On Sat, 2010-06-12 at 19:39 -1000, Mitch Bradley wrote: > Minimally, OFW needs to own some memory that the kernel won't steal. > OFW on ARM is position-independent, so it can be tucked up at the top of > memory > fairly easily.
Amen :-) > To call back into OFW, the virtual mapping for that memory needs to be > reestablished. That's a nasty part unless ARM provides a usable "real mode" which allows MMIO accesses, which I -think- it does. I don't remember the details that much. Maybe we could define a binding tho where we could somewhat standardize the portion of the virtual address space used by OF. IE. from the top of the address space down to the max size it requires. It might require some games to play with the fixmap on ARM side tho... Another option would be something more RTAS-like where a specific call can be done by the OS itself to 'relocate' (not physically but virtually in this case) OF into the OS preferred location. Be prepared to have multiple of these called though as kernels kexec into one another. > Or perhaps the MMU and caches can be turned off for the duration of the > callback. > I don't have the details of ARM MMUs and caches reloaded into my head > yet. Maybe next week... Forgot most of it too. Looks like it's about time I read the ARM architecture again, this sounds like fun :-) BTW. I notice no ARM list is CCed on this discussion ... maybe we should fix that ? > Also, for debugging, OFW typically needs access to a UART. If the OS is > using the UART, it's often possible for OFW to use it just by turning off > interrupts and > polling the UART. That might not be a big deal unless the OS plays with the clocks which it -does- tend to do. It might be worth defining some kind of property OF puts in the UART node to inform the OS not to play games and keep that one enabled, though that could affect power management, so might need to be conditional on some nvram option (debug-enabled?) > The use case for a dynamic device tree is not compelling. Right, generally not, except in virtualized environments (see my other response). Now, the -one- thing that WILL happen if we have something like OF that remains alive is of course vendors will try to use it as a HAL. You know as well as I do that it -will- happen :-) There's two reasons that typically happen. The misguided "good" one which is to think it helps keeping a single/more maintainable kernel image by stuffing the horrible details of nvram, rtc, etc.. access, poweron/off GPIOs, clock control, etc... in there. The "bad" one which is to stash code you don't want to show the source code for (codec control, etc...). This is bad for so many reasons that I don't think I need to even start listing them :-) So that's something that will have to be strongly kept in check and fought I suspect. To some extent, in fact, doing that sort of stuff in OF or even in RTAS like we do on power is even worse than ACPI-like tables. At least with those tables, the interpreter is in the operating system, thus can run with interrupts on, scheduling on, etc... With RTAS, or client interface calls, you'll end up with potentially huge slumps of CPU time "lost" into the bowels of the firmware. Cheers, Ben. _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
