How can I determin if execution takes place in thread mode or if an exception is active? (ARMv7-A architecture)

Question

I am using FreeRTOS on an ARM Cortex A9 CPU und I'm desperately trying to find out if it is possible to determin if the processor is executing a normal thread or an interrupt service routine. It is implemented in V7-a architecture.

I found some promising reference hinting the ICSR register (-> VECTACTIVE bits), but this only exist in the cortex M family. Is there a comparable register in the A family as well? I tried to read out the processor modes in the current processor status register (CPSR), but when read during an ISR I saw that the mode bits indicate supervisor mode rather than IRQ or FIQ mode.

Looks a lot like there is no way to determine in which state the processor is, but I wanted to ask anyway, maybe I missed something...

The processor has a pl390 General Interrupt Controller. Maybe it is possible to determine the if an interrupt has been triggered by reading some of it's registers?

If anybody can give me a clue I would be very greatfull!

Edit1: The IRQ Handler of FreeRTOS switches the processor to Superviser mode:

And subsequently switches back to system mode:

Can I just check if the processor is in supervisor mode and assume that this means that the execution takes place in an ISR, or are there other situations where the kernel may switches to supervisor mode, without being in an ISR?

Edit2: On request I'll add an overal background description of the solution that I want to achieve in the first place, by solving the problem of knowing the current execution context.

I'm writing a set of libraries for the CortexA9 and FreeRTOS that will access periphery. Amongst others I want to implement a library for the available HW timer from the processor's periphery.

In order to secure the access to the HW and to avoid multiple tasks trying to access the HW resource simultaneously I added Mutex Semaphores to the timer library implementation. The first thing the lib function does on call is to try to gain the Mutex. If it fails the function returns an error, otherwise it continouses its execution.

Lets focus on the function that starts the timer:

   static ret_val_e TmrStart(tmr_ctrl_t * pCtrl)
   {
   ret_val_e retVal = RET_ERR_DEF;
   BaseType_t  retVal_os = pdFAIL;
   XTtcPs * pHwTmrInstance = (XTtcPs *) pCtrl->pHwTmrInstance;

   //Check status of driver
   if(pCtrl == NULL)
   {
       return RET_ERR_TMR_CTRL_REF;
   }else if(!pCtrl->bInitialized )
   {
       return RET_ERR_TMR_UNINITIALIZED;
   }else
   {
       retVal_os = xSemaphoreTake(pCtrl->osSemMux_Tmr, INSTANCE_BUSY_ACCESS_DELAY_TICKS);

       if(retVal_os != pdPASS)
       {
           return RET_ERR_OS_SEM_MUX;
       }
   }

   //This function starts the timer
   XTtcPs_Start(pHwTmrInstance);

   (...)

Sometimes it can be helpful to start the timer directly inside an ISR. The problem that appears is that while the rest of function would support it, the SemaphoreTake() call MUST be changed to SemaphoreTakeFromISR() - moreover no wait ticks are supported when called from ISR in order to avoid a blocking ISR.

In order to achieve code that is suitable for both execution modes (thread mode and IRQ mode) we would need to change the function to first check the execution state and based on that invokes either SemaphoreTake() or SemaphoreTakeFromISR() before proceeding to access the HW.

That's the context of my question. As mentioned in the comments I do not want to implement this by adding a parameter that must be supplied by the user on every call which tells the function if it's been called from a thread or an ISR, as I want to keep the API as slim as possible.

I could take FreeRTOS approch and implement a copy of the TmrStart() function with the name TmrStartFromISR() which contains the the ISR specific calls to FreeRTOS's system resources. But I rather avoid that either as duplicating all my functions makes the code overall harder to maintain.

So determining the execution state by reading out some processor registers would be the only way that I can think of. But apparently the A9 does not supply this information easily unfortunately, unlike the M3 for example.

Another approch that just came to my mind could be to set a global variable in the assembler code of FreeRTOS that handles exeptions. In the portSAVE_CONTEXT it could be set and in the portRESTORE_CONTEXT it could be reset. The downside of this solution is that the library then would not work with the official A9 port of FreeRTOS which does not sound good either. Moreover you could get problems with race conditions if the variable is changed right after it has been checked by the lib function, but I guess this would also be a problem when reading the state from a processor registers directly... Probably one would need to enclose this check in a critical section that prevents interrupts for a short period of time.

If somebody sees some other solutions that I did not think of please do not hesitate to bring them up.

Also please feel free to discuss the solutions I brought up so far. I'd just like to find the best way to do it.

Thanks!

Answer 1

On a Cortex-A processor, when an interrupt handler is triggered, the processor enters IRQ mode, with interrupts disabled. This is reflected in the state field of CPSR. IRQ mode is not suitable to receive nested interrupts, because if a second interrupt happened, the return address for the first interrupt would be overwritten. So, if an interrupt handler ever needs to re-enable interrupts, it must switch to supervisor mode first.

Generally, one of the first thing that an operating system's interrupt handler does is to switch to supervisor mode. By the time the code reaches a particular driver, the processor is in supervisor mode. So the behavior you're observing is perfectly normal.

A FreeRTOS interrupt handler is a C function. It runs with interrupts enabled, in supervisor mode. If you want to know whether your code is running in the context of an interrupt handler, never call the interrupt handler function directly, and when it calls auxiliary functions that care, pass a variable that indicates who the caller is.

void code_that_wants_to_know_who_called_it(int context) {
    if (context != 0)
        // called from an interrupt handler
    else
        // called from outside an interrupt handler
}

void my_handler1(void) {
    code_that_wants_to_know_who_called_it(1);
}
void my_handler2(void) {
    code_that_wants_to_know_who_called_it(1);
}

int main(void) {
    Install_Interrupt(EVENT1, my_handler1);
    Install_Interrupt(EVENT2, my_handler1);
    code_that_wants_to_know_who_called_it(0);
}