How to use SPI from Linux userland with spidev

1 Article purpose

Linux^® SPI framework offers several ways to access SPI peripherals. Among them, the spidev framework enables to easily control an SPI peripheral straight from Linux^® user space.
Before going further in this document, the reader might be interested in having a look at the SPI overview article that describes how to use an SPI when the peripheral is assigned to Linux^®:

• How to configure an SPI device through the board device tree (example using "spidev")
• How to perform data transfers in userland

2 SPI hardware tests (loopback MOSI / MISO)

Short-circuit the SPI bus MISO and MOSI lines to create a loopback enables the bus to receive the same data it is sending. This is an interesting solution to quickly perform basic tests as well as performance tests.

On the STM32MP157X-DKX discovery board, MOSI and MISO signals are accessible via the D12 and D11 pins of the STM32MP157x-DKx_-_hardware_description#Arduino_Uno_connector.

3 DT and kernel configuration

To be able to control the SPI device from Linux^® user space, the User mode SPI device driver support must be enabled. Its configuration is described in the SPI_overview#Kernel_configuration.
In addition, the device tree must be customized to expose the SPI peripheral via the spidev framework. The overall device tree configuration is described in SPI_device_tree_configuration.

In our example of MOSI/MISO loopback on the STM32MP157X-DKX Discovery board, the stm32mp157a-dk1.dts device tree, which already includes the board skeleton for spi4, must be customized as follows:

• Activate the SPI controller by setting its status to okay.
• Add a spidev child node.
  • Enable spidev by adding a compatible spidev.
  • Add a reg property, required for the SPI framework but not meaningful in this case since chip select is not defined and loopback is used.
  • Configure the bus speed for SPI communications by setting the spi-max-frequency property.

&spi4 {
    pinctrl-names = "default", "sleep";
    pinctrl-0 = <&spi4_pins_a>;
    pinctrl-1 = <&spi4_sleep_pins_a>;
    status = "okay";
 
    spidev@0{
        compatible = "spidev";
        reg = <0>;
        spi-max-frequency = <4000000>;
    };
};

4 SPI unitary tests using spidev_test

spidev_test, available within the Linux^® kernel, is a test tool enabling to perform tests via the spidev interface.

4.1 Source code

The Linux^® kernel spidev_test tool source code can be found under tools/spi directory:

• tools/spi/spidev_test.c

4.2 Installation on your target

The Linux^® kernel SPI tools are not embedded by default in OpenSTLinux distribution. They can be compiled independently and then installed on the target (see How to build Linux kernel user space tools).

4.3 List of spidev options

The spidev_test tool has the following options:

Board $> spidev_test -h
Usage: spidev_test [-DsbdlHOLC3vpNR24SItx]
  -D --device   device to use (default /dev/spidev1.1)
  -s --speed    max speed (Hz)
  -d --delay    delay (usec)
  -b --bpw      bits per word
  -i --input    input data from a file (e.g. "test.bin")
  -o --output   output data to a file (e.g. "results.bin")
  -l --loop     loopback
  -H --cpha     clock phase
  -O --cpol     clock polarity
  -L --lsb      least significant bit first
  -C --cs-high  chip select active high
  -3 --3wire    SI/SO signals shared
  -v --verbose  Verbose (show tx buffer)
  -p            Send data (e.g. "1234\xde\xad")
  -N --no-cs    no chip select
  -R --ready    slave pulls low to pause
  -2 --dual     dual transfer
  -4 --quad     quad transfer
  -S --size     transfer size
  -I --iter     iterations
  -t --txonly   simplex tx transfer
  -r --rxonly   simplex rx transfer

4.4 Example of 32-byte transfer in Full-duplex with loopback

Board $> spidev_test -D /dev/spidev0.0 -v
			spi mode: 0x0
			bits per word: 8
			max speed: 500000 Hz (500 KHz)
			TX | FF FF FF FF FF FF 40 00 00 00 00 95 FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF F0 0D  | ......@.... .................. .
			RX | FF FF FF FF FF FF 40 00 00 00 00 95 FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF F0 0D  | ......@.... .................. .