[U-Boot] U-Boot as a next bootloader in a chain - reading the kernel boot args provided by the previous stage bootlader?

[Wojciech Zabolotny]

Hi,

I need to use U-Boot (or barebox) as a bootloader in a Raspberry Pi
based system to extend booting flexibility.
The problem however is that certain kernel boot arguments are prepared
by the previous stage bootloader (start.elf)
basing on the properties of the individual board.
For example in one of the boards I use, the kernel command line when a
standard kernel is booted, looks as follows (MAC address and serial
number are partially masked for privacy):

dma.dmachans=0x7f35 bcm2708_fb.fbwidth=720 bcm2708_fb.fbheight=480
bcm2708.boardrev=0xd bcm2708.serial=0x6f15XXXX
smsc95xx.macaddr=B8:27:EB:XX:XX:XX bcm2708_fb.fbswap=1
sdhci-bcm2708.emmc_clock_freq=250000000 vc_mem.mem_base=0x1ec00000
vc_mem.mem_size=0x20000000  console=ttyAMA0 root=/dev/mmcblk0p2
rootwait

Only the "console=ttyAMA0 root=/dev/mmcblk0p2 rootwait" is provided by
the user (in the cmdline.txt file). The rest is generated by the
start.elf.

Of course when I boot the u-boot.bin instead of zImage, the same
parameters are passed to it, but the U-Boot is not able to read them
and to pass them to the finally booted kernel.
As U-Boot shares a lot of code with the Linux kernel it should be
possible to add necessary functions.
It could be useful in all applications where U-Boot is used as an
additional stage in the boot chain e.g., to add new booting
functionalities (booting from the network, booting from the flash disk
etc.).

I have found a nice description, how the paremeters are passed in ARM
architecture:
http://www.simtec.co.uk/products/SWLINUX/files/booting%5Farticle.html
but of course the solution should be probably portable (or implemented
for each platform independently with unified API).

--
Regards,
Wojtek


Wojciech M. Zabołotny
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