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From:   Florian Fainelli <florian@openwrt.org>
To:     Geert Uytterhoeven <geert@linux-m68k.org>
Subject: Re: Converting MIPS to Device Tree
Date:   Sun, 12 Jun 2011 20:20:15 +0200
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Cc:     David VomLehn <dvomlehn@cisco.com>,
        Grant Likely <grant.likely@secretlab.ca>,
        Ralf Baechle <ralf@linux-mips.org>, linux-mips@linux-mips.org,
        Imre Kaloz <kaloz@openwrt.org>,
        Gabor Juhos <juhosg@openwrt.org>,
        John Crispin <blogic@openwrt.org>,
        "Dezhong Diao (dediao)" <dediao@cisco.com>,
        Thomas Gleixner <tglx@linutronix.de>
References: <20110606010753.GA16202@linux-mips.org> <20110607230218.GA23552@dvomlehn-lnx2.corp.sa.net> <BANLkTikjgj-QH=8u6NeGbWHy5hi1jiiU6Q@mail.gmail.com>
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On Wednesday 08 June 2011 09:15:14 Geert Uytterhoeven wrote:
> On Wed, Jun 8, 2011 at 01:02, David VomLehn <dvomlehn@cisco.com> wrote:
> > I took a look at the issue of passing device trees to the kernel and
> > started by surveying the methods currently in use for passing
> > information from the bootloader to the kernel. I came up with the ten
> > approaches:
> > 
> > How MIPS Bootloaders Pass Information to the Kernel
> > ---------------------------------------------------
> > Apologies for any errors; this was meant more to be a quick survey
> > rather than a detailed analysis.
> > 
> > 6.      a0 - argc
> >        a1 - argv
> >        a2 - non-standard envp
> >        Command line created by concatenating argv strings, starting at
> >        argv[1]. The envp is a pointer to a list of char ptr to name/char
> >        ptr pairs.
> >        Platforms: txx9
> 
> This depends on the actual boot loader. My rbtx4927 has a VxWorks boot
> loader, which just doesn't pass anything.
> 
> Cfr. commit 97b0511ce125b0cb95d73b198c1bdbb3cebc4de2 ("MIPS: TXx9:
> Make firmware parameter passing more robust").

Thinking about this more, on platforms for which we do not have control about 
the bootloader, we can usually still get it to boot a wrapper. Such a wrapper 
could do the following:

- embedd the kernel
- embedd the appropriate dtb
- copy the relevant a0-a3 values and pass to the kernel a pointer to the valid 
dtb in the aX register

such a wrapper already exists, which is the code responsible for decompressing 
the kernel in arch/mips/boot/compressed.

If we want to support multiple flavors of the same SoC, using the same kernel 
image, we only need to relink the boot wrapper with the correct dtb (and 
kernel of course).
--
Florian