Porting RTOS Device Drivers to Embedded Linux
Porting RTOS code to embedded Linux does not differ conceptually from enterprise application migration. After the logistics of porting have been addressed (make/build scripts and methods, compiler compatibility, location of include files and so on), code-level porting challenges turn on the issues of application architecture and API usage.
For the purposes of the discussion at hand, let us assume that the application part (everything except I/O-specific code) migrates from the RTOS-based system into a single Linux process. RTOS tasks map to Linux threads and intertask IPCs map to Linux inter-process and inter-thread equivalents.
Although the basic shape of the port is easy to understand, the devil is in the details. And the most salient details are the RTOS APIs in use and how to accommodate them with Linux constructs.
If your project is not time-constrained, and if your goal is to produce portable code for future project iterations, then you want to spend some time analyzing the current structure of your RTOS application and how/if it fits into the Linux paradigm. For RTOS application code, you want to consider the viability of one-to-one mapping of RTOS tasks onto Linux process-based threads and whether to repartition the RTOS application into multiple Linux processes. Depending on that decision, you should review the RTOS IPCs in use to determine proper intra-process vs. inter-process scope.
On the driver level, you definitely want to convert any informal in-line RTOS code to proper drivers. If your legacy application already is well partitioned, either using RTOS I/O APIs or at least segregated into a distinct layer, your task becomes much easier. If ad hoc I/O code is sprinkled liberally throughout your legacy code base, you've got your work cut out for you.
Developers in a hurry to move off a legacy RTOS or those trying to glue together a prototype are more likely to attempt to map or convert as many RTOS APIs to Linux equivalents in situ. Entities in common, such as comparable APIs, IPCs and system data types, port nearly transparently. Others can be addressed with #define redefinition and macros. Those remaining need to be recoded, ideally as part of an abstraction layer.
You can get a head start on API-based porting by using emulation libraries that accompany many embedded Linux distributions (including MontaVista's libraries for Wind River VxWorks and pSOS) or by using third-party API-mapping packages from companies such as MapuSoft.
Most projects take a hybrid approach, mapping all comparable or easily translatable APIs, re-architecting where it doesn't slow things down and playing Whack-a-Mole with the remaining code until it builds and runs.
For both intensive re-architecting and for quicker-and-dirtier API approaches, you still have to (re)partition your RTOS application and I/O code to fit the Linux kernel and user-space paradigm. Table 1 illustrates how Linux is much stricter about privileged operations than a legacy RTOS and helps guide you in the (re)partitioning process.
Table 1. Privileged Operations in Linux and Legacy RTOSes
|RTOS Application||Queues, Signals, Mailboxes Informal Shared Memory||Semaphores, Mutexes||Full RTOS Tasking Repertoire||Full Application, Libraries and System (Link-Time)|
|RTOS Driver||Queues, Signals, Mailboxes Informal Shared Memory||Semaphores, Mutexes||Full RTOS Tasking Repertoire||Full Application, Libraries and System (Link-Time)|
|Linux Application||Queues, Signals, Pipes Intra-Process Shared Memory Shared System Memory||Semaphores, Mutexes||Process and Threads APIs||Local Process, Static and Shared Libraries|
|Linux Driver (Static)||Shared System Memory Read/Write Process Memory||Kernel Semaphores Spinlocks||Kernel Threads, Tasklets||Full Kernel|
|Linux Module (Dynamic)||Shared System Memory Read/Write Process Memory||Kernel Semaphores Spinlocks||Kernel Threads, Tasklets||Module-Local and Exported Kernel Symbols|
Two important distinctions are called out in Table 1:
RTOSes are egalitarian, letting application and I/O code touch any address and perform almost any activity, whereas Linux is much more hierarchical and restrictive.
Legacy RTOS code can see every symbol or entry point in the system, at least at link time, whereas Linux user code is isolated from and built separately from kernel code and its accompanying namespace.
The consequences of the Linux hierarchy of privileged access is normally only kernel code (drivers) actually accesses physical memory. User code that also does so must run as root.
In general, user-space code is isolated from the Linux kernel and can see only explicitly exported symbols as they appear in /proc/ksyms. Moreover, visible system calls to the kernel are not invoked directly but by calls to user library code. This segregation is intentional, enhancing stability and security in Linux.
When you write a driver, the opposite is true. Statically linked drivers are privy to the entire kernel namespace, not only exports, but have zero visibility into user-space process-based symbols and entry points. And, when you encapsulate driver code in run-time loadable modules, your program can leverage only those interfaces explicitly exported in the kernel by the *EXPORT_SYMBOL* macro.