TriCore: 从RTOS内核的角度看CSA

今天尝试从RTOS内核的角度来看看 TriCore 的 CSA。

CSA的细节信息可以参考前一篇文章 TriCore User Manual 笔记 1-CSDN博客

CSA 的全称是 Context Save Area，顾名思义就是专门用来保存上下文的一块存储区域。

既然是上下文使用，那必然要求低延迟，因此一般将其部署在 fast ram 区域，具体到 TriCore 中，一般将其置于 DSPR 区域。在使用时，在内核启动之前，要将用作 CSA 的内存区域进行初始化，将其串成一个 Free CSA List，并将该区域的首尾地址分别赋值给 FCX 和 LCX 两个 CSA 寄存器，保证程序运行时能正常通过两个寄存器使用 Free CSA List。然后在创建任务时，为每个任务分配初始 CSA，保存任务初始运行状态信息。

下面，结合 Aurix Studio IDE 和 FreeRTOS 来看下上面所说的每一步分别怎么实现。

1. 部署 csa，放置到 DSPR 区域

#define LCF_DSPR0_SIZE  240k
#define LCF_CSA0_OFFSET     (LCF_DSPR0_SIZE - 1k - LCF_CSA0_SIZE)
     
 
    memory dsram0 // Data Scratch Pad Ram
    {
        mau = 8;
        size = 240k;
        type = ram;
        map (dest=bus:tc0:fpi_bus, dest_offset=0xd0000000, size=240k, priority=8);
        map (dest=bus:sri, dest_offset=0x70000000, size=240k);
    }
 
    group (ordered)
    {
         // 将 CORE0 CSA  部署到 DSPR 区域
         group (align = 64, attributes=rw, run_addr=mem:dsram0[LCF_CSA0_OFFSET]) 
         reserved "csa_tc0" (size = LCF_CSA0_SIZE);
         "__CSA0":=        "_lc_ub_csa_tc0";
         "__CSA0_END":=    "_lc_ue_csa_tc0";
    }
 

2. 初始化 CSA， 创建 Free CSA List，并初始化 FCX 和 LCX 寄存器

    #define IFX_SSW_INIT_CONTEXT()                                                   \
    {                                                                            \
        /* Load user stack pointer */                                            \
        Ifx_Ssw_setAddressReg(a10, __USTACK(0));                                 \
        Ifx_Ssw_DSYNC();                                                         \
                                                                                 \
        /*Initialize the context save area for CPU0. Function Calls Possible */  \
        /* Setup the context save area linked list */                            \
        Ifx_Ssw_initCSA((unsigned int *)__CSA(0), (unsigned int *)__CSA_END(0)); \
        /* Clears any instruction buffer */                                      \
        Ifx_Ssw_ISYNC();                                                         \
    }
 
 
IFX_SSW_INLINE void Ifx_Ssw_initCSA(unsigned int *csaBegin, unsigned int *csaEnd)
{
    unsigned int  k;
    unsigned int  nxt_cxi_val = 0U;
    unsigned int *prvCsa      = 0U;
    unsigned int *nxtCsa      = csaBegin;
    unsigned int  numOfCsa    = (((unsigned int)csaEnd - (unsigned int)csaBegin) / 64U);
 
    for (k = 0U; k < numOfCsa; k++)
    {
        nxt_cxi_val = ((unsigned int)((unsigned int)nxtCsa & ((unsigned int)0XFU << 28U)) >> 12U) | \
                      ((unsigned int)((unsigned int)nxtCsa & ((unsigned int)0XFFFFU << 6U)) >> 6U);
 
        if (k == 0U)
        {
            Ifx_Ssw_MTCR(CPU_FCX, nxt_cxi_val);   /* store the new pcxi value to LCX */
        }
        else
        {
            *prvCsa = nxt_cxi_val;
        }
 
        if (k == (numOfCsa - 3U))
        {
            Ifx_Ssw_MTCR(CPU_LCX, nxt_cxi_val);   /* Last but 2 context save area is pointed in LCX to know if there is CSA depletion */
        }
 
        prvCsa  = (unsigned int *)nxtCsa;
        nxtCsa += IFX_SSW_CSA_SIZE; /* next CSA */
    }
 
