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SocketCAN 中的非 FD（“传统”）CAN 帧具有以下格式：

struct can_frame {
        canid_t can_id;  /* 32 bit CAN_ID + EFF/RTR/ERR flags */
        __u8    can_dlc; /* frame payload length in byte (0 .. 8) */
        __u8    __pad;   /* padding */
        __u8    __res0;  /* reserved / padding */
        __u8    __res1;  /* reserved / padding */
        __u8    data[8] __attribute__((aligned(8)));
};

帧ID、长度和数据都有明确的位置，加上一些我们不用担心的填充。但是，对于 CAN-FD 帧，有一个额外的字段：

struct canfd_frame {
        canid_t can_id;  /* 32 bit CAN_ID + EFF/RTR/ERR flags */
        __u8    len;     /* frame payload length in byte (0 .. 64) */
        __u8    flags;   /* additional flags for CAN FD */
        __u8    __res0;  /* reserved / padding */
        __u8    __res1;  /* reserved / padding */
        __u8    data[64] __attribute__((aligned(8)));
};

该flags字段看起来很有用，但我没有找到关于它实际包含什么的文档。它是内部的（即由内核设置）？可能的标志是什么，它们是什么意思？

谢谢！

4

1 回答 1

3

我在这里找到了一些信息：

http://elixir.free-electrons.com/linux/latest/source/include/uapi/linux/can.h#L112

/*
 * defined bits for canfd_frame.flags
 *
 * The use of struct canfd_frame implies the Extended Data Length (EDL) bit to
 * be set in the CAN frame bitstream on the wire. The EDL bit switch turns
 * the CAN controllers bitstream processor into the CAN FD mode which creates
 * two new options within the CAN FD frame specification:
 *
 * Bit Rate Switch - to indicate a second bitrate is/was used for the payload
 * Error State Indicator - represents the error state of the transmitting node
 *
 * As the CANFD_ESI bit is internally generated by the transmitting CAN
 * controller only the CANFD_BRS bit is relevant for real CAN controllers when
 * building a CAN FD frame for transmission. Setting the CANFD_ESI bit can make
 * sense for virtual CAN interfaces to test applications with echoed frames.
 */

如果我理解这一点，ESI 位仅对虚拟 CAN 接口的测试有用。但是，BRS 位是相当低级的，它并没有指定硬件是否会自动设置或取消设置它。

于 2017-06-30T17:00:37.247 回答