linux-kernel - Re: [PATCH] Open Firmware device tree virtual filesystem

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Message-Id: <bb0d56f649449edb8b5cc0e1c12ede29@kernel.crashing.org>
Date:	Tue, 2 Jan 2007 22:28:21 +0100
From:	Segher Boessenkool <segher@...nel.crashing.org>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	linux-kernel@...r.kernel.org, devel@...top.org,
	David Miller <davem@...emloft.net>, David Kahn <dmk@...x.com>,
	Mitch Bradley <wmb@...mworks.com>, jg@...top.org
Subject: Re: [PATCH] Open Firmware device tree virtual filesystem

>> 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

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