Confused by Run* state of STMH7 and the startup code that jumps to ExitRun0Mode

[Torsten_Robitzki]

According to RM0433 Rev 8:

The Run* mode is entered after a POR reset and a wakeup from Standby. In Run*
mode, the performance is limited and the system supply configuration shall be
programmed in PWR control register 3 (PWR_CR3). The system enters Run
mode only when the ACTVOSRDY bit in PWR control status register 1
(PWR_CSR1) is set to 1.

and

As long as ACTVOSRDY indicates that voltage levels are invalid, the system is in
Run* mode, write accesses to the RAMs are not permitted and VOS shall not be
changed.

Now, the startup code first jumps (bl ExitRun0Mode) to the C function `ExitRun0Mode()` which probably might involves writes to stack.

Is this construct (calling a C function from ASM) just relying on the compiler not touching the stack or am I'm reading the docs wrongly?

best regards

Torsten

[KnarfB]

The bl instruction places the return address in the lr register. If ExitRun0Mode is a leaf function (does not call other functions), there is no need for the compiler to save lr on the stack, but just to generate a bx lr instruction for return. The current implementation is indeed a leaf function that just sets PWR->CR3 and waits for the PWR_CSR1_ACTVOSRDY condition. No need to touch the stack.

But, interestingly, when compiled in Debug configuration (-O0 optimization), it will use the stack for saving r7 (callers frame pointer):

08000434 <ExitRun0Mode>:
 8000434:	b480      	push	{r7}
 8000436:	af00      	add	r7, sp, #0
 8000438:	4b09      	ldr	r3, [pc, #36]	@ (8000460 <ExitRun0Mode+0x2c>)
 800043a:	68db      	ldr	r3, [r3, #12]
 800043c:	4a08      	ldr	r2, [pc, #32]	@ (8000460 <ExitRun0Mode+0x2c>)
 800043e:	f023 0302 	bic.w	r3, r3, #2
 8000442:	60d3      	str	r3, [r2, #12]
 8000444:	bf00      	nop
 8000446:	4b06      	ldr	r3, [pc, #24]	@ (8000460 <ExitRun0Mode+0x2c>)
 8000448:	685b      	ldr	r3, [r3, #4]
 800044a:	f403 5300 	and.w	r3, r3, #8192	@ 0x2000
 800044e:	2b00      	cmp	r3, #0
 8000450:	d0f9      	beq.n	8000446 <ExitRun0Mode+0x12>
 8000452:	bf00      	nop
 8000454:	bf00      	nop
 8000456:	46bd      	mov	sp, r7
 8000458:	f85d 7b04 	ldr.w	r7, [sp], #4
 800045c:	4770      	bx	lr
 800045e:	bf00      	nop
 8000460:	58024800 	.word	0x58024800

This is pointless as the caller is the startup code that does not use r7 at that time.

Hmm.

hth

KnarfB

[Torsten_Robitzki]

I think it's not surprising, that the compiler generate sub-optimal code without any optimization.

Do you think, I read the documentation correctly and the startup code is broken?