Random comments on LinuxBIOS

[Deepak Kotian]

I think, the  paper at this link is also good
http://www.lysator.liu.se/upplysning/fa/linuxbios.pdf
A newbie would like to read this first ..
May like to add it or update more on this and put it somewhere in
linuxbios site.

Regards
Deepak






----- Original Message -----
From: "McMechan, James W CIV" <james.mcmechan at navy.mil>
To: <linuxbios at clustermatic.org>
Sent: Thursday, April 17, 2003 3:11 AM
Subject: Random comments on LinuxBIOS


> Here is what I wrote while trying to catch up to list, though there is a
FAQ I produced this while working my way through the emails perhaps some of
it could be added to the FAQ
>
> LinuxBIOS is a project to replace the original BIOS on motherboards with
open source.
> It appears that the two most common uses for LinuxBIOS are for
Supercomputer clusters and embedded systems.
>
> LinuxBIOS no longer always uses a real OS to load the OS.
>
> LinuxBIOS currently can boot several payloads including:
> 1 Linux which is what started all this, hence the name.
> 2 Plan9 Ron Minnich seems to like this one.
> 3 VxWorks Felix Radensky said he had success with a older boot method
> 4 EtherBoot which despite its name can also boot from IDE hard disks with
a patch or the dev branch
> 5 ADLO is a shim to allow the Boch emulator's BIOS to be used on top of
LinuxBIOS to boot some versions of that other OS or to run VGA controller
ROMs which seem to assume that a BIOS is present.
> ?
>
> The Supercomputer people appear to want remote control, remote booting and
fast boot times without user interaction. For example "Press F2" is a very
inconvenient task on a system with 100 computers
>
> The embedded people appear to want full control of the boot process and
very fast boot times. For example who wants to watch a memory test go by on
their VCR/MP3 player or see "Press F2" without a keyboard.
>
> LinuxBIOS is installed into the flash ROM in place of the original BIOS.
>
> Most flash ROMs are currently 2 Megabits in size, i.e. a 29SF020 which is
only 256 Kilobytes of storage, or some might be 4 Mbit ROM which provides
512 Kbytes and so could allow for very squeezed down Linux kernel to also be
included in the ROM, though I don't know of a useful kernel that small.
> It would be nice to add a larger flash ROM but many motherboards do not
connect enough address lines to the Flash ROM socket, for even the 4 Mbit
part or they solder the chip down.
>
> Another popular if grumbled about option is the disk on a chip the DoC
Millennium which is a paged flash part that can have LinuxBIOS in the first
part and up to 8 Mbytes of other data on other pages of the chip. Very short
~3-10 seconds boot times should be possible with this method.
>
> The BIOS Flash can usually be programmed in place by either just
reflashing the original BIOS or by the hotswap method also known as the
dental floss method, referring to putting dental floss under a PLCC chip to
make it easy to remove without requiring metal tools in a powered on system.
The hotswap method is not for the faint of heart but not all that difficult
either, the system is booted with a working ROM which is then pulled out and
the ROM to be programmed inserted in its place, then the ROM flashing
program is run, and when done a reboot can be tried, this does not require a
programmer as it uses the motherboards ability to update its BIOS as the
programmer, if the reboot fails the original ROM is reinserted and the
system rebooted again, all this requires, is some spare flash ROMs it is a
good idea to make and label a couple backups incase something goes wrong, I
have put ROMs in DIP sockets backwards destroying the ROMs, and ESD is a
worry even if it is not a commo!
> n problem.
>
> Some people also use a very nice device called the BIOS savior which
allows for two BIOS ROMS to be switched by the user with a switch.
>
> Compact Flash IDE adaptors appear to be popular as the CF does not need
the spin up time of a real hard disk. Very short boot times of ~3-10 seconds
are claimed for this configuration also.
>
> Each model of motherboard currently needs its own build information due to
a lack of standards in how the circuitboards are laid out between models and
manufactures.
>
> If you have a motherboard that is not listed, a "lspci" will show what
chipsets are in use and you can then check to see if they are in the
LinuxBIOS tree, if not some research may be needed to get the documentation
for that chipset.
>
> The kexec patches date back to the original LinuxBIOS with the Linux
kernel in the BIOS ROM and allowed for new kernels to be loaded and executed
without rebooting back to the BIOS this would allow for a minimal kernel in
the BIOS and a full featured kernel to be loaded by the first kernel. Also
the BIOS ROMs have a limited programming life and limited space so a small
unchanging kernel that then loads the final kernel from something else
prolongs the useful life of the ROMs.
>
> Ron Minnich at LANL uses similar feature called "two kernel monte"
http://sourceforge.net/projects/monte/
>
> LinuxBIOS now has the ability to make use of the CMOS space for
fallback/normal boot image selection, serial port baud rates, and boot order
for Etherboot, Ron Minnich also used it  authentication information for a
Plan9 cluster. See the cmos.layout files for information.
>
