SPI writes that don't stick

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.

uint8_t spi_xfer(uint8_t b)
{
    while (!(SPI1->SR & SPI_SR_TXE)) { }
    *(volatile uint8_t *)&SPI1->DR = b;
    while (!(SPI1->SR & SPI_SR_RXNE)) { }
    return *(volatile uint8_t *)&SPI1->DR;
}

void sensor_write(uint8_t reg, const uint8_t *d, int n)
{
    CS_LOW();
    SPI1->DR = reg;
    for (int i = 0; i < n; i++) {
        while (!(SPI1->SR & SPI_SR_TXE)) { }
        SPI1->DR = d[i];
    }
    while (!(SPI1->SR & SPI_SR_TXE)) { }
    CS_HIGH();
}

uint8_t sensor_read(uint8_t reg)
{
    CS_LOW();
    spi_xfer(reg | 0x80u);
    uint8_t v = spi_xfer(0);
    CS_HIGH();
    return v;
}

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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