Re: [PATCH] Open Firmware device tree virtual filesystem

From: Segher Boessenkool
Date: Tue Jan 02 2007 - 16:29:57 EST

Not single thread -- but a "global OF lock" yes. Not that
it matters too much, (almost) all property accesses are init
time anyway (which is effectively single threaded).

Not that true anymore. A lot of driver probe is being threaded nowadays,
either bcs of the new multithread probing bits, or because they get
loaded by userland from some initramfs etc..

The kernel doesn't care if one CPU is in OF land while the others
are doing other stuff -- well you have to make sure the OF won't
try to use a hardware device at the same time as the kernel, true.

Finally, you can't have something as simple as powerpc's get_property()
(that just returns a pointer to the property content) with direct OF
access unless you use some magic static buffer or some crap around
those
lines, or add passing of a buffer in, so from a driver pointer of view,
the interface provided by powerpc/sparc is nicer.

But since you have a _put() function anyway, for your reference
counting, having magic (automatically allocated, not static of
course) buffers works just fine.

You can maybe create an in-memory cache of all properties in a node from
whatever function that returns a node pointer and dispose of them on
_put(), that would allow to keep the get_property semantics as-is with a
"live" tree, but I still don't see the point

I'm a bit concerned about the 100kB or so of data duplication
(on a *quite big* device tree), and the extra code you need
(all changes have to be done to both tree copies). Maybe
I shouldn't be worried; still, it's obviously not a great
idea to *require* any arch to get and keep a full copy of
the tree -- it's wasteful and unnecessary.


Segher

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/