On Sat, Jun 12, 2010 at 11:48 PM, Benjamin Herrenschmidt
[off-list ref] wrote:

On Sat, 2010-06-12 at 23:07 -0600, Grant Likely wrote:

quoted

What is needed to keep OFW alive? =A0I've got no problem with doing so
if it isn't invasive, and as long as the same boot entry interface can
be used.

Well, no. OF has a well defined "client interface" which is what gets us
in prom_init.c on powerpc when r5 is non NULL. You can use it to do
things like access devices or the device-tree.

We use that to suck the device-tree, which we flatten, and then re-enter
the kernel with the "common" entry interface.

I don't think I want to do the same on ARM.  I'd rather have the
prom_init stuff in a boot wrapper, or have OFW itself generate the
flat representation before booting the kernel.  I'm trying to
constrain the number of things that could go wrong by defining only
one way for getting the device tree data into the kernel.

If you want to keep OF alive, you -can- probably do that too, but it's
more tricky. You need to keep the memory where OF resides reserved, OF
might need special MMU translations, you maybe need to install hooks in
OF itself for it to be able to manipulate some of those, etc...

quoted

What is the use-case for having a dynamic device tree? =A0I can see
keeping OFW alive being useful for some debug facilities, but once the
kernel has started, I'm really not interested in relying on firmware
to manage the hardware. =A0(but then again it's no secret that I'm
suspicious of anything that depends on runtime interaction with
firmware).

As we all are :-)

:-)

I see one genuine use of dynamic device-tree though, which is in a
virtualized environment (where the host can be Linux itself). Hotplug
can then be a matter of getting new nodes & properties fed down to the
OS by the hypervisor/firmware.

pSeries somewhat works like that, but using weirdo rtas calls. We could
do something saner for kvm or whatever other hypervisors we have some
amount of control onto, where bits of compiled dtb can be handed down
representing a sub-hierarchy of the tree.

Right.  We don't need to use OFW/RTAS to handle this use case.

g.