> Steven James had nice comments on Normal/Fallback images:
> LinuxBIOS for that board (and many others) supports two copies of
> LinuxBIOS, fallback and primary. The fallback image is intended to be old
> and stable and it's purpose is to act as a sort of rescue image. The
> primary image is the one that gets updated to the latest and greatest, and
> is meant to be more versatile and complete.
>
> The fallback image is loaded at the top of the flash, and always receives
> control at power on or reset. It does very minimal setup (switch to 32bit
> flat memory protected mode), checks and clears the CMOS boot bit, and if
> it was set, jumps to the primary image. This way, if the primary fails,
> hitting reset will boot from the fallback image alone.
>
> From your config, you were building a primary image. the missing bytes are
> for the fallback image to be appended. It didn't work because you didn't
> have a fallback image at the top of the flash (the end of the image file).
>
> Ron Minnich expanded on fallback vs. primary:
>
> You need to build TWO linuxbioses, a fallback and a primary. The fallback
> is 64k. The primary is (romsize-64k).
>
> To build a romimage, you build the fallback, build the primary, then:
>
> cat primary/romimage fallback/romimage > final_romimage
>
> flash_rom final_romimage
>
> Steve Gehlbach wrote about using BOOT_IDE:
> See pcchips787.config in util/config for complete configuration.
>
> option BOOT_IDE=1
> This enables booting from IDE, the file to use is linux.bin.gz:
>
> option IDE_BOOT_DRIVE=2
> If you do not use drive 0  (default), then you can set which drive to
> boot; (0,1,2,3) are the four standard PC drives:
>
> option ONE_TRACK=32
> The linux.bin.gz file is put in raw form at partition 1, ie, the first
> partition on the disk.  This is located just past the partition table.
> The partition table size varies, it is "one track" from the beginning of
> the disk.  "one track" in c/h/s notation is "s" or the number of sectors
> per track.  ONE_TRACK is in sectors, the software multiplies by 512.
> Most disks are 63 sectors per track (the default), but my CF is 32
> sectors per track.  eg, the partion table is 63x512 or 32x512 bytes.
>
> You can partition your disk as you want, but Linux goes raw in partition
> 1; just make sure partition 1 is big enough, not a problem on today's
> disks.  You could put the Linux root file system on partition 2, for
> example.  In pcchips787.config, I put the Linux root file system on IDE
> 0, partition 2 (I was experimenting with Linux in partition 1), but I
> eventually put Linux on drive 2 using CF.  You are right, copying of
> linux.bin.gz raw to the partition is dangerous, and something like "cat
> linux.bin.gz > /dev/hda1" will definitely screw the disk if you put the
> wrong disk or partition.  I recommend a shell script, fingers cannot be
> trusted.  You can also use "dd" but "cat" works.
>
> Greg Watsons had some good comments on on the boot process and file
layout:
> There seem to be two main parts to linuxbios. The first is
> arch/{arch}/config/ctr0.base which does the very low level
> initialization, like turning on memory, etc. The second is
> arch/{arch}/lib/c_start.S which does whatever else is necessary to
> call the C function hardwaremain(). hardwaremain() then does whatever
> else is necessary to load Linux.
>
> c_start.S is linked with linuxbios.a, a library containing generic
> support routines (those found in the lib directory) and anything
> specified using the 'object'  directive in a Config file (and other
> stuff). The resultant 'executable' is called linuxbios_c. The loader
> script used to link linuxbios_c is config/linuxbios_c.ld, and is
> configured to be loaded relative to _RAMBASE.
>
> crt0.base is not linked against anything. Any additional assembly
> routines you need must be specified using the 'mainboardinit'
> directive in a Config file. This causes the specified assembly file
> to be added to "crt0_includes.h" which is in turn included at the
> start of crt0.base (or at the end in the case of the ppc version).
> The loader script used to link crt0.base is in
> arch/{arch}/config/ldscript.base. The resultant 'executable' is
> called linuxbios and will be loaded at _ROMBASE. The tricky thing is
> that this loader script will also load the linuxbios_c 'executable'
> at a location called _payload in this file. The main task of
> crt0.base is then to initialize enough hardware so that this payload
> can be copied from ROM into ram (which may also involve uncompressing
> code). Then control is transferred to _start, which is the first
> location in linuxbios_c.
>
> To get an idea of how crt0.base works, look at the following files.
> This is the order of execution specified by the configuration file
> for sis735.
>
> cpu/i386/entry16.inc
> cpu/i386/entry32.inc
> superio/sis/950/setup_serial.inc
> pc80/serial.inc
> arch/i386/lib/console.inc
> cpu/k7/earlymtrr.inc
> northsouthbridge/sis/735/raminit.inc
> arch/i386/config/crt0.base
>
> Next look at c_start.S which will show you what happens once control
> is transferred to _start. Finally, look at
> arch/{arch}/lib/hardwaremain.c to see what other stuff is done to get
> Linux loaded.
>
> Most other files are specific to particular hardware, so it can be
> pretty confusing to just browse the tree.
>
> Hope this helps,
>
> Greg
>
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