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Observations and Comparison with Linux Porting Efforts

Porting NetBSD to this SoC turned out to be a very straightforward effort, with very few "potholes". I've done a few Linux ports before, and the adaptation of NetBSD went at least as smoothly as any of the Linux efforts. In fact, I found NetBSD to have several advantages that made it substantially easier to port to a new platform than Linux, especially for a "newbie" like myself with no previous NetBSD porting experience.

NetBSD builds its own NetBSD-specific toolchain as part of the build process. With Linux, you need to acquire or build yourself a toolchain that's appropriate for the processor architecture and C library variant you're using. While there are a number of nice aids for this process (e.g., Dan Kegel's crosstool), it can still be a significant pain - for example, use the wrong binutils variant and things break in mysterious ways. With NetBSD, just run the included build script and the toolchain's there.

The NetBSD source includes the full system source - kernel and applications. While this can reduce the flexibility somewhat (for example, there's only one C library provided), it makes porting and building much easier. With Linux, one of the main initial tasks is pulling together the source for the components you want in your root filesystem, and creating the build tree and scripts to create the desired filesystem images. On the flip side, however, there are some Linux distributions available (both free and commercial) that provide full system source and a build environment - for example, the Snapgear distribution, or the MontaVista offering - and one advantage these have is that they generally provide a broad selection of library and application choices, and tend to be more easily configurable than the standard NetBSD distribution in selecting the components to be included.

However, NetBSD has a few drawbacks (some significant), particularly for use in embedded systems.

There is at the time of this writing no Flash Translation Layer or dedicated Flash filesystem that provides journaling or wear-leveling to support Flash memory (NOR or NAND). You need to have the root filesystem on a disk or embedded as a RAM filesystem in the kernel image (or mounted via NFS). Since the embedded space is extremely Flash-centric, this is a significant hole. There are apparently efforts to correct this, but in early 2007 it's not there yet. (With the recent release of Apple's iPhone, which runs OSX, whose kernel, Darwin, is BSD-based, and which uses Flash as its storage medium, it looks like there is some solution that exists, but whether this is open-source at this point I don't know.)

The NetBSD kernel doesn't provide the option to build itself as a self-extracting compressed image (a zImage in Linux parlance). The kernel image can be built as a compressed image, but it relies on the bootloader to be able to uncompress it. However, NetBSD does provide a utility called gzboot, which decompresses a gzipped kernel to RAM and jumps to its entry point. This provides essentially the same functionality as the Linux zImage. (Thanks to Allen Briggs for the gzboot tip.)

Both Linux and NetBSD benefit tremendously from being open-source systems. As noted in the introduction, it's because of the work of others, and their openness in sharing it, that NetBSD and Linux and most other open-source systems are quite straightforward to adapt to new platforms. And there's nothing quite so satisfying as finally seeing on your console (debug messages and warts and all):

NetBSD/evbarm (Vx115 VEP) booting...

physmemory: 3840 pages at 0x24100000 -> 0x24ffffff

Allocating page tables

freestart = 0x24100000, free_pages = 512 (0x00000200)

IRQ stack: p0x242f2000 v0xc01f2000

ABT stack: p0x242f1000 v0xc01f1000

UND stack: p0x242f0000 v0xc01f0000

SVC stack: p0x242ee000 v0xc01ee000

Creating L1 page table at 0x242fc000

Mapping kernel

pmap_map_chunk: pa=0x24300000 va=0xc0200000 size=0x183000 resid=0x183000 prot=0x3 cache=1

SLLLLLLLLPPP

pmap_map_chunk: pa=0x24483000 va=0xc0383000 size=0x56d000 resid=0x56d000 prot=0x3 cache=1

PPPPPPPPPPPPPLLLLLLLSSSSLLLLLLLLLLLLLLL

Constructing L2 page tables

pmap_map_chunk: pa=0x242f2000 va=0xc01f2000 size=0x1000 resid=0x1000 prot=0x3 cache=1

pmap_map_chunk: pa=0x242f1000 va=0xc01f1000 size=0x1000 resid=0x1000 prot=0x3 cache=1

pmap_map_chunk: pa=0x242f0000 va=0xc01f0000 size=0x1000 resid=0x1000 prot=0x3 cache=1

pmap_map_chunk: pa=0x242ee000 va=0xc01ee000 size=0x2000 resid=0x2000 prot=0x3 cache=1

