RTAI编程指南：掌握实时Linux内核与应用开发

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"DIAPM_RTAI_prog_guide.pdf - RTAI实时Linux编程指南1.0版" 这篇文档是关于Real-Time Application Interface (RTAI) 的编程指南，它是一份详细阐述如何在Linux环境下进行实时系统编程的经典教程。RTAI是一个开源的、用于Linux的硬实时扩展，它提供了一个低延迟、高确定性的执行环境，使得Linux系统能够满足严格的实时需求，如工业自动化、航空航天和嵌入式系统的应用。 RTAI的核心在于其调度器和内存管理机制，这些设计使得它能够在不影响常规Linux进程的同时，为实时任务提供确定性的响应时间。通过RTAI，开发者可以创建并运行需要严格时间限制的任务，确保它们能在预设的时间内完成。编程指南中可能涵盖了以下关键知识点： 1. **RTAI架构**：介绍RTAI的内核模块、用户空间接口以及它与标准Linux内核的交互方式。这包括了如何加载和卸载RTAI模块，以及如何配置实时任务的优先级和调度策略。 2. **实时任务管理**：详细讲解如何创建、管理以及调度实时任务。这包括任务的生命周期管理，如任务的启动、暂停、恢复和删除，以及如何设置任务的执行周期和时限。 3. **内存管理**：RTAI提供了专用的实时内存区域，以确保低延迟的内存访问。这部分会解释如何分配和释放这些内存，以及如何确保内存访问的安全性。 4. **中断处理**：在实时系统中，中断处理是至关重要的。RTAI允许开发者注册中断处理程序，并提供了中断上下文下的操作指南。 5. **同步与通信机制**：介绍RTAI提供的各种同步原语（如信号量、互斥锁）和通信机制（如消息队列、管道），这些都是实时系统中不同任务间协作的关键。 6. **设备驱动开发**：RTAI如何支持实时设备驱动的编写，以及如何使设备驱动与实时任务协同工作。 7. **案例研究和示例代码**：为了帮助读者更好地理解和应用RTAI，文档中可能会包含实际的编程示例，展示如何实现一个简单的实时应用程序。 8. **安全性和稳定性**：讨论在实时环境中保证系统稳定和避免死锁等问题的策略。 9. **调试与性能分析**：提供关于如何调试RTAI应用程序和评估系统性能的工具和技巧。此文档还强调，Lineo公司对内容不做任何明示或暗示的保证，读者应理解并接受可能存在错误或不准确之处。同时，Lineo公司保留随时修改软件而不通知的权利。这意味着读者需要持续关注RTAI的更新和改进，以确保其代码的兼容性和最佳性能。这份RTAI编程指南是学习和掌握实时Linux系统编程的重要参考资料，适合对实时系统有深入需求的开发者阅读。通过深入学习，读者将能够利用RTAI开发出高效、可靠的实时应用程序。

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RTAI Programming Guide

‘Hard real-time’ is illustrated by the control system of a high-speed missile

because it relies on guaranteed and repeatable system responses of

thousandths or millionths of a second. Since these control deadlines can never

be missed, a hard real-time system cannot use average case performance to

compensate for worst-case performance. Thus, hard real-time systems are

required for the most technically challenging tasks.

Since an embedded system often performs only a single task, the differences

between soft and hard real-time for these applications are not as critical as one

would think. However, as true multi-tasking operating systems, such as Linux,

are adopted for use in increasingly complex systems, the need for hard

real-time often becomes apparent.

To further confuse the real-time issue, the general term “Real-Time Operating

System (RTOS)” is used to refer to one that can provide either hard or soft

real-time capabilities but not necessarily both. Thus all operating systems

labeled as “RTOS” are not created equally.

The Real-Time Linux Solution

The real-time Linux scheduler treats the Linux operating system kernel as the

idle task. Linux only executes when there are no real time tasks to run, and the

real time kernel is inactive. The Linux task can never block interrupts or

prevent itself from being preempted. The mechanism that makes this possible

is the software emulation of interrupt control hardware. When any code in

Linux tries to disable interrupts, the real time system intercepts the request,

records it, and returns it to Linux. In fact, Linux is not permitted to ever really

disable hardware interrupts, and hence, regardless of the state of Linux, it

cannot add latency to the interrupt response time of the real time system. When

an interrupt occurs, the real time kernel intercepts the interrupt and decides

what to dispatch. If there is a real time handler for the interrupt, the appropriate

handler is invoked. If there is no real time interrupt handler, or if the handler

indicates that it wants to share the interrupt with Linux, then the interrupt is

marked as pending. If Linux has requested that interrupts be enabled, any

pending interrupts are enabled, and the appropriate Linux interrupt handler

invoked - with hardware interrupts re-enabled. Regardless of the state of Linux:

running in kernel mode; running a user process; disabling or enabling

interrupts; the real-time system is always able to respond to an interrupt.

