[U-Boot] Mips, U-Boot and ramdisk

[Matthew Lear <matt@bubblegen.co.uk>]

Hi Robert.
> I set the bootargs variable to: root=\dev\ram (I used: set bootargs 
> root=/dev/ram)
> But when I'm trying to start the Linux with the
> 
> bootm 81000000 81FFFFC0
> 
> the Linux can't find the ramdisk. It write out:
> 
> Initrd not found or empty - disabling initrd

Do you see U-Boot detecting and loading the ram disk image once you invoke your
bootm command above? eg:

## Loading RAMDisk Image at 00500000 ...
   Image Name:   uboot ext2 ramdisk rootfs
   Created:      2009-06-15  14:39:13 UTC
   Image Type:   M68K Linux RAMDisk Image (gzip compressed)
   Data Size:    5219290 Bytes =  5 MB
   Load Address: 00000000
   Entry Point:  00000000
   Verifying Checksum ... OK
   Loading Ramdisk to 4fa79000, end 4ff733da ... OK

I believe that for U-Boot to pass the ram disk image information to the kernel,
it needs to be able to detect the ram disk image in the first place. You can use
U-Boot's mkimage utility to add a header onto your ram disk image.

> But when I set its address into the bootargs (so the bootargs: 
> root=/dev/ram rd_start=0x82000000 rd_size=0x191160), it works well; it 
> successfully find the image, and can mount it.

This is because you're explicitly telling the kernel where to find the ram disk
image in memory. Take a look at drivers/block/brd.c in the kernel src.

> How does the U-Boot pass the ramdisk information? It sets some kind of 
> environment variables in the bootm.c. But it doesn't work for me. Why? 
> (I could use the bootargs solution in this case, but I'm afraid, it 
> can't pass other arguments too, like ethernet address, etc.)

This is arch specific in U-Boot but I'd also check that your MIPS kernel has
support for a) correctly parsing the U-Boot environment provided to it and b)
providing the required data to other parts of the kernel for utilisation of the
ram disk, eg initrd_start / initrd_end as an example.

If you're struggling to pass other args to the kernel then it sounds like there
is more of a fundamental issue somewhere, though. Maybe just double check
Documentation/kernel-parameters.txt to make sure you're passing syntax in a form
that the kernel will recognise?

Hope that helps.

Cheers,
--  Matt