How does boot loader pass initrd address / size to kernel?

How does boot loader pass initrd address / size to kernel?

[Rajat Jain]

Hi,

I'm running an ancient Linux kernel 2.4.20 (please don't ask me why
:-( ) on a MIPS 4KEC. I am experimenting with initrd and my initrd
fails to mount. My bootloader (U-BOOT) coorectly loads the initrd into
RAM as I can see.

I am wondering how does the kernel get to know the address at which
the initrd is loaded by boot loader? How does the boot loader
communicate this to the kernel?

I can see that when emebedding root filesystem into kernel image, the
symbols __rd_start and __rd_end are defined by the linker script and
hence the kernel gets to know. However, how does this happen when
bootloader loads the ramdisk and needs to tell the kernel?

Any code references will be appreciated.

Thanks,

Rajat

Re: How does boot loader pass initrd address / size to kernel?

[Thiemo Seufer]

Rajat Jain wrote:
> Hi,
> 
> I'm running an ancient Linux kernel 2.4.20 (please don't ask me why
> :-( ) on a MIPS 4KEC. I am experimenting with initrd and my initrd
> fails to mount. My bootloader (U-BOOT) coorectly loads the initrd into
> RAM as I can see.
> 
> I am wondering how does the kernel get to know the address at which
> the initrd is loaded by boot loader? How does the boot loader
> communicate this to the kernel?
> 
> I can see that when emebedding root filesystem into kernel image, the
> symbols __rd_start and __rd_end are defined by the linker script and
> hence the kernel gets to know. However, how does this happen when
> bootloader loads the ramdisk and needs to tell the kernel?

http://www.linux-mips.org/wiki/Kernel_Command_Line_Arguments mentions
rd_start and rd_size which are used for this purpose.


Thiemo

Re: How does boot loader pass initrd address / size to kernel?

[Thiemo Seufer]

Thiemo Seufer wrote:
> Rajat Jain wrote:
> > Hi,
> > 
> > I'm running an ancient Linux kernel 2.4.20 (please don't ask me why
> > :-( ) on a MIPS 4KEC. I am experimenting with initrd and my initrd
> > fails to mount. My bootloader (U-BOOT) coorectly loads the initrd into
> > RAM as I can see.
> > 
> > I am wondering how does the kernel get to know the address at which
> > the initrd is loaded by boot loader? How does the boot loader
> > communicate this to the kernel?
> > 
> > I can see that when emebedding root filesystem into kernel image, the
> > symbols __rd_start and __rd_end are defined by the linker script and
> > hence the kernel gets to know. However, how does this happen when
> > bootloader loads the ramdisk and needs to tell the kernel?
> 
> http://www.linux-mips.org/wiki/Kernel_Command_Line_Arguments mentions
> rd_start and rd_size which are used for this purpose.

... and they IIRC didn't exist in 2.4.20, but e.g. Debian's 2.4 mips
kernels carry a patch to add such support.


Thiemo

Re: How does boot loader pass initrd address / size to kernel?

[Rajat Jain]

On 3/15/07, Thiemo Seufer <ths@networkno.de> wrote:
> Thiemo Seufer wrote:
> > Rajat Jain wrote:
> > > Hi,
> > >
> > > I'm running an ancient Linux kernel 2.4.20 (please don't ask me why
> > > :-( ) on a MIPS 4KEC. I am experimenting with initrd and my initrd
> > > fails to mount. My bootloader (U-BOOT) coorectly loads the initrd into
> > > RAM as I can see.
> > >
> > > I am wondering how does the kernel get to know the address at which
> > > the initrd is loaded by boot loader? How does the boot loader
> > > communicate this to the kernel?
> > >
> > > I can see that when emebedding root filesystem into kernel image, the
> > > symbols __rd_start and __rd_end are defined by the linker script and
> > > hence the kernel gets to know. However, how does this happen when
> > > bootloader loads the ramdisk and needs to tell the kernel?
> >
> > http://www.linux-mips.org/wiki/Kernel_Command_Line_Arguments mentions
> > rd_start and rd_size which are used for this purpose.

Ah thanks a lot ... this is what I was looking for.

Rajat