Porting RTOS Device Drivers to Embedded Linux
As indicated above, porting character and block device drivers to Linux is a straightforward if time-consuming activity. Porting network drivers, though, can seem much more daunting.
Remember that while Linux grew up with TCP/IP, most RTOSes had networking grafted onto them in the late 1990s. As such, legacy networking often only presents bare-bones capabilities, such as being able to handle only a single session or instance on a single port or to support only a physical interface on a single network medium. In some cases, networking architecture was generalized after the fact, as with Wind River VxWorks MUX code to allow for multiple interfaces and types of physical connection.
The bad news is that you likely have to rewrite most or all of your existing network interfaces. The good news is that re-partitioning for Linux is not hard and you have dozens of open-source network device driver examples to choose from.
Your porting task is to populate the areas at the bottom of Figure 4 with suitable packet formatting and interface code.
Writing network drivers is not for beginners. Because, however, many RTOS network drivers actually were derived from existing GPL Linux interfaces, you might find the process facilitated by the code itself. Moreover, there is a large and still-growing community of integrators and consultants focused on making a business of helping embedded developers move their applications to Linux, for reasonable fees.
The goal of this article has been to give embedded developers some insight into both the challenges they will face and benefits they will realize from moving their entire software stack from a legacy RTOS to Linux. The span of 2,800 words or so is too brief to delve into many of the details of driver porting (driver APIs for bus interfaces, address translation and so on), but the wealth of existing open-source GPL driver code serves as both documentation and a template for your migration efforts. The guidelines presented here should help your team scope the effort involved in a port of RTOS to Linux and provide heuristics for re-partitioning code for the best native fit to embedded Linux.
As Director of Strategic Marketing and Technology Evangelist when he wrote this article, Bill focused his 17+ years of industry experience on advancing MontaVista and embedded Linux in today's dynamic pervasive computing marketplace. His background includes extensive embedded and real-time experience with expertise in OS, tools, software licensing and manufacturing.