Fixed task switching

[alexcekay]

Debugging NuttX reveals that it stores an incorrect SP at regs[0] during context switching.

Right before the context switch via SVC call:

0x08014410 in sys_call2 (nbr=2, parm2=537382140, parm1=537028520) at /home/alex/PX4-Autopilot/platforms/nuttx/NuttX/nuttx/arch/arm/include/syscall.h:186 186 __asm__ __volatile__ => 0x08014410 <arm_switchcontext+10>: 00 df svc 0

sp is as follows:
sp 0x20036d4c 0x20036d4c

In the exception entry (due to SVC)

After the SVC happens, NuttX calculates the previous sp (before exc) and stores it to r2
157 add r2, #HW_XCPT_SIZE /* R2=MSP/PSP before the interrupt was taken */ => 0x0800821a <exception_common+17>: 02 f1 68 02 add.w r2, r2, #104 ; 0x68

r2 is as follows:
r2 0x20036d48 537095496

Workaround

The workaround below solves the problem by adding the offset to the sp stored by NuttX. Afterwards the bt works again. A complete solution is to discuss the topic with the NuttX guys.

[niklaut]

Thanks!