Porting RTOS Device Drivers to Embedded Linux
Most RTOSes ship with a customized standard C run-time library, such as pREPC for pSOS, and selectively patched C libraries (libc) from compiler ISVs. They do the same for glibc. Thus, at a minimum, most RTOSes support a subset of standard C-style I/O, including the system calls open, close, read, write and ioctl. In most cases, these calls and their derivatives resolve to a thin wrapper around I/O primitives. Interestingly, because most RTOSes did not support filesystems, those platforms that do offer file abstractions for Flash or rotating media often use completely different code and/or different APIs, such as pHILE for pSOS. Wind River VxWorks goes further than most RTOS platforms in offering a feature-rich I/O subsystem, principally to overcome hurdles in integration and generalization of networking interfaces/media.
Many RTOSes also support a bottom-half mechanism, that is, some means of deferring I/O processing to an interruptible and/or preemptible context. Others do not but may instead support mechanisms such as interrupt nesting to achieve comparable ends.
A typical I/O scheme (input only) and the data delivery path to the main application is diagramed in Figure 1. Processing proceeds as follows:
A hardware interrupt triggers execution of an ISR.
The ISR does basic processing and either completes the input operation locally or lets the RTOS schedule deferred handling. In some cases, deferred processing is handled by what Linux would call a user thread, herein an ordinary RTOS task.
Whenever and wherever the data ultimately is acquired (ISR or deferred context), ready data is put into a queue. Yes, RTOS ISRs can access application queue APIs and other IPCs—see the API table.
One or more application tasks then read messages from the queue to consume the delivered data.
Output often is accomplished with comparable mechanisms—instead of using write() or comparable system calls, one or more RTOS application tasks put ready data into a queue. The queue then is drained by an I/O routine or ISR that responds to a ready-to-send interrupt, a system timer or another application task that waits pending on queue contents. It then performs I/O directly, either polled or by DMA.
The queue-based producer/consumer I/O model described above is one of many ad hoc approaches employed in legacy designs. Let us continue to use this straightforward example to discuss several possible (re)implementations under embedded Linux.
Developers who are reticent to learn the particulars of Linux driver design, or who are in a great hurry, likely try to port most of a queue-based design intact to a user-space paradigm. In this driver-mapping scheme, memory-mapped physical I/O occurs in user context by way of a pointer supplied by mmap():
#include <sys/mman.h> #define REG_SIZE 0x4 /* device register size */ #define REG_OFFSET 0xFA400000 /* physical address of device */ void *mem_ptr; /* de-reference for memory-mapped access */ int fd; fd=open("/dev/mem",O_RDWR); /* open physical memory (must be root) */ mem_ptr = mmap((void *)0x0, REG_AREA_SIZE, PROT_READ+PROT_WRITE, MAP_SHARED, fd, REG_OFFSET); /* actual call to mmap() */
A process-based user thread performs the same processing as the RTOS-based ISR or deferred task would. It then uses the SVR4 IPC msgsnd() call to queue a message for receipt by another local thread or by another process by invoking msgrcv().
Although this quick-and-dirty approach is good for prototyping, it presents significant challenges for building deployable code. Foremost is the need to field interrupts in user space. Projects such as DOSEMU offer signal-based interrupt I/O with SIG (the silly interrupt generator), but user-space interrupt processing is quite slow—millisecond latencies instead of tens of microseconds for a kernel-based ISR. Furthermore, user-context scheduling, even with the preemptible Linux kernel and real-time policies in place, cannot guarantee 100% timely execution of user-space I/O threads.
It is highly preferable to bite the bullet and write at least a simple Linux driver to handle interrupt processing at kernel level. A basic character or block driver can field application interrupt data directly in the top half or defer processing to a tasklet, a kernel thread or to the newer work-queue bottom-half mechanism available in the 2.6 kernel. One or more application threads/processes can open the device and then perform synchronous reads, just as the RTOS application made synchronous queue receive calls. This approach will require at least recoding consumer thread I/O to use device reads instead of queue receive operations.
To reduce the impact of porting to embedded Linux, you also could leave a queue-based scheme in place and add an additional thread or dæmon process that waits for I/O on the newly minted device. When data is ready, that thread/dæmon wakes up and queues the received data for use by the consuming application threads or processes.