9.1. FreeRTOS Usage and Migration Guidelines¶

9.1.3.1.1. Overview¶

• FreeRTOS has the ability to create tasks with the below parameters:
  • Entry function
  • One void * entry function argument
  • Stack memory (when NULL, FreeRTOS uses the default heap to allocate the stack memory)
  • Stack size in units of “stack words”, i.e 32b or 4 bytes in case of R5F and C66x.
  • Priority (0 is lowest, configMAX_PRIORITY-1 is the highest)
    • PDK default config is 16 priorities to match SysBIOS priorities
• Static task object memory allocation is supported
  • PDK examples and OSAL by default use static alloc APIs
• Scheduler policy is configurable
  • configUSE_PREEMPTION, PDK default is 1 to match SysBIOS scheduler policy
    • This enables pre-emptive priority-based scheduling
  • configUSE_TIME_SLICING, PDK default is 0, i.e. disabled, to match SysBIOS scheduler policy
    • If enabled, when two tasks are of the same priority, FreeRTOS will time-slice between the tasks at units of the timer tick
    • NOTE, this is unlike SysBIOS where unless `Task_yeild` is called scheduler will not switch to another task at the same priority

9.1.3.1.2. Initial Tasks¶

• Two tasks are created inside FreeRTOS on startup, idle task and timer task

• Idle task is similar to any other task, only it runs at lowest priority.

  • User can configure a “hook” function to call inside of IDLE, e.g, WFI can be called here.

  • PDK default config is shown below, vApplicationIdleHook calls a periodic load update API

    /* in FreeRTOSConfig.h file */
    #define configUSE_IDLE_HOOK                     (1)
    
    /* in port.c file for R5F and C66x */
    void vApplicationIdleHook( void )
    {
    #if (configLOAD_UPDATE_IN_IDLE==1)
        void vApplicationLoadHook();
    
        vApplicationLoadHook();
    #endif
    }
    

  • NOTE, unlike SysBIOS, only one idle hook function (`vApplicationIdleHook`) is supported in FreeRTOS

    • SysBIOS supported multiple functions to be called when no other Tasks are running.

• Timer task (PDK default config enables timer task to match SysBIOS),

  • Timer callbacks and “deferred” interrupt handler functions are called in the context of the timer task

  • NOTE, unlike SysBIOS, SW timer callbacks are not called in ISR context

  • “deferred” handler functions, can be used as an equivalent of SW interrupt of SysBIOS

  • NOTE, unlike SysBIOS, there is no SWI module in FreeRTOS

  • PDK config keeps the priority of this task to the highest so that the timer function will run immediately after an ISR

  • All of above can be controlled via FreeRTOS Config, PDK defaults are listed below

    #define configUSE_TIMERS               (1) /* enable timer task and SW timers */
    #define configTIMER_TASK_PRIORITY      (configMAX_PRIORITIES - 1) /* highest priority */
    #define configTIMER_TASK_STACK_DEPTH   (256) /* 1KB */
    

9.1.3.1.3. Task load¶

• To measure task and CPU load, in FreeRTOS config, we need to set configGENERATE_RUN_TIME_STATS to 1 (PDK default is 1 when configOPTIMIZE_FOR_LATENCY is 0)
• When load measurement is enabled, portGET_RUN_TIME_COUNTER_VALUE() is called to get a high resolution timer counter value.
• In PDK FreeRTOS porting layer, we implement portGET_RUN_TIME_COUNTER_VALUE() using PMU Counter for R5F and Time Stamp Counter for C66x to return time stamp in units of usecs
• In FreeRTOS Kernel, the load measurement counter will overflow after 32b, i.e around 1 hr with usec resolution timer.
• To avoid this, in FreeRTOS OSAL, a periodic load update API is called in IDLE task to accumulate the measured task load and take care of overflow condition.
• To get task load with overflow condition taken care of, it is recommended to create the task using OSAL TaskP APIs
• Total CPU load is calculated by measuring the time spent in IDLE task and then subtracting it from 100%, i.e CPU load = 100 - CPU idle load

9.1.3.1.4. Task FPU context¶

• When configFLOATING_POINT_CONTEXT is enabled, save/restore of R5F FPU Registers will be performed during each task switch.
• In this case all tasks are created with floating point context and usage of portTASK_USES_FLOATING_POINT/vPortTaskUsesFPU() doesn’t have any effect.
• PDK default is configFLOATING_POINT_CONTEXT enabled (when configOPTIMIZE_FOR_LATENCY is 0).
  • This matches the SYSBIOS implementation.
  • FreeRTOS C66x port as well save/restore the FPU registers by default.
  • C7x doesn’t have separate set of FPU registers.

