Interfacing with SPI Devices from User Space

Execute full-duplex communication with SPI-based hardware components by configuring bus parameters and managing transaction buffers in your application.

In this lesson, you will learn how to orchestrate full-duplex communication with SPI peripherals by configuring bus parameters and managing transaction buffers through the Linux SPI character device.

Unlike I2C, which is often byte-oriented, the Serial Peripheral Interface (SPI) is a synchronous, full-duplex protocol where data is shifted out of the master and into the slave simultaneously at every clock cycle. In Embedded Linux, user-space applications interact with this bus via the /dev/spidevX.Y device node.

Configuring Bus Parameters

Before initiating a transfer, you must use ioctl commands to define the operational characteristics of the bus. You set the SPI mode (clock polarity and phase), the bits-per-word, and the maximum clock frequency. These parameters ensure your application matches the specific timing requirements of your peripheral, such as an ADC or a sensor:

struct spi_ioc_transfer tr = {
    .tx_buf = (unsigned long)tx_buffer,
    .rx_buf = (unsigned long)rx_buffer,
    .len = ARRAY_SIZE(tx_buffer),
    .speed_hz = 1000000,
    .bits_per_word = 8,
};

Managing Transaction Buffers

The core of SPI interfacing in user space is the spi_ioc_transfer structure. Because SPI is full-duplex, every transaction is a swap: for every byte you send in the tx_buf, you receive a byte in the rx_buf. Even if you only intend to read from a device, you must provide a buffer of "dummy" bytes to clock the data out.

When you issue the SPI_IOC_MESSAGE(1) ioctl, the kernel handles the low-level signal toggling. It manages the Chip Select (CS) line, shifts the data through the hardware controller, and returns the results to your application once the transaction is complete. You must ensure that your tx_buf and rx_buf remain valid and are not modified by your application code until the kernel confirms the transaction is finished. This approach allows you to perform complex protocol interactions by chaining multiple transfers into a single message, ensuring the CS line remains asserted for the duration of the entire sequence.

Key Takeaway: SPI communication in user space relies on the spi_ioc_transfer structure to perform full-duplex swaps, where every outbound byte necessitates a corresponding inbound buffer to capture simultaneous hardware responses.