Linux Realtime and Time-Triggert-Systems

This repository is my playground around the linux realtime kernel scheduler in general and realtime in special. Micheal J. Pont (http://www.safetty.net/about) describes in his book ERES (http://www.safetty.net/publications/the-engineering-of-reliable-embedded-systems) a time triggert os/system/environment to build reliable embedded systems. As a linux and open-source developer i try to reflect the ideas to linux. The linux kernel already provides all needed parts like SCHED_FIFO/SCHED_RR and so the environment (time_triggert_env) is more a configuration of them and a little bit around it.

Dont expect to much from this repository because its only a starting point and more a playground than something serious (https://tjohann.wordpress.com/category/embedded-realtime/). With this repository I also want to learn more about linux realtime capabilities.

Everthing useful will make it into libbaalue (https://github.com/tjohann/libbaalue.git).

Prerequisites

I use libcap-ng to get rid of root rights.

Basic idea configuration

The basic idea is to split the task into different functions:

/* build table of functions for a fiber */
int build_sched_table(fiber_element_t fiber_array[], int count);
/* set scheduling properties of a fiber */
int set_sched_props(fiber_element_t fiber_array[], int count);
/* start a fiber -> wait for pthread_exit */
int start_sched_table(fiber_element_t fiber_array[], int count);

The scheduling entity is a fiber (cooperative thread or preemptive thread)

void *fiber(void *arg);

Example

The basic example example1 shows the usage:

size_t num_fiber_elements = 2;
fiber_element_t fiber_array[] =
{
	{
		.func = fiber_1,
		.sched_param = { .sched_priority = 89,
		},
		.cpu = 0,
		.policy = SCHED_FIFO,
		.dt = MS_TO_NS(10),
	},
	{
		.func = fiber_2,
		.sched_param = { .sched_priority = 90,
		},
		.cpu = 1,
		.policy = SCHED_RR,
		.dt = MS_TO_NS(100),
	}
};

With fiber_array i define an array with all needed properties.

A fiber is defined like:

static void
fiber_2 (void)
{
	function_2();
	function_2();
	function_3();
	function_4();
}

With the properities defined in fiber_array[x] we have a SCHED_FIFO/SCHED_RR thread (fiber_2) with priority 90 at CPU 1 (0...x) called every 100ms. Within this fiber we call function_2 to function_4.

Example for function_2:

static void
function_2()
{
	printf("in function_2\n");
}

Examples

ttenv_simple -> simple example to show usage
ttenv_gpio_simple -> ...
ttenv_gpio -> ...
ttenv_gpio -> ...

ttenv_simple

ttenv_gpio(s)

To use that example you need my gpio driver (https://github.com/tjohann/mydriver.git). So pls clone, build and install the gpio_driver.

Possible extenstion

1. At the moment the maximal cyclic time is 999ms, but it should be possible have values >=1s.
2. Check for overruns (within SCHED_FF)
3. Check for overruns (within a fiber)
4. Correct signal-handling (SIGINT ...)

Test environment

Actually I use a void-linux (http://www.voidlinux.eu/) with PREEMPT kernel on a Bananapi-M1 (http://linux-sunxi.org/LeMaker_Banana_Pi). It has a dual-core armv7hf processor@1GHz. I support that device with a RT-PREEMPT patched kernel via my a20_sdk (https://github.com/tjohann/a20_sdk).

When it comes to GPIO example I use my driver examples/scripts (https://github.com/tjohann/mydriver). On of the examples will be to toogle a PIN with a fixed sequence. For that I will use the my gpio driver. Here's my really simple workbench: