CAN receive filter and handler

The code

Embedded C for Cortex-M. It compiles. There are 3 real bugs in it: the kind that pass a bench test and fail in the field.

typedef struct { uint32_t id; uint8_t ide; uint8_t dlc; uint8_t data[8]; } can_frame_t;
static uint8_t node_state[16][8];

void can_filter_init(void)
{
    CAN1->FMR |= 1u;                 /* filter init mode */
    CAN1->FS1R |= 1u;                /* bank 0: 32-bit */
    CAN1->FM1R &= ~1u;               /* mask mode */
    CAN1->sFilterRegister[0].FR1 = 0x100u << 21;
    CAN1->sFilterRegister[0].FR2 = 0x00Fu << 21;   /* low 4 bits: any node */
    CAN1->FA1R |= 1u;
    CAN1->FMR &= ~1u;
}

void CAN1_RX0_IRQHandler(void)
{
    can_frame_t f;
    can_read_fifo0(&f);
    can_release_fifo0();
    uint8_t node = f.id & 0x0F;
    memcpy(node_state[node], f.data, f.dlc);
    if (f.id == 0x100) {
        apply_command(f.data);
    }
}

What an interviewer expects

• Point to each line you'd stop in a code review.
• Say what actually breaks: the failure mode, not just the rule it violates.
• Fix it, then defend the fix when asked the follow-up.

The bugs, reference fixes and the follow-up question are in the practice app, after you've tried it.

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