• For optimized performance the config can be disabled.
  • In this case all task will start without a floating point context.
  • A task that uses the floating point hardware must call portTASK_USES_FLOATING_POINT/vPortTaskUsesFPU() before executing any floating point instructions.

9.1.3.1.5. Important tips¶

• Task function should not return in FreeRTOS, instead, it should call vTaskDelete(NULL) to destroy itself, e.g

  void myTaskMain(void *args)
  {
      /* ... do something ... */
  
      vTaskDelete(NULL);
      /* do not call “return” */
  }
  

• On task delete, FreeRTOS will free any memory allocated internally, if dynamically memory allocation mode was used. This memory free is done in the “IDLE” task, so “IDLE” needs to get the opportunity to run at some point.

• vTaskStartScheduler() starts FreeRTOS and the highest priority task is executed first.

  • Tasks, semaphores and OS objects can be created before vTaskStartScheduler() is called.
  • Recommend to create one task and call vTaskStartScheduler(). Then do everything from within that task including creating other tasks.
  • `vTaskStartScheduler()` is similar to `BIOS_start()`, i.e it never returns and switches to the created tasks or idle task
  • .stack is the stack used by code before FreeRTOS scheduler is started, i.e from _c_int00 to vTaskStartSchedular()

9.1.3.2.1. Overview¶

• Interrupt handler is not directly invoked by FreeRTOS Kernel, porting layer handles this
  • In theory, FreeRTOS can work without interrupts. To switch tasks, interrupts are not needed. However, that’s not very useful.
• Porting layer does below
  • Setup (any) one timer to be configured at configTICK_RATE_HZ, typically 1ms
  • For R5F, we use one of the SOC level general-purpose DM timers.
  • Timer ISR is outside FreeRTOS kernel
  • When the timer ISR happens the porting layer calls xTaskIncrementTick() FreeRTOS API to maintain the FreeRTOS timer tick state
• Porting layer also implements the common interrupt entry and exit logic
  • Esp before interrupt exit, porting layer needs to invoke a task switch if during ISR handling a semaphore was posted that needed a task switch on ISR exit
• Interrupt nesting is also taken care of by this common interrupt handler
• Porting layer also implements functions to protect the critical sections of FreeRTOS via the below APIs
  • portENTER_CRITICAL, disable interrupt, track nesting of calls
  • portEXIT_CRITICAL, re-enable interrupt, if nesting call depth is 0
  • portSET_INTERRUPT_MASK_FROM_ISR, interrupt disable and return old interrupt state
  • portCLEAR_INTERRUPT_MASK_FROM_ISR, restore interrupt state

9.1.3.2.2. Nested interrupts and ISR stack¶

• On R5F,

  • When an interrupt is triggered, the CPU switches to IRQ mode and uses the IRQ stack.
  • IRQ interrupts are disabled by HW at this point.
  • In the ISR handler, some CPU state is saved to IRQ stack and mode is switched to SVC mode and therefore SVC stack
  • IRQs are then enabled, i.e nested interrupts are enabled
  • The user ISR code is executed
  • At this point, more IRQs can occur to interrupt the user ISR code
  • After all IRQs are handled, IRQ is disabled
  • Now mode is switched back to IRQ mode
  • If a task switch was requested by any of the ISRs, tasks are switched, and control returns from IRQ to the highest priority pending task
  • If a task switch was NOT requested, control returns from IRQ to the interrupted task

• On R5F,

  • Thus the user ISR code gets called with SVC stack, so the size of the SVC stack MUST be kept large enough to handle worst-case ISR requirement.

  • The IRQ stack size itself can be kept small since it only saves few bytes (8 bytes) of state for every nested IRQ invocation.

