SPI from Linux with spidev

Revision and Notes

Date	Owner	Revision	Notes
24 Feb 2022		1.0	Initial release

Introduction

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. The following provides a guide for testing the SPI bus with spidev.

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.

Attached here the MikroBus schematics:

Note: To be able to control the SPI1 device, the DT must be customized accordingly. See the above schematics to get the SPI1 Som side Pad Names.

DT and kernel configuration

In our example of MOSI/MISO loopback, the DT 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>;
    };
}

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.

Source code

The Linux® kernel spidev_test tool source code can be found under tools/spi[2] directory:

• tools/spi/spidev_test.c

List of spidev options

The spidev_test tool has the following options:

Usage: spidev_test [-DsbdlHOLC3vpNR24SI]
  -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
  -8 --octal    octal transfer
  -S --size     transfer size
  -I --iter     iterations

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

root@imx8mpsolidrun:~# spidev_test -D /dev/spidev1.0 -v
spi mode: 0x4
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  | ......@.... .................. .

• To make sure you can detect your SPI devices, use the following:

ls /dev/spi*

Two available SPI devices are detected: