Subject: Re: Request review of device tree documentation

From: Benjamin Herrenschmidt <[email-masked]>
Date: Sun, 13 Jun 2010 15:59:33 +1000

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.