{ ARMV7M_R1, 0x6c, 32 }, /* r1 */
{ ARMV7M_R2, 0x70, 32 }, /* r2 */
{ ARMV7M_R3, 0x74, 32 }, /* r3 */
{ ARMV7M_R4, 0x08, 32 }, /* r4 */
{ ARMV7M_R5, 0x0c, 32 }, /* r5 */
{ ARMV7M_R6, 0x10, 32 }, /* r6 */
{ ARMV7M_R7, 0x14, 32 }, /* r7 */
{ ARMV7M_R8, 0x18, 32 }, /* r8 */
{ ARMV7M_R9, 0x1c, 32 }, /* r9 */
{ ARMV7M_R10, 0x20, 32 }, /* r10 */
{ ARMV7M_R11, 0x24, 32 }, /* r11 */
{ ARMV7M_R12, 0x78, 32 }, /* r12 */
{ ARMV7M_R13, -2, 32 }, /* sp */
{ ARMV7M_R14, 0x7c, 32 }, /* lr */
{ ARMV7M_PC, 0x80, 32 }, /* pc */
{ ARMV7M_XPSR, 0x84, 32 }, /* xPSR */
};

static const struct stack_register_offset rtos_standard_cortex_r4_stack_offsets[] = {
{ 0, 0x08, 32 }, /* r0 (a1) */
{ 1, 0x0c, 32 }, /* r1 (a2) */
Expand Down Expand Up @@ -196,6 +251,24 @@ static target_addr_t rtos_standard_cortex_m4f_fpu_stack_align(struct target *tar
stack_ptr, XPSR_OFFSET);
}

static target_addr_t rtos_standard_cortex_m33_stack_align(struct target *target,
const uint8_t *stack_data, const struct rtos_register_stacking *stacking,
target_addr_t stack_ptr)
{
const int XPSR_OFFSET = 0x44;
return rtos_cortex_m_stack_align(target, stack_data, stacking,
stack_ptr, XPSR_OFFSET);
}

static target_addr_t rtos_standard_cortex_m33_fpu_stack_align(struct target *target,
const uint8_t *stack_data, const struct rtos_register_stacking *stacking,
target_addr_t stack_ptr)
{
const int XPSR_OFFSET = 0x84;
return rtos_cortex_m_stack_align(target, stack_data, stacking,
stack_ptr, XPSR_OFFSET);
}


const struct rtos_register_stacking rtos_standard_cortex_m3_stacking = {
.stack_registers_size = 0x40,
Expand All @@ -221,6 +294,22 @@ const struct rtos_register_stacking rtos_standard_cortex_m4f_fpu_stacking = {
.register_offsets = rtos_standard_cortex_m4f_fpu_stack_offsets
};

const struct rtos_register_stacking rtos_standard_cortex_m33_stacking = {
.stack_registers_size = 0x48,
.stack_growth_direction = -1,
.num_output_registers = ARMV7M_NUM_CORE_REGS,
.calculate_process_stack = rtos_standard_cortex_m33_stack_align,
.register_offsets = rtos_standard_cortex_m33_stack_offsets
};

const struct rtos_register_stacking rtos_standard_cortex_m33_fpu_stacking = {
.stack_registers_size = 0xd0,
.stack_growth_direction = -1,
.num_output_registers = ARMV7M_NUM_CORE_REGS,
.calculate_process_stack = rtos_standard_cortex_m33_fpu_stack_align,
.register_offsets = rtos_standard_cortex_m33_fpu_stack_offsets
};

const struct rtos_register_stacking rtos_standard_cortex_r4_stacking = {
.stack_registers_size = 0x48,
.stack_growth_direction = -1,
Expand Down
2 changes: 2 additions & 0 deletions src/rtos/rtos_standard_stackings.h
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Original file line number Diff line number Diff line change
Expand Up @@ -13,6 +13,8 @@
extern const struct rtos_register_stacking rtos_standard_cortex_m3_stacking;
extern const struct rtos_register_stacking rtos_standard_cortex_m4f_stacking;
extern const struct rtos_register_stacking rtos_standard_cortex_m4f_fpu_stacking;
extern const struct rtos_register_stacking rtos_standard_cortex_m33_stacking;
extern const struct rtos_register_stacking rtos_standard_cortex_m33_fpu_stacking;
extern const struct rtos_register_stacking rtos_standard_cortex_r4_stacking;
target_addr_t rtos_generic_stack_align8(struct target *target,
const uint8_t *stack_data, const struct rtos_register_stacking *stacking,
Expand Down