On Mon, Nov 17, 2014 at 8:13 AM, Arnd Bergmann <arnd@xxxxxxxx> wrote: > This is not just DT, it's actually an implementation of a boot > interface. The situation here seems much more to what we had on > PowerPC a long time ago than what we had on ARM before the DT > conversion. I think the best approach here would be to move the > platform specific bits into the decompressor code, and allow > multiple implementations of that. This way you can have the > generic vmlinux file that has a common DT parser, and you wrap > that into one decompressor per platform, some of which can have > their own board detection logic or pre-boot setup where necessary. > > To be honest, I think having multiple DT files linked into the > kernel is a really bad idea, because it doesn't solve the > scalability problem at all. What we did on ARM was to force those > hacks out into external projects such as the PXA impedence > matcher [https://github.com/zonque/pxa-impedance-matcher]. This > can handle all weird boot protocol and adapt them to the normal > well-defined interfaces we have in the kernel. To some extent this is how BCM3384 was done[1]. There is a tradeoff here: to add support for the older platforms it is easy to build a new DTB file into the kernel image, but it is a lot of trouble to write a new 3rd stage bootloader. Do we want to maximize our list of supported boards, or are we shooting for a super clean kernel implementation right off the bat? >> And unless there is one, having a >> multiplatform kernel does not make much sense, as there is no sane way >> to tell apart different platforms on boot. > > How do you normally tell boards apart on MIPS when you don't use DT? On BCM7xxx (STB) kernels, we could assume the chip ID was in a known register, and also we could call back into the bootloader to get a somewhat-accurate board name. On BCM63xx there is logic in arch/mips/bcm63xx/cpu.c to try to guess the chip identity from the CPU type/revision (because the latter can be read directly from CP0). These systems were never really designed to support multiplatform kernels. The ARM BCM7xxx variants, by contrast, were. [1] https://github.com/Broadcom/aeolus
