轉載自：https://blog.csdn.net/qwaszx523/article/details/54984544

/**

* 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);

}

}

---------------------

作者：qwaszx523

來源：CSDN

原文：https://blog.csdn.net/qwaszx523/article/details/54984544

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