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On Friday 19 December 2003 7:15 pm, Eric W. Biederman wrote:
Yes we are reaching the point where we can converge on
some of these
things. LAB might be the right framework. And if it is something good it
will save me the trouble of starting my own project. But it takes more
than a hyper active 2 year old to convince me. It might take a hyperactive
2 year old to remind me about interesting ideas though.
Right, well then you should
see it in action. If you're in the Boston area
sometime soon I can give you a demo on an iPAQ, perhaps.
If you can
have it in the kernel you can have it in LAB. LAB's command
line API is really easy - I've ported userland apps (mtderase) to LAB in
By and large I don't want a command line interface. I
don't want a
hardware monitor. I want a configurable but non-interactive boot.
that's where our autoboot stuff comes in. You can have it mount a
partition by default, do whatever you want by default if no key has been
struck on the serial line within 3 seconds.
Compression wise there is not a difference between
bzImage and zImage.
zImage on x86 has a 640K limit because it loads below 1MB. bzImage breaks
that ancient barrier.
Right, I just read about that. Mea culpa.
512k is with a
few ARM-specific drivers, and jffs2. It does not have
networking. This is with kernel 2.6.
Hmm. I am pretty certain I have gotten 2.6
down some smaller. Our
practical limit with LinuxBIOS etc is in the neighborhood of 384KB.
2.4 in 256k, but it's rather useless like that. If you do not plan
to load modules at runtime, you can shave a good bit more off of it. If you
write bzip2 compression support (or upport the stuff from kernel 2.4), you
can shave even more off of it. I've pulled off 50k with bzip2 (not actually
written the code, just did a bzip2 -9 < piggy > piggy.bz2). If you don't plan
to have a framebuffer, you can shave some off of it. If you don't plan to
have jffs2 you can shave a lot more off. Little tidbits here and there make
the world go 'round.
Well I think I have run finally convinced to use the
Mostly I prefer to flash from a production kernel rather than a
bootloader, there are more recover options but anyway.
Ah yes, the ancient problem.
Instead of read/modify/erase/write, it often
turns into read/modify/erase/poweroff. That's Bad.
I will see. Does LAB restrict it's kernel to a
very small subset of
memory? Or do you use something like kexec?
To boot a secondary kernel I use some
code I wrote called armboot, although
it's not very arm specific. It does something like this:
1) Load the new kernel into a contiguous vmalloced block.
2) We allocate 64k for a list of things that need to be relocated. We call
this a pointer of type "struct physlist", which is 32 bytes. It has four
ints: the new address, the old address, the block size, and whether this is
the last block.
2) In blocks of the maximum kmalloc size (these blocks have to be contiguous),
we kmalloc space for the kernel, and memcpy the kernel into those blocks. We
then fill in a struct physlist, and move on to the next struct physlist. We
can do this because kmalloc is always contiguous, and we can always map it
3) We set up another kmalloced block for the tagged list of boot parameters
that you need on ARM.
4) We set up one more kmalloced block and copy an assembler function into it,
to make sure we don't wipe ourself out while relocating.
5) We flush our data caches.
6) We call the relocated assembler function, which turns off the MMU, jumps
into the relocated assembler function's physical address, and does actual
relocating. Then we jump into our newly moved zImage. Confused yet?
7) If at any point we failed, the system could be in an inconsistent state.
You will want to panic() if you fail, because you're leaking memory like a
sieve, and if you failed there's probably something bigger wrong.
This looks more difficult than it actually is. The C segment is only about 170
lines, and the assembler bit is 90 lines.
The reason that this works is that kmalloc should allocate from the top of
memory down. You need a fair bit of ram - say, 8MB - to prevent the tail from
running over other important structures, such as the list of addresses to
relocate. But it seems to work well enough, and it looks like it should be
fairly portable. The important code is in handhelds.org
linux/kernel26, files drivers/bootldr/armboot.c and
Joshua Wise | www.joshuawise.com
GPG Key | 0xEA80E0B3
Quote | <lilo> I akilled *@* by mistake
In memoriam | Whiskers the hamster, 2001 - Dec 15, 2003
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