Linux and Initrd used to access disk : how does it work ?

Linux and Initrd used to access disk : how does it work ?

[Paul Rolland]

Hello,

I've been fighting for a few days with binary modules from some manufacturer
for hardware support of disk controler, and I'm at a point where I need
some more understanding.

1 - My machine contains an Adaptec SATA Raid based on Marvel 88SX60xx
    so I need to used the aar81xxx binary module,

2 - This module is presented as required to access the disk (when 
    installing a RH kernel, it says that no disk is present unless the
    module is loaded),

3 - When booting the kernel from disk after installation, the module is 
    loaded so the machine can access the disk...

... BUT ... how can the machine /
 - boot the kernel,
 - access the initrd image and uncompress it,
 - read the binary module inside and load it
BEFORE loading the module itself, if it is mandatory to access the disk.

And if it is not, then how can I do the installation ?

I suspect this should be fairly trivial, but I've been thinking about
for long, and it looks like chicken and egg to me...

Any help ?

Regards,
Paul

Paul Rolland, rol(at)as2917.net
ex-AS2917 Network administrator and Peering Coordinator

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Re: Linux and Initrd used to access disk : how does it work ?

[Richard B. Johnson]

On Tue, 24 May 2005, Paul Rolland wrote:

> Hello,
>
> I've been fighting for a few days with binary modules from some manufacturer
> for hardware support of disk controler, and I'm at a point where I need
> some more understanding.
>
> 1 - My machine contains an Adaptec SATA Raid based on Marvel 88SX60xx
>    so I need to used the aar81xxx binary module,
>
> 2 - This module is presented as required to access the disk (when
>    installing a RH kernel, it says that no disk is present unless the
>    module is loaded),
>
> 3 - When booting the kernel from disk after installation, the module is
>    loaded so the machine can access the disk...
>
> ... BUT ... how can the machine /
> - boot the kernel,
> - access the initrd image and uncompress it,
> - read the binary module inside and load it
> BEFORE loading the module itself, if it is mandatory to access the disk.
>
> And if it is not, then how can I do the installation ?
>
> I suspect this should be fairly trivial, but I've been thinking about
> for long, and it looks like chicken and egg to me...
>
> Any help ?
>
> Regards,
> Paul

initrd means Initial RAM disk.

The boot image is put onto your Hard disk. The hard disk needs
to be accessible from the BIOS to boot from it. Most controllers
have a BIOS module that lets it be accessed during boot so you
don't need a chicken before the egg to start the boot.
Then, when booting, LILO or grub starts linux which uncompresses
a RAM disk and mounts it instead of your hard disk. A special
version of `init` (called nash) gets started which reads a script
called linuxrc. It contains commands like:

#!/bin/nash
mount -t proc /proc /proc
setquiet
echo Mounted /proc filesystem
echo Mounting sysfs
mount -t sysfs none /sys
echo "Loading scsi_mod.ko module"
insmod /lib/scsi_mod.ko 
echo "Loading sd_mod.ko module"
insmod /lib/sd_mod.ko 
echo "Loading aic7xxx.ko module"
insmod /lib/aic7xxx.ko 
echo "Loading libata.ko module"
insmod /lib/libata.ko 
echo "Loading ata_piix.ko module"
insmod /lib/ata_piix.ko 
echo "Loading jbd.ko module"
insmod /lib/jbd.ko 
echo "Loading ext3.ko module"
insmod /lib/ext3.ko 
echo Creating block devices
mkdevices /dev
echo Creating root device
mkrootdev /dev/root
umount /sys
echo 0x0100 > /proc/sys/kernel/real-root-dev
echo Mounting root filesystem
mount -o defaults --ro -t ext3 /dev/root /sysroot
pivot_root /sysroot /sysroot/initrd
umount /initrd/proc

In this case, it loads my SCSI disk (aic7xxx) module and other
stuff necessary to access the disk and its filesystem.

The module(s) are in the /lib directory of the RAM disk.
They are put there by a build-script that installs initrd.
This script gets executed when you do 'make install' in
the kernel directory. If you have private modules, you
can copy them to the appropriate /lib/modules/`uname -r`/kernel/drivers
directory. You put the module(s) name(s) you need to boot
in /etc/modprobe.conf or /etc/modules.conf (depends upon the
module tools version) as:

alias scsi_hostadapter aic7xxx
alias scsi_hostadapter1 ata_piix

... any scsi_hostadapter stuff will be put into initrd when
you execute the script, /sbin/mkinitrd.


Cheers,
Dick Johnson
Penguin : Linux version 2.6.11.9 on an i686 machine (5537.79 BogoMips).
  Notice : All mail here is now cached for review by Dictator Bush.
                  98.36% of all statistics are fiction.

Re: Linux and Initrd used to access disk : how does it work ?

[Paul Rolland]

Hello Dick,

> initrd means Initial RAM disk.
> 
> The boot image is put onto your Hard disk. The hard disk needs
> to be accessible from the BIOS to boot from it. Most controllers
> have a BIOS module that lets it be accessed during boot so you
> don't need a chicken before the egg to start the boot.

Ok, basically, the trick is that the BIOS knows how to access the
disk, and Linux doesn't because it doesnt use the BIOS ? Or is it
some more subtle (though I doubt) thing in which only a part of the
disk can be accessed by the BIOS.

> Then, when booting, LILO or grub starts linux which uncompresses
> a RAM disk and mounts it instead of your hard disk. A special
> version of `init` (called nash) gets started which reads a script
> called linuxrc. It contains commands like:
Yes, I've seen that in my .img file...

