Reading the STM32F407-DISCO MEMS Accelerometer over SPI

static void spi_test(BaseSequentialStream *chp, int argc, char **argv)
{
    //select LED15 to signify entry
    palSetPadMode(GPIOD, 15, PAL_MODE_OUTPUT_PUSHPULL);
    palSetPad(GPIOD, 15);

    //start the SPI driver
    const SPIConfig cfg =
    {
        NULL,
        GPIOE,      // port
        GPIOE_CS_SPI,   // /SS
        SPI_CR1_BR_0 | SPI_CR1_BR_1 |
        SPI_CR1_CPOL | SPI_CR1_CPHA
    };
    uint8_t tx[2], rx[2];
    int32_t x, y, z;
    spiStart(&SPID1, &cfg);
    spiSelect(&SPID1);
    //X, Y, Z are in 0x29, 0x2B and 0x2D
    //Control bytes for init are 0x20, 0x21, 0x22
    tx[0] = 0x20;
    tx[1] = 0x73;
    spiSend(&SPID1, 2, tx);
    spiUnselect(&SPID1);
    spiSelect(&SPID1);
    tx[0] = 0x21;
    tx[1] = 0x00;
    spiSend(&SPID1, 2, tx);
    spiUnselect(&SPID1);
    spiSelect(&SPID1);
    tx[0] = 0x22;
    tx[1] = 0x00;
    spiSend(&SPID1, 2, tx);
    spiUnselect(&SPID1);

    //init done, now read out values
    tx[0] = 0x80 | 0x29;
    tx[1] = 0xff;
    spiSelect(&SPID1);
    spiExchange(&SPID1, 2, tx, rx);
    spiUnselect(&SPID1);
    x = (int8_t) rx[1];

    tx[0] = 0x80 | 0x2B;
    tx[1] = 0xff;
    spiSelect(&SPID1);
    spiExchange(&SPID1, 2, tx, rx);
    spiUnselect(&SPID1);
    y = (int8_t) rx[1];

    tx[0] = 0x80 | 0x2D;
    tx[1] = 0xff;
    spiSelect(&SPID1);
    spiExchange(&SPID1, 2, tx, rx);
    spiUnselect(&SPID1);
    //__asm__("bkpt");
    z = (int8_t) rx[1];

    chThdSleepMilliseconds(50);
    chprintf(chp, "Accelerometer: (%d,%d,%d)\r\n", x, y, z);

    spiUnselect(&SPID2);
    palClearPad(GPIOD, 15);

}