Messages in this thread | | | From | Segher Boessenkool <> | Subject | Re: [PATCH] Open Firmware device tree virtual filesystem | Date | Wed, 3 Jan 2007 01:48:02 +0100 |
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>>>> 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. > > True, but at the very least you have to prevent multiple cpus > from enterring OFW. In fact this is very important.
Yes. "Global OF lock".
> OFW is not multi-threaded
You are not _guaranteed_ it is multithreaded, and you don't know it's threading model (or how to do thread synchronisation).
> therefore you can't let multiple CPUs call > into OFW at one time. You must use some kind of locking mechanism, > and that locking mechanism is not simple because it has to not just > stop the other cpus, it has to be able to stop the other cpus yet > still allow them to receive SMP cross-calls from the firmware if the > OFW call is 'stop' or similar.
YOu don't need to *stop* the other CPUs, you just need to prevent them from entering the client interface. Put a lock in front.
>> 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. > > The largest amount of memory I've ever seen consumed on sparc64 > was 76K and this is 1) 64-bit and 2) an ENORMOUS machine with > lots of cpus and devices. And I know because sparc64 prints > a kernel message at boot which states how much memory was > consumed by the in-kernel device tree copy.
The in-OF tree uses a bit more memory, depending on implementation. It's hard to tell though, it contains so much more than the properties-only tree, perhaps you're right.
> Please let's get over this memory consumption non-issue and move > on to more productive talk.
Okay -- so answer the second part of my concern please: if you keep a copy, you need to keep both in sync -- that means every change by the kernel has to be done twice, and you won't ever be told about changes by the OF, so you have to get a full fresh copy every single time you return from an OF client call that could have changed a property.
Segher
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