> The module(s) are in the /lib directory of the RAM disk.
> They are put there by a build-script that installs initrd.
> This script gets executed when you do 'make install' in
> the kernel directory. If you have private modules, you
> can copy them to the appropriate /lib/modules/`uname 
> -r`/kernel/drivers
> directory. You put the module(s) name(s) you need to boot
> in /etc/modprobe.conf or /etc/modules.conf (depends upon the
> module tools version) as:
> 
> alias scsi_hostadapter aic7xxx
> alias scsi_hostadapter1 ata_piix
> 
> ... any scsi_hostadapter stuff will be put into initrd when
> you execute the script, /sbin/mkinitrd.
> 

Hmmm... I didn't know this part, thanks for the details.

So, it seems that I'm stuck with my binary module unless I can find a
way to tell the kernel to use the BIOS to access the disk ;-)))

Cheers,
Paul

Re: Linux and Initrd used to access disk : how does it work ?

[Richard B. Johnson]

On Tue, 24 May 2005, Paul Rolland wrote:

> Hello Dick,
>
>> initrd means Initial RAM disk.
>>
>> The boot image is put onto your Hard disk. The hard disk needs
>> to be accessible from the BIOS to boot from it. Most controllers
>> have a BIOS module that lets it be accessed during boot so you
>> don't need a chicken before the egg to start the boot.
>
> Ok, basically, the trick is that the BIOS knows how to access the
> disk, and Linux doesn't because it doesnt use the BIOS ? Or is it
> some more subtle (though I doubt) thing in which only a part of the
> disk can be accessed by the BIOS.
>

Linux needs to load a driver before it can access the disk. The
BIOS only works for 16-bit real-mode access.

>> Then, when booting, LILO or grub starts linux which uncompresses
>> a RAM disk and mounts it instead of your hard disk. A special
>> version of `init` (called nash) gets started which reads a script
>> called linuxrc. It contains commands like:
> Yes, I've seen that in my .img file...
>
>> The module(s) are in the /lib directory of the RAM disk.
>> They are put there by a build-script that installs initrd.
>> This script gets executed when you do 'make install' in
>> the kernel directory. If you have private modules, you
>> can copy them to the appropriate /lib/modules/`uname
>> -r`/kernel/drivers
>> directory. You put the module(s) name(s) you need to boot
>> in /etc/modprobe.conf or /etc/modules.conf (depends upon the
>> module tools version) as:
>>
>> alias scsi_hostadapter aic7xxx
>> alias scsi_hostadapter1 ata_piix
>>
>> ... any scsi_hostadapter stuff will be put into initrd when
>> you execute the script, /sbin/mkinitrd.
>>
>
> Hmmm... I didn't know this part, thanks for the details.
>
> So, it seems that I'm stuck with my binary module unless I can find a
> way to tell the kernel to use the BIOS to access the disk ;-)))
>

No. You are "stuck with a driver" just like everybody else. You
either have drivers built-in (getting rare, because there are so
many), or you load a driver while booting. This is the usual
way. From the boot perspective, it doesn't matter if the module
is a 'binary' one or a GPL one.

> Cheers,
> Paul
>
>

Cheers,
Dick Johnson
Penguin : Linux version 2.6.11.9 on an i686 machine (5537.79 BogoMips).
  Notice : All mail here is now cached for review by Dictator Bush.
                  98.36% of all statistics are fiction.

Re: Linux and Initrd used to access disk : how does it work ?

[Karl Essinger]

On Tuesday 24 May 2005 10:19 am, Paul Rolland wrote:

> Ok, basically, the trick is that the BIOS knows how to access the
> disk, and Linux doesn't because it doesnt use the BIOS ? Or is it
> some more subtle (though I doubt) thing in which only a part of the
> disk can be accessed by the BIOS.
>

I'm not an expert in this area, but . . . For lilo I think the boot loader 
just records the physical address where the initrd image resides and loads it 
directly into memory thru the bios. It doesn't need to understand the device 
or filesystem -- it's just retrieving a chunk of data -- as long as the bios 
understands how to read raw (real-mode) data from the device. That would be 
why you have to rerun lilo after changing any boot files -- because the 
addresses probably changed. 

I believe grub understands filesystems but I've also had trouble using grub to 
boot from devices which need module loading, i.e. USB flash drives. I don't 
use grub much though.

Re: Linux and Initrd used to access disk : how does it work ?

[Brian Gerst]

Paul Rolland wrote:
> Hello,
> 
> I've been fighting for a few days with binary modules from some manufacturer
> for hardware support of disk controler, and I'm at a point where I need
> some more understanding.
> 
> 1 - My machine contains an Adaptec SATA Raid based on Marvel 88SX60xx
>     so I need to used the aar81xxx binary module,
> 
> 2 - This module is presented as required to access the disk (when 
>     installing a RH kernel, it says that no disk is present unless the
>     module is loaded),
> 
> 3 - When booting the kernel from disk after installation, the module is 
>     loaded so the machine can access the disk...
> 
> ... BUT ... how can the machine /
>  - boot the kernel,
>  - access the initrd image and uncompress it,
>  - read the binary module inside and load it
> BEFORE loading the module itself, if it is mandatory to access the disk.
> 
> And if it is not, then how can I do the installation ?
> 
> I suspect this should be fairly trivial, but I've been thinking about
> for long, and it looks like chicken and egg to me...
> 
> Any help ?

The kernel and initrd images are loaded by the bootloader (grub, lilo, 
etc.), which uses the BIOS to read from the disk.  So as long as your 
controller has BIOS support you should be ok.

--
				Brian Gerst