PP

pmap_map_chunk: pa=0x242fc000 va=0xc01fc000 size=0x4000 resid=0x4000 prot=0x3 cache=2

PPPP

pmap_map_chunk: pa=0x242fb000 va=0xc01fb000 size=0x1000 resid=0x1000 prot=0x3 cache=2

pmap_map_chunk: pa=0x242fa000 va=0xc01fa000 size=0x1000 resid=0x1000 prot=0x3 cache=2

pmap_map_chunk: pa=0x242f9000 va=0xc01f9000 size=0x1000 resid=0x1000 prot=0x3 cache=2

pmap_map_chunk: pa=0x242f8000 va=0xc01f8000 size=0x1000 resid=0x1000 prot=0x3 cache=2

pmap_map_chunk: pa=0x242f7000 va=0xc01f7000 size=0x1000 resid=0x1000 prot=0x3 cache=2

pmap_map_chunk: pa=0x242f6000 va=0xc01f6000 size=0x1000 resid=0x1000 prot=0x3 cache=2

pmap_map_chunk: pa=0x242f5000 va=0xc01f5000 size=0x1000 resid=0x1000 prot=0x3 cache=2

pmap_map_chunk: pa=0x242f4000 va=0xc01f4000 size=0x1000 resid=0x1000 prot=0x3 cache=2

devmap: 70000000 -> 700fffff @ fd000000

pmap_map_chunk: pa=0x70000000 va=0xfd000000 size=0x100000 resid=0x100000 prot=0x3 cache=0

devmap: fc000000 -> fc1fffff @ fc000000

pmap_map_chunk: pa=0xfc000000 va=0xfc000000 size=0x200000 resid=0x200000 prot=0x3 cache=0

SS

freestart = 0x249f0000, free_pages = 1552 (0x610)

switching to new L1 page table  @0x242fc000...done!

bootstrap done.

init subsystems: stacks vectors undefined page pmap done.

Loaded initial symtab at 0xc038b3f4, strtab at 0xc03b81b0, # entries 11404

abcdefg

Copyright (c) 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005

    The NetBSD Foundation, Inc.  All rights reserved.

Copyright (c) 1982, 1986, 1989, 1991, 1993

    The Regents of the University of California.  All rights reserved.

pmap_postinit: Allocated 35 static L1 descriptor tables

NetBSD 3.0.1 (VX115_VEP) #8: Tue Jan  2 19:17:21 EST 2007

        root@localhost.localdomain:/home/agere/vx115_netbsd/build/usr/src/sys/arch/evbarm/compile/obj/VX115_VEP

total memory = 15360 KB

avail memory = 5404 KB

mainbus0 (root)

cpu0 at mainbus0: ARM926EJ-S rev 5 (ARM9E-S core)

cpu0: WB enabled EABT

cpu0: 16KB/32B 4-way Instruction cache

cpu0: 16KB/32B 4-way write-back-locking-C Data cache

vx115_ahb0 at mainbus0

vx115_apb0 at mainbus0

vx115_clk0 at vx115_apb0 addr 0x700c5000-0x700c5067 intr 9

vx115_pic0 at vx115_apb0 addr 0x700c1000-0x700c114b

vx115_com0 at vx115_apb0 addr 0x700e2000-0x700e207b intr 19

vx115_com0: major = 110: console

md0: internal 5000 KB image area

boot device: <unknown>

root on md0a dumps on md0b

mountroot: trying msdos...

mountroot: trying ffs...

root file system type: ffs

WARNING: CHECK AND RESET THE DATE!

init: copying out flags `-s' 3

init: copying out path `/sbin/init' 11

Jan  3 00:17:46 init: /etc/pwd.db: No such file or directory

Enter pathname of shell or RETURN for /bin/sh:

# ls

bin     dev     etc     lib     libexec sbin    usr

# ps

ps: warning: /var/run/dev.db: No such file or directory

PID TTY STAT    TIME COMMAND

  5 ?   Ss   0:00.13 -sh

  7 ?   R+   0:00.02 ps


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