  • This stack size is specified in the linker command file, a sample snippet is shown below,

    __IRQ_STACK_SIZE = 256; /* This is the size of stack when R5 is in IRQ mode */
    __FIQ_STACK_SIZE = 256;
    __SVC_STACK_SIZE = 4096; /* This is the size of stack when R5 is in SVC mode */
    __ABORT_STACK_SIZE = 256;  /* This is the size of stack when R5 is in ABORT mode */
    __UNDEFINED_STACK_SIZE = 256;  /* This is the size of stack when R5 is in UNDEF mode */
    

9.1.3.2.3. Interrupts outside of FreeRTOS¶

• On R5F,
  • When FreeRTOS enter its critical section, it only disables IRQ but not FIQ
  • Hence, FreeRTOS API calls MUST NOT be done inside FIQ.
  • FIQs can be used for extremely low latency interrupt bypassing the OS completely.
  • NOTE: this is same as the case with SysBIOS

9.1.3.2.4. Additional important tips¶

• Only FreeRTOS APIs that end with FromISR can be called from the ISR context

  BaseType_t xHigherPriorityTaskWoken = 0;
  
  xSemaphoreGiveFromISR(pSemaphore->semHndl, &xHigherPriorityTaskWoken);
  portYIELD_FROM_ISR(xHigherPriorityTaskWoken);
  

• The FromISR API returns a flag xHigherPriorityTaskWoken to indicate task switch should be requested, portYIELD_FROM_ISR requests the task switch. The actual task switch happens after all nested ISRs have been executed.

• This is different from SysBIOS, where the same APIs can be used inside ISR and outside ISR.

• The OSAL APIs will take care of this internally as below, e.g,

  SemaphoreP_post(...)
  {
    ....
    if( xPortInIsrContext() )
    {
        BaseType_t xHigherPriorityTaskWoken = 0;
  
        xSemaphoreGiveFromISR(pSemaphore->semHndl, &xHigherPriorityTaskWoken);
        portYIELD_FROM_ISR(xHigherPriorityTaskWoken);
    }
    else
    {
        xSemaphoreGive(pSemaphore->semHndl);
    }
    ...
  }
  

9.1.3.3.1. Overview¶

• FreeRTOS has a textbook implementation of binary semaphore, counting semaphore, mutex with priority inheritance, and recursive mutex
• Timeouts can be specified when waiting on a semaphore/mutex, with timeout error code if semaphore/mutex is not available
• Timeout is specified in units of OS ticks. Use pdMS_TO_TICKS() to convert from msec to OS ticks

9.1.3.3.2. Important tips¶

• Binary and counting semaphores should be used to signal from ISR to task and task to task
  • PDK default config enables binary and counting semaphores
• Mutex should be used for mutual exclusion of critical sections.
  • PDK default config enables mutex and recursive mutexes
• Once again, only APIs that end with FromISR can be used from within ISR. These are non-blocking APIs.

9.1.3.4.1. Overview¶

• Task notification is low overhead API to signal a task from ISR or another task
• We see about a 25% reduction in cycles needed for task switch using task notification vs using semaphores in ideal standalone conditions.
• This is a very freertos specific API and PDK drivers and OSAL will NOT use this.

9.1.3.4.2. Important tips¶

• In practical use-cases, typically cache effects due to cache miss on the larger application code/data, will have a higher effect than overheads of task switch in the smaller OS code.
• So we recommend to NOT use this feature unless really the savings is critical to the end application.
• Here one needs to know the task handle to signal, so it’s difficult to use this API from within a driver library since driver ISR for example does not know the task in which the driver API is called
• Thus use these APIs in applications after thorough analysis and profiling on benefit for the use-case.

9.1.3.5.1. Overview¶

• An event group is a set of event bits. Event bits are used to indicate if an event has occurred or not. Event bits are often referred to as event flags.
• The number of bits (or flags) stored within an event group is 24 bits

9.1.3.5.2. Important tips for application writers¶

• Unlike “Event” in SysBIOS where one could give a “AND” mask and “OR” mask to wait on, in FreeRTOS one can only give one “event mask” and then tell if one should use “AND” wait or “OR” wait on the single event mask
• When “set bits” is called from ISR, actual action of setting bits is done within the “Timer task” as a “deferred” function call.

9.1.3.6.1. Overview¶

• Allows users to create one-shot or periodic callbacks at timer resolution of one OS tick, typically 1ms.
• Similar to “Clock” module in SysBIOS
• A timer task executes the callbacks. This timer task is like any other task, nothing special about it.

