/** * struct can_frame - basic CAN frame structure * @can_id:  CAN ID of the frame and CAN_*_FLAG flags, see canid_t definition * @can_dlc: frame payload length in byte (0 .. 8) aka data length code *           N.B. the DLC field from ISO 11898-1 Chapter 8.4.2.3 has a 1:1 *           mapping of the 'data length code' to the real payload length * @data:    CAN frame payload (up to 8 byte) */struct can_frame { canid_t can_id;  /* 32 bit CAN_ID + EFF/RTR/ERR flags */ __u8    can_dlc; /* frame payload length in byte (0 .. CAN_MAX_DLEN) */ __u8    data[CAN_MAX_DLEN] __attribute__((aligned(8)));};

can_id定義如下所示，是一個無符號的32位整形數

typedef __u32 canid_t;

can_id數據組織形式如下

/* * Controller Area Network Identifier structure * * bit 0-28 : CAN identifier (11/29 bit) * bit 29 : error message frame flag (0 = data frame, 1 = error message) * bit 30 : remote transmission request flag (1 = rtr frame) * bit 31 : frame format flag (0 = standard 11 bit, 1 = extended 29 bit) */

0-28位為標識符，如果是擴展幀，則高11位為標準ID

29位標識是數據幀還是錯誤消息

30位說明是否是遠程幀

31位說明是標準幀還是擴展幀。

以下是在處理can_frame時用到的掩碼和標識符：

/* special address description flags for the CAN_ID */#define CAN_EFF_FLAG 0x80000000U /* EFF/SFF is set in the MSB */#define CAN_RTR_FLAG 0x40000000U /* remote transmission request */#define CAN_ERR_FLAG 0x20000000U /* error message frame */

/* valid bits in CAN ID for frame formats */#define CAN_SFF_MASK 0x000007FFU /* standard frame format (SFF) */#define CAN_EFF_MASK 0x1FFFFFFFU /* extended frame format (EFF) */#define CAN_ERR_MASK 0x1FFFFFFFU /* omit EFF, RTR, ERR flags */

對can_frame的處理是在mcp251x_hw_tx中進行的，如下：

static void mcp251x_hw_tx(struct spi_device *spi, struct can_frame *frame,     int tx_buf_idx){ struct mcp251x_PRiv *priv = spi_get_drvdata(spi); u32 sid, eid, exide, rtr; u8 buf[SPI_TRANSFER_BUF_LEN];

 exide = (frame->can_id & CAN_EFF_FLAG) ? 1 : 0; //取can_id的31位，判斷是標準幀還是擴展幀 if (exide)  sid = (frame->can_id & CAN_EFF_MASK) >> 18;//如果是擴展幀，can_id的0-28位為ID，其中高11位為標準ID else  sid = frame->can_id & CAN_SFF_MASK; /* Standard ID */ eid = frame->can_id & CAN_EFF_MASK; /* Extended ID */

 rtr = (frame->can_id & CAN_RTR_FLAG) ? 1 : 0; /* 是否是遠程幀*/

 buf[TXBCTRL_OFF] = INSTRUCTION_LOAD_TXB(tx_buf_idx);    //發送緩沖器控制寄存器地址 buf[TXBSIDH_OFF] = sid >> SIDH_SHIFT;        //發送緩沖器標準ID高8位

//5-7位存放發送緩沖器低3位,3位存放幀格式，0-1位存放擴展標識符低18位的高兩位（16-17） buf[TXBSIDL_OFF] = ((sid & SIDL_SID_MASK) << SIDL_SID_SHIFT) |  (exide << SIDL_EXIDE_SHIFT) |  ((eid >> SIDL_EID_SHIFT) & SIDL_EID_MASK); buf[TXBEID8_OFF] = GET_BYTE(eid, 1);     //存放擴展標識符低18位的8-15位 buf[TXBEID0_OFF] = GET_BYTE(eid, 0);    //擴展標識符低18位的低8位（0-7） buf[TXBDLC_OFF] = (rtr << DLC_RTR_SHIFT) | frame->can_dlc; //6位存放遠程幀標識符，0-3存放數據長度碼 memcpy(buf + TXBDAT_OFF, frame->data, frame->can_dlc);//拷貝要發送的數據 mcp251x_hw_tx_frame(spi, buf, frame->can_dlc, tx_buf_idx);

 /* use INSTRUCTION_RTS, to avoid "repeated frame problem" */ priv->spi_tx_buf[0] = INSTRUCTION_RTS(1 << tx_buf_idx); mcp251x_spi_trans(priv->spi, 1);}

static void mcp251x_hw_rx_frame(struct spi_device *spi, u8 *buf,    int buf_idx){ struct mcp251x_priv *priv = spi_get_drvdata(spi);

 if (mcp251x_is_2510(spi)) {  int i, len;

  for (i = 1; i < RXBDAT_OFF; i++)   buf[i] = mcp251x_read_reg(spi, RXBCTRL(buf_idx) + i);

  len = get_can_dlc(buf[RXBDLC_OFF] & RXBDLC_LEN_MASK);  for (; i < (RXBDAT_OFF + len); i++)   buf[i] = mcp251x_read_reg(spi, RXBCTRL(buf_idx) + i); } else {  priv->spi_tx_buf[RXBCTRL_OFF] = INSTRUCTION_READ_RXB(buf_idx);  mcp251x_spi_trans(spi, SPI_TRANSFER_BUF_LEN);  memcpy(buf, priv->spi_rx_buf, SPI_TRANSFER_BUF_LEN); }}