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

[Albert ARIBAUD <albert.u.boot@aribaud.net>]

Hello Wojciech,

On Mon, 16 Nov 2015 13:09:43 +0100, Wojciech Zabolotny
<wzab01@gmail.com> wrote:
> 2015-11-16 11:52 GMT+01:00 Albert ARIBAUD <albert.u.boot@aribaud.net>:
> > Hello Wojciech,
> >
> > On Mon, 16 Nov 2015 10:42:50 +0100, Wojciech Zabolotny
> > <wzab01@gmail.com> wrote:
> >> 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).
> >
> > Not sure what kind of answer you're asking for here. Do you want to
> > know if what you're suggesting is feasible? Are you looking for someone
> > to implement it? Are you going to implement it yourself but asking for
> > feedback?
> >
> >> --
> >
> > (aside: if the above should be a signature delimiter, then it lacks a
> > space after the dashes)
> >
> > Amicalement,
> > --
> > Albert.
> 
> Dear Albert,
> 
> Yes the first question is if this feature is feasible.

In software, just about anything is feasible; ask any PHB. :)

Specifically, catching information passed to U-Boot's entry point is
something that e.g. OMAPs do already. It is probably not going to be
portable, though, because the passing method is inherently specific to
your platform and pre-U-Boot loader.

> If yes, then I'd like to propose such functionality as a possible improvement.
> I think that may be more people interested in such a feature.
>
> I'll appreciate any sugestions/pointers regarding the possible implementation.
> 
> Probably I can try to implement it. Of course if there are other
> interested users
> who can do it, I'll be definitely happy to discuss that with them.

Best is that you post a patch with a working implementation which can
be build above current u-boot/master branch; people interested in it
will comment.

> Regards,

Amicalement,
-- 
Albert.