• PDK default is to set highest priority for this task so that after all ISRs this task will execute

• A SW queue is used to post callbacks or functions to execute on timer expiry, the queue depth is the config option. PDK default is 16

9.1.3.6.2. Important tips¶

• Since a single task executes all the multiple callbacks, the callbacks are recommended to not block and spend too much time, even though in theory blocking calls are allowed.
• Use xTimerPendFunctionCall() to do “deferred ISR” handling, i.e do the most critical work in ISR and do the rest of the work in another function call which gets called right after all ISRs are done. This feature can be used as alternative to SWI in SysBIOS
• Note, do not use FromISR APIs in callbacks, use regular FreeRTOS APIs

9.1.3.7.1. Default heap¶

• FreeRTOS kernel has multiple heap implementations as defined in portable/MemMang
• A port should compile one of the below heap_{n}.c files to pick the required heap implementation
  • heap_1.c
    • linear heap allocation, free() not allowed. Typically used when true dynamic alloc is prohibited.
    • Obsolete now and not used anymore, since FreeRTOS natively support static alloc mode
  • heap_2.c
    • Linked list based heap but adjacent free blocks not merged
    • Obsolete now and not used anymore, heap_4.c is a better implementation
  • heap_3.c
    • Uses compiler provided, malloc() and free(), FreeRTOS only makes the calls thread-safe
    • Heap size specified via –heap compiler option and placed in .heap section in the linker command file.
  • heap_4.c PDK default
    • Linked list based heap with adjacent free block merging
    • More or less similar to malloc() from compiler and SysBIOS heap implementation
    • Memory needs to be provided as a global static array, and placed appropriately in the linker command file.
  • heap_5.c
    • Same as heap_4.c, + it allows memory to be specified as multiple memory blocks When memory in one block is full, it will alloc from the next block and so on.

9.1.3.7.2. User defined heaps¶

• Unlike SysBIOS, FreeRTOS does not support user defined heaps at arbitrary user defined memory locations
• On TI SOCs with multiple levels of memory like L2, L3, DDR, it is very convenient to have ability to create multiple application specified heaps.
• heap_4.c almost does this, but it uses a global variable for heap memory base and size
• Therefore, PDK OSAL implements heap APIs to match SysBIOS as below
  • PDK has adapted heap_4.c to take memory base and size as input parameters during heap create. This allows creation of arbitrary number of application heaps to match SysBIOS features
• HeapP.h is the user API
• HeapP_freertos.c, HeapP_freertos_internal.c is the implementation (Note, FreeRTOS kernel code is not modified)

9.1.3.7.3. Important tips¶

• Use –heap in linker command file to specify heap size and place .heap section appropriate memory in the linker command file. This is used by FreeRTOS(heap_4.c) when it needs dynamic memory allocation.
• In general recommend to use FreeRTOS static alloc APIs for tasks, semaphores to keep the application deterministic.
  • PDK OSAL uses FreeRTOS static alloc APIs
• Use PDK provided HeapP.h to create arbitrary application heaps as needed.

9.1.3.8.1. Queues¶

• Highly efficient SW queue implementation
• Lowest level primitive in FreeRTOS used internally by almost all modules like tasks, semaphores, mutex
• Thread/ISR safe
• Blocking/non-blocking with timeouts
• Fixed queue depth and fixed queue element size at queue create
• Can be used by application users

9.1.3.8.2. Queue Sets, Event Groups, Stream Buffers, Co-routines¶

• Croutines, used when tasks are deemed too expensive to use, almost unlikely we will ever use these
• Queue Sets, more higher level than queues, built using queues, semaphores. FreeRTOS recommends to use these very carefully. https://www.freertos.org/Pend-on-multiple-rtos-objects.html
• Stream buffers, built using queues, semaphores
• PDK default config keeps these disabled

