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@xxxxxxxxxxxxxx> 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
