使用mmap讀取/寫入寄存器

Question

我正在嘗試學習使用mmap來讀寫一些寄存器。 我有以下代碼。

#define MY_BASE_ADDRESS  0xC0000000 //Base Address for the PCIe
#define LED_ADDRESS      0x00010000 //Offset for LEDS  0x00010000
#define MAPPED_FILE_SIZE  (50 * sizeof(int)) //Guess
#define PAGE_SIZE (sysconf( _SC_PAGESIZE));

void *mapped_region, *mapped_LED_base;

off_t dev_base = (  MY_BASE_ADDRESS | LED_ADDRESS);
unsigned long readback = 0;

首先，我打開/ dev / mem

//The O_SYNC option prevents Linux from caching the contents of /dev/mem
memoryFileDescriptor = open("/dev/mem", O_RDWR | O_SYNC);
    if (memoryFileDescriptor == -1)
        {
        printf("Can't open /dev/mem. %d\n", memoryFileDescriptor );
        exit(0);
        }

// Map one page of memory into user space such that the device is in that page, but  
//it    may not
// be at the start of the page.
mapped_region = mmap(NULL,
                    MAPPED_FILE_SIZE,  //How to know size?
                    PROT_READ | PROT_WRITE,
                    MAP_SHARED,  //File may not be updated until msync or munmap is 
                                                                          // called.
                    memoryFileDescriptor,
                    dev_base);  //How to know the offset?

// get the address of the device in user space which will be an offset from the base
// that was mapped as memory is mapped at the start of a page
mapped_LED_base = mapped_region + dev_base;

然后，我將寫信給該地址：

*((volatile unsigned long *) (mapped_LED_base)) = 0xFFFFF;

並閱讀

readback = *((volatile unsigned long *) (mapped_LED_base));

我無法知道MAP_SIZE及其偏移量是什么？ 該文檔對此不太清楚。 當前代碼給出了分段錯誤錯誤。

我正在使用Linux和C ++

Answer 1

該大小是硬件寄存器的大小，因此當前您是說有50 x 32位寄存器。 在實踐中，此數字會四舍五入為體系結構的頁面大小，通常為4KB。

如果只有一個寄存器是無符號長的，則應將其設置為sizeof(unsigned long) -不應映射超出實際需要的內容-即使系統將其“增長”到4KB。