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

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.

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

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

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

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