Porting NetBSD To A New ARM SoC
This article describes experiences in porting NetBSD to a new
ARM-based SoC. The specific chip addressed is the ARM9-based Agere
Vx115 cellular telephony digital baseband
chip, and the Vx115 VEP evaluation board hosting the chip. The
information presented here is intended to assist with the porting of
NetBSD to any new ARM-based platform.
Porting Variants
There are a couple of variants possible when porting NetBSD depending on the level of support already available in the kernel.
- Processor architecture port
This involves adapting the NetBSD
kernel to run on a new processor architecture type (like MIPS, PowerPC,
ARM, etc., though those types are already supported). This is a major
undertaking that requires deep understanding of both the NetBSD core
and the processor architecture.
This involves porting NetBSD to a new
SoC which uses as its processor core a type already supported by NetBSD
but provides some unique set of on-chip peripheral devices. This
involves defining the bus structures needed to interconnect the devices
on the SoC, as well as drivers for the peripherals.
This involves developing code to allow
NetBSD to run on a particular platform (evaluation board) that uses a
chip (SoC) already supported by the NetBSD kernel. This involves creating
appropriate platform startup code, as well as support for off-chip
peripherals on the board, as well as an appropriate root filesystem
with the platform applications and libraries.
This document describes the latter two activities. The Vx115 SoC is
based on an ARM9 processor core already supported by NetBSD (though the
exact variant, the ARM926EJ-S, was not directly supported in the kernel
version used and required minor adaptation), so a processor port was
not needed, but the SoC provides a uniques set of on-chip peripherals,
and was provided on an evaluation board with various supporting
hardware components.
Scope
The focus of this porting effort was to get NetBSD running on the
platform with only the minimum support needed to host a console and
shell on the serial port. Only those on- and off-chip peripherals
needed to support this operation were addressed; the SoC provides many
other peripherals which would need support (drivers) to comprise
anything near a full board-support package.
Such a "basic" port requires addressing a relatively small number of issues, described in the following sections.
Each of these involves creating various configuration, source and header
files, and the contents of these files is discussed in the subsequent
sections.
Acknowledgments and References
As is standard practice with open-source systems, much of the
required functionality can be found in corresponding files for existing
ports. This project makes full use of this available source code (with
thanks to all of the original authors). In fact, it's because of the
work of others, and their openness in sharing it, that NetBSD and other
open-source systems are quite straightforward to adapt to new platforms.
There's a great deal of information available on the NetBSD Project website on building and working with NetBSD systems. Of particular interest for building is the NetBSD Guide, especially the sections on building and cross-compiling.
Also invaluable is the web-based source cross-referencing tool initially provided as the Linux Cross-Reference by the LXR Project. This tool has been applied to the NetBSD source (as well as many others) by Robert Watson in his FreeBSD and Linux Kernel Cross-Reference.
Comments/questions: jsevy@cs.drexel.edu