9.1.3.9.1. Debug Hook Functions¶

• FreeRTOS provides “hook” function callbacks specified in FreeRTOS config.
• These callbacks are invoked by FreeRTOS at specific points or when certain conditions are met
• Stack overflow check, configCHECK_FOR_STACK_OVERFLOW PDK default enabled
  • Enables stack overflow checks, two options to detect overflows,
    • Option 1: Check if the stack pointer has gone beyond the limit
    • Option 2: Put a pattern at top of the stack and if the pattern is overwritten then call out stack overflow PDK default
    • NOTE: Depending on the nature of stack overflow, this logic may or may not catch all stack overflows
• Malloc failed hook, configUSE_MALLOC_FAILED_HOOK PDK default disabled
  • Called when pvPortMalloc fails inside FreeRTOS, not very useful, since all APIs return error on memory alloc failure.
• Assert, configASSERT PDK default enabled
  • Called when unrecoverable assert condition happens inside FreeRTOS or porting layer
• PDK defines a new config flag configOPTIMIZE_FOR_LATENCY, this can be used to disable all debug hooks and some other configs in FreeRTOS config to minimize FreeRTOS overheads due to debug hooks,
• Recommend to enable all hooks in debug mode and disable all hooks in release mode after applications are reasonably sure assert and overflows will not happen.
  • By default configOPTIMIZE_FOR_LATENCY is set to 0, i.e disabled

9.1.3.9.2. Statistics Hook Functions¶

• FreeRTOS provides APIs to query and return time spent in each task and output the results in a nice formatted table
• The hook functions for this feature are enabled in PDK default config when configOPTIMIZE_FOR_LATENCY is 0.
• It uses the same timer as the tick timer to measure task load.
• See also https://www.freertos.org/rtos-run-time-stats.html

9.1.3.9.3. Trace Hooks Functions¶

• FreeRTOS provides more than 70+ hook macros at key points in the kernel to log trace info
• Example,
  • traceTASK_SWITCHED_IN, called when a task is being switched into
  • traceTASK_SWITCHED_OUT, called when a task is being switched to another task
• This is used by tools like Tracealyzer to log and then visualize task execution and a lot more in a GUI tool, see https://www.freertos.org/FreeRTOS-Plus/FreeRTOS_Plus_Trace/FreeRTOS_Plus_Trace.html
• Not enabled in PDK default config

9.1.4.3.1. Important tips¶

On R5F, Following are major updates required for creating a custom linker files for FreeRTOS Build compared to that for SysBIOS

• Include the following Linker Settings

  --retain="*(.bootCode)"
  --retain="*(.startupCode)"
  --retain="*(.startupData)"
  --retain="*(.irqStack)"
  --retain="*(.fiqStack)"
  --retain="*(.abortStack)"
  --retain="*(.undStack)"
  --retain="*(.svcStack)"
  
  --fill_value=0
  --stack_size=0x4000
  --heap_size=0x8000
  --entry_point=_freertosresetvectors
  
  -stack  0x4000  /* SOFTWARE STACK SIZE */
  -heap   0x8000  /* HEAP AREA SIZE      */
  
  /*-------------------------------------------*/
  /*       Stack Sizes for various modes       */
  /*-------------------------------------------*/
  __IRQ_STACK_SIZE   = 0x1000;
  __FIQ_STACK_SIZE   = 0x0100;
  __ABORT_STACK_SIZE = 0x0100;
  __UND_STACK_SIZE   = 0x0100;
  __SVC_STACK_SIZE   = 0x0100;
  

• The FreeRTOS default linker file places the reset vectors directly at 0x0(ATCM).
• In case of placing the reset vectors at some different address, configCOPY_RESET_VECTORS in FreeROTSConfig.h should be enabled.

9.1.4.4.1. Important tips for creating Custom MPU/Cache Settings¶

• The default settings gCslR5MpuCfg defined in PDK_INSTALL_DIR/packages/ti/csl/arch/r5/src/startup/startup.c is a weak definition and applications can override the defaults by simply redefining the same.
• Typical use-cases may include adding a new section and/or modifying the default attributes of a memory region.
• Applications may include a new source file which redefines the gCslR5MpuCfg as per the required configuration.

9.1.4.5.1. Important tips¶

• In case of SysBIOS, this was setup via XDC cfg and it directly plug in the passed hook function to the reset vectors.
• On contrast with FreeRTOS, the data abort exception ends up in a default exception handler and will in-turn redirect to callback function(if registered)
  • As a result the callback function can be a normal function, i.e. it shouldn’t take care of returning from an interrupt, etc.