On Sun, Jun 05, 2011 at 11:41:10PM -0500, Grant Likely wrote:
> On Sun, Jun 5, 2011 at 7:07 PM, Ralf Baechle <ralf@linux-mips.org> wrote:
> > Over the past few days I've started to convert arch/mips to use DT.
>
> Nice!
>
> > So
> > far none of the platforms (except maybe PowerTV?) seems to have a
> > firmware that is passing a DT nor is there any 2nd stage bootloader that
> > could do so.
>
> FWIW, U-Boot now has pretty generic support for manipulating and
> passing a dtb at boot. That doesn't do much good for existing
> deployed systems though.
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.
1. a0 - argc
a1 - argv
Strings are concatenated to create the command line, starting at
argv[0].
Platforms: ath79
2. a0 - argc
a1 - argv
a2 - envp
Strings are concatenated to create the command line, starting at
argv[0].
Platforms: pnx8550, rb532
3. a0 - argc
a1 - argv
Command line created by concatenating argv strings, starting at
argv[1].
Platforms: emma, fw, jz4740, lasat, pnx833x, vr41xx
4. a0 - argc
a1 - argv
a2 - envp
Command line created by concatenating argv strings, starting at
argv[1].
Platforms: alchemy, ar7, loongson, mti-malta, pmc-sierra
5. a0 - unused
a1 - unused
a2 - unused
Boot descriptor in a3.
Platforms: cavium-octeon
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
7. a0 - argc
a1 - argv
a2 - magic
a3 - prom_vec
Command line created by concatenating argv strings, starting at
either argv[1] or argv[2], depending on the value passed in magic.
Platforms: dec
8. a0 - argc
a1 - unused
a2 - envp
a3 - prom_vec
Interpretation depends on the values passed, but command line is
a single string in the cfe environment.
Platforms: bcm47xxx, sibyte
9. a0 - bits 30-16: memory size, bits 15-0, argc
a1 - argv
Command line created by concatenating argv strings, starting at
argv[1].
Platforms: cobalt
10. a0 - argc
a1 - argv
a2 - unused
a3 - memory size
The command line is assumed to already be a single string, pointed
to by argv.
Platforms: powertv
It seems like everything ultimately does create a command line. We could then
use a parameter like "devtree=<virtual-address>" on the command line, passed
in any way the bootloader likes. In this case, the <virtual-address> will be
a kseg0 address so we don't have to set up any mappings. If we allow multiple
device trees to be built in or appended to the end of the kernel, we can use
the existing "dtb_compat" command line parameter to select which one to use.
I would propose that "devtree" take precedence over "dtb_compat", but that's
really just a desire to pick one over the other, whichever is the preferred
one.
--
David VL
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