linux-kernel - Re: [PATCH 1/2] metag: copy devicetree to non-init memory

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Date:	Thu, 21 Feb 2013 16:48:22 +0530
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	James Hogan <james.hogan@...tec.com>
CC:	<linux-kernel@...r.kernel.org>,
	Grant Likely <grant.likely@...retlab.ca>,
	Arnd Bergmann <arnd@...db.de>,
	Rob Herring <rob.herring@...xeda.com>
Subject: Re: [PATCH 1/2] metag: copy devicetree to non-init memory

On Thursday 21 February 2013 03:04 PM, James Hogan wrote:
> Hi Vineet,
>
> On 21/02/13 09:08, Vineet Gupta wrote:
>> On Wednesday 20 February 2013 08:22 PM, James Hogan wrote:
>>> Make a copy of the device tree blob in non-init memory. It is required
>>> when using built-in device tree files that the platform code copies the
>>> blob to non-init memory prior to calling unflatten_device_tree(),
>>> otherwise the strings that the device tree refer to will get poisoned
>>> and potentially reused, breaking later reading of the device tree
>>> post-init (such as compatible matching in modules, debugfs, and the
>>> procfs interface).
>> While the patch conceptually looks correct, I'm not sure why any user of DT -
>> post-init would refer to DT bindings using of_fdt_* API which use the flat tree,
>> instead of the binary tree (more efficient in space/usage). Is this to support
>> some in-transition drivers and other code.
> The strings aren't copied when the devicetree is unflattened, so the
> unflattened version still points into initdata, so all the strings "in"
> the unflattened version are wiped when it's freed too.

You are absolutely right, so ARC port is infested with same ticking time bomb !
Thanks for the heads up - I'll queue up a fix - but that will go in after the
first pull request as I want any change, however minimal/local to cook in -next
for some time (for paranoid sake at this stage)

> Documentation/kbuild/makefiles.txt has this to say:
>>     dtc
>> 	Create flattend device tree blob object suitable for linking
>> 	into vmlinux. Device tree blobs linked into vmlinux are placed
>> 	in an init section in the image. Platform code *must* copy the
>> 	blob to non-init memory prior to calling unflatten_device_tree().
> Other architectures using the builtin dtb also do the copy. I presume
> it's in initdata in the first place to avoid keeping the built-in one
> around if one is provided by the bootloader instead.

Makes sense !

-Vineet
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