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