    *prvCsa = 0U;                   /* Store null pointer in last CSA (= very first time!) */
 
    Ifx_Ssw_DSYNC();
}

3. 创建任务时分配csa

StackType_t * pxPortInitialiseStack( StackType_t * pxTopOfStack,
                                     TaskFunction_t pxCode,
                                     void * pvParameters )
{
    uint32_t * pulUpperCSA = NULL;
    uint32_t * pulLowerCSA = NULL;
 
    /* 16 Address Registers (4 Address registers are global), 16 Data
     * Registers, and 3 System Registers.
     *
     * There are 3 registers that track the CSAs.
     *  FCX points to the head of globally free set of CSAs.
     *  PCX for the task needs to point to Lower->Upper->NULL arrangement.
     *  LCX points to the last free CSA so that corrective action can be taken.
     *
     * Need two CSAs to store the context of a task.
     *  The upper context contains D8-D15, A10-A15, PSW and PCXI->NULL.
     *  The lower context contains D0-D7, A2-A7, A11 and PCXI->UpperContext.
     *  The pxCurrentTCB->pxTopOfStack points to the Lower Context RSLCX matching the initial BISR.
     *  The Lower Context points to the Upper Context ready for the return from the interrupt handler.
     *
     * The Real stack pointer for the task is stored in the A10 which is restored
     * with the upper context. */
 
    /* Have to disable interrupts here because the CSAs are going to be
     * manipulated. */
    portENTER_CRITICAL();
    {
        /* DSync to ensure that buffering is not a problem. */
        _dsync();
 
        /* Consume two free CSAs. */
        pulLowerCSA = portCSA_TO_ADDRESS( __MFCR( $FCX ) );
 
        if( NULL != pulLowerCSA )
        {
            /* The Lower Links to the Upper. */
            pulUpperCSA = portCSA_TO_ADDRESS( pulLowerCSA[ 0 ] );
        }
 
        /* Check that we have successfully reserved two CSAs. */
        if( ( NULL != pulLowerCSA ) && ( NULL != pulUpperCSA ) )
        {
            /* Remove the two consumed CSAs from the free CSA list. */
            _disable();
            _dsync();
            _mtcr( $FCX, pulUpperCSA[ 0 ] );
            _isync();
            _enable();
        }
        else
        {
            /* Simply trigger a context list depletion trap. */
            _svlcx();
        }
    }
    portEXIT_CRITICAL();
 
    /* Clear the upper CSA. */
    memset( pulUpperCSA, 0, portNUM_WORDS_IN_CSA * sizeof( uint32_t ) );
 
    /* Upper Context. */
    pulUpperCSA[ 2 ] = ( uint32_t ) pxTopOfStack;      /* A10; Stack Return aka Stack Pointer */
    pulUpperCSA[ 1 ] = portSYSTEM_PROGRAM_STATUS_WORD; /* PSW  */
 
    /* Clear the lower CSA. */
    memset( pulLowerCSA, 0, portNUM_WORDS_IN_CSA * sizeof( uint32_t ) );
 
    /* Lower Context. */
    pulLowerCSA[ 8 ] = ( uint32_t ) pvParameters; /* A4;  Address Type Parameter Register */
    pulLowerCSA[ 1 ] = ( uint32_t ) pxCode;       /* A11; Return Address aka RA */
 
    /* PCXI pointing to the Upper context. */
    pulLowerCSA[ 0 ] = ( portINITIAL_PCXI_UPPER_CONTEXT_WORD | ( uint32_t ) portADDRESS_TO_CSA( pulUpperCSA ) );
 
    /* Save the link to the CSA in the top of stack. */
    pxTopOfStack = ( uint32_t * ) portADDRESS_TO_CSA( pulLowerCSA );
 
    /* DSync to ensure that buffering is not a problem. */
    _dsync();
 
    return pxTopOfStack;
}