Real-time Linux decouples the mechanisms of the real time kernel from the

mechanisms of the general purpose Linux kernel so that each can be optimized

independently and so that the real-time kernel can be kept small and simple.

Introduction

From a maintenance perspective, this de-coupling allows the Real-Time Linux

kernel to be easily and quickly adapted to follow changes in the mainstream

Linux kernel.

Real-time Linux has been designed so that the real time kernel never waits for

the Linux side to release any resources. The real time kernel does not directly

request memory, share spin locks, or synchronize data structures, except in

tightly controlled situations. For example, the communication links that are

used to transfer data between real time tasks and Linux processes are

non-blocking on the real time side. There is never a case where the real time

task waits to queue or dequeue data.

One of the fundamental design philosophies of Real-time Linux is to let the

Linux operating system do as much as is practicable. Typical examples include

system and device initialization, and blocking dynamic resource allocation.

Any thread of execution that can be blocked when there are no available

resources cannot have real time constraints. Real-time Linux relies on the

Linux loadable module mechanism to install components of the Real-Time

system, which keeps it extensible and modular. Loading a Real-Time module

is not a real-time operation, and so Linux can do it. The primary function of the

Real-Time kernel is to provide direct access to the raw hardware for real time

tasks so that they can execute with minimal latency and maximal processing

resource, when required.

There are two primary variants of hard real-time Linux available: RTLinux and

RTAI. RTLinux was developed at the New Mexico Institute of Technology by

Michael Barabanov under the direction of Professor Victor Yodaiken.

Real-Time Application Interface (RTAI) was developed at the Dipartimento

di Ingeneria Aerospaziale, Politecnico di Milano by Professor Paolo

Mantegazza.

Under both RTLinux and RTAI, all interrupts are initially handled by the real

time kernel and are passed to Linux only when there are no active real time

tasks. Changes to the Linux kernel are minimized by providing the kernel with

a software emulation of the interrupt control hardware. Thus, when Linux has

disabled interrupts, the emulation software will queue interrupts that have been

passed on by the real time kernel. This is achieved by installing a layer of

emulation software between the Linux kernel and the interrupt controller

hardware, and replacing all occurrences of cli, sti, and iret in the Linux source

code with emulating macros. When the Linux kernel would normally disable

interrupts, this event is logged by the emulation software, but interrupts are not

actually disabled. When an interrupt occurs, the emulation software checks to

see whether Linux has interrupts enabled, if so the interrupt is delivered to the

Linux kernel. If not, the interrupt is held pending the Linux kernel re-enabling

interrupts. In this way, the Linux kernel does not have direct control over

RTAI Programming Guide

interrupts, and cannot delay the processing of real time interrupts, as these

interrupts do not pass through the emulation software. Instead, they are

delivered direct to the real time kernel. This also means that the scheduling of

real time tasks cannot be delayed by Linux.

Fundamentally, RTAI, RTLinux and applications written to take advantage of

them operate in the same way. The Real Time kernel, all their component parts,

and the real time application are all run in Linux kernel address space as kernel

modules. As each kernel module is loaded it initializes itself ready for system

operation. The kernel modules can be removed from the kernel on completion

of the real time system operation. Kernel modules can be loaded and unloaded

dynamically, either by an application or by taking advantage of the automatic

module loading features of Linux itself.

The advantages of running the real time system in Linux kernel address space

is the task switch time for the real time tasks is minimized, and Translation

Look-aside Buffer (TLB) invalidation is kept to a minimum as are protection

level changes. Another advantage of making use of kernel loadable modules is

that it aids system modularity. For example, if the scheduler is unsuitable for

a particular application, then the scheduler module can be replaced by one that

meets the needs of the application.

One of the main disadvantages of running in Linux kernel address space is that

a bug in a real time task can crash the whole system, as there is no separate

protected memory space for an individual RT task.

As mentioned earlier, there are two current ‘flavours’ of Real-Time Linux:

RTAI and RTLinux. The remainder of this document is devoted to describing

RTAI.

The RTAI Solution

RTAI provides deterministic and preemptive performance in addition to

allowing the use of all standard Linux drivers, applications and functions.

RTAI was initially developed by The Dipartimento di Ingeneria Aerospaziale

Politecnico di Milano (DIAPM) as a variant of the New Mexico Institute of

Technology's (NMT) RTLinux, at a time when neither floating point support

nor periodic mode scheduling were provided by RTLinux. Since then, RTAI

has added many new features without compromising performance.

RTAI's growing list of features now includes:

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RTAI编程指南：掌握实时Linux内核与应用开发

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RTAI_DEV="$(apt-cache search -n rtai-modules | head -1 | awk '{print $1}')" RTAI_ARCHITECTURE="i386 amd64" if [ ! -z "$RTAI_DEV" ]; then RTAI_DEV="$RTAI_DEV [i386], $RTAI_DEV [amd64]" fi

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