Kernel cares where it should reside?

2009-04-02 Thread David Harel 

Hi there,

Using Gentoo (and have to configure kernel).
When I do the command - file - on kernel file I just built it says, 
among other things, root_dev 0x801.

The working kernel says - root_dev 0x10.
Where is this parameter determined and since when does the kernel care 
where it resides?


--
Regards.

David Harel,

==

Home office +972 77 7657645
Cellular:   +972 54 4534502
Snail Mail: Amuka
   D.N Merom Hagalil
   13802
   Israel
Email:  harel...@ergolight-sw.com


___
Linux-il mailing list
Linux-il@cs.huji.ac.il
http://mailman.cs.huji.ac.il/mailman/listinfo/linux-il

Re: Kernel cares where it should reside?

2009-04-02 Thread Yedidyah Bar-David 
On Thu, Apr 02, 2009 at 12:32:53PM +0300, David Harel wrote:
 Hi there,

 Using Gentoo (and have to configure kernel).
 When I do the command - file - on kernel file I just built it says,  
 among other things, root_dev 0x801.
 The working kernel says - root_dev 0x10.
 Where is this parameter determined

Not sure exactly, somewhere in the build process. You can change it with
'rdev', but you do not need to:

 and since when does the kernel care  
 where it resides?

It's not 'since', but 'until when'. It was so since the very beginning.
The kernel had a small boot loader in its start. You could 'cat zImage 
/dev/fd0' and get a bootable floppy. No standard boot loader (lilo,
grub) care{s,ed} about this. IIRC today it does not work anymore,
perhaps never worked with bzImage.
The reason it cared is obvious - you had to tell it which FS to mount as
root. And had no config file to write 'root=something'.
-- 
Didi


___
Linux-il mailing list
Linux-il@cs.huji.ac.il
http://mailman.cs.huji.ac.il/mailman/listinfo/linux-il

Re: Kernel cares where it should reside?

2009-04-02 Thread Oleg Goldshmidt 
David Harel harel...@gmail.com writes:

 Hi there,
 Using Gentoo (and have to configure kernel).
 When I do the command - file - on kernel file I just built it says, among
 other things, root_dev 0x801.
 The working kernel says - root_dev 0x10.
 Where is this parameter determined and since when does the kernel care where
 it resides?

I don't think it is related to where the kernel resides, but to where
the root filesystem resides - this is the rootfs device number - and
the kernel does care, obviously.

You can add this as a boot parameter with root=/dev/sda1 or sth of the
kind.

The root device may change (more than once) during boot. E.g., if you
use initrd then the initrd root device may point to the ramdisk
(/dev/ram0 or similar). Then at some point you'll want the regular
root device to be engaged - the pivot_root syscall (for more info, see
the pivot_root(2), pivot_root(8), rdev(8), initrd(4) man pages, the
kernel Documentation, etc.) will change the root device as appropriate
(/dev/sda1 or whatever).

This may explain why your just-built kernel and the working kernel
have different root_dev. I *think* there is a registry for device
numbers in Documentation/devices.txt, if you want to find out what
0x801 or 0x10 are.

Hope it helps,

-- 
Oleg Goldshmidt | p...@goldshmidt.org

___
Linux-il mailing list
Linux-il@cs.huji.ac.il
http://mailman.cs.huji.ac.il/mailman/listinfo/linux-il