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Raspberry Pi kernel GPIO benchmarking test

Abstract

rpi-gpio-test is a test to benchmark and compare different approaches to performing GPIO on Raspberry Pi from within a Linux kernel module.

Motivation

There are quite a few GPIO benchmarking tests for Raspberry Pi available on the web in general and GitHub in particular. The absolute most of those just do write-only GPIO and measure performance of that. In reality, however, it's often crucial to measure the speed of a two-way communication to estimate the expected latency of a response to some external event.

Another thing is, when we benchmark a two-way GPIO communication, it's important to test and compare both interrupt-based and polling-based approaches.

This repo provides a codebase for a more complete testing matrix.

Testing matrix

Test Description
write-only a simple busy loop which constantly changes the state of a GPIO line from HIGH to LOW and back
read/write there are two parties, A and B; each one is both a reader and a writer. Each reader waits for its GPIO line to drop, resets it to back to HIGH, then drops the GPIO line of its counterpart. One of the GPIO lines is dropped initially to start the loop.

When a reader waits for its GPIO to drop, it can do either of the following:

Method Description
IRQ-based use an interrupt handler to be triggered on lowering of a GPIO line
polling-based run a thread which would constantly poll the state of a GPIO line in a busy loop

For reading and writing the states of GPIO lines, two approaches may be used:

Method Description
GPIO registers map BCM-2836-specific GPIO registers into kernel address space and use those to read and write GPIO lines
gpio_get_value / gpio_set_value use portable, but less performant GPIO API available in Linux kernel

Usage

  1. Install Linux kernel headers for Raspberry Pi

    $ sudo apt install raspberrypi-kernel-headers

  2. Select testing configuration using defines in rpi-gpio-test.c

    Testing scenario can be changed using:

    #define

    Default

    Description

    USE_GPIO_REGS

    1

    use BCM-2836 GPIO registers (otherwise, use the gpio_get_value / gpio_set_value API)

    USE_RW_IRQ

    0

    use interrupts for read-write benchmark

    USE_RW_POLL

    1

    use polling for read-write benchmark

    USE_RW_YIELD

    0

    yield CPU with schedule () during polling

    USE_WO_BLOCKING

    1

    perform blocking write-only benchmark in module_init ()

    USE_WO_THREADED

    0

    perform write-only benchmark in a dedicated thread

    USE_AFFINITY

    1

    assign threads to different CPU cores

    GPIO pins can be changed using:

    #define

    Default

    Description

    GPIO_A_OUT

    17

    output pin used to trigger reader A

    GPIO_A_IN

    18

    reader A of the read-write test

    GPIO_B_OUT

    23

    output pin used to trigger reader B

    GPIO_B_IN

    24

    reader B of the read-write test

    GPIO_C_OUT

    22

    used for write-only test

    For read-write tests, connect pins GPIO_A_OUT ↔ GPIO_A_IN and GPIO_B_OUT ↔ GPIO_B_IN.

    Iteration counts can be changed using:

    #define

    Default (API)

    Default (regs)

    Description

    RW_IRQ_ITERATION_COUNT

    50000

    500000

    IRQ-based read-write test

    RW_POLL_ITERATION_COUNT

    500000

    5000000

    polling-based read-write test

    WO_ITERATION_COUNT

    1000000

    10000000

    write-only test

  3. Build kernel module

    $ cd src
$ make
make -C /lib/modules/4.9.80-v7+/build/ M=/home/pi/Projects/rpi-gpio-test/src modules
make[1]: Entering directory '/usr/src/linux-headers-4.9.80-v7+'
  CC [M]  /home/pi/Projects/rpi-gpio-test/src/rpi-gpio-test.o
  Building modules, stage 2.
  MODPOST 1 modules
  CC      /home/pi/Projects/rpi-gpio-test/src/rpi-gpio-test.mod.o
  LD [M]  /home/pi/Projects/rpi-gpio-test/src/rpi-gpio-test.ko
make[1]: Leaving directory '/usr/src/linux-headers-4.9.80-v7+'

  4. Load kernel module

    $ sudo insmod rpi-gpio-test

    This will launch the testing scenario chosen with the pre-processor definitions during step 2.

  5. Inspect the results

    $ dmesg

  6. Unload kernel module

    $ sudo rmmod rpi-gpio-test

  7. Repeat from step 2

Results

The following results are obtained on Raspberry Pi 2 Model B V1.1

The Linux version is:

$ uname -a
Linux raspberrypi 4.9.80-v7+ #1098 SMP Fri Mar 9 19:11:42 GMT 2018 armv7l GNU/Linux
Test Frequency (API) Frequency (regs)
Write-only test 1.3 MHz 41 MHz
Read-write test (IRQ) 110 KHz 140 KHz
Read-write test (poll) 370 KHz 2.7 MHz