linux-kernel - Re: [PATCH v2 4/6] arm/dt: probe for platforms via the device tree

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Date:	Mon, 31 Jan 2011 16:03:24 -0700
From:	Grant Likely <grant.likely@...retlab.ca>
To:	Nicolas Pitre <nicolas.pitre@...aro.org>
Cc:	Russell King <linux@....linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	devicetree-discuss@...ts.ozlabs.org,
	lkml <linux-kernel@...r.kernel.org>,
	Lennert Buytenhek <buytenh@...tstofly.org>,
	Olof Johansson <olof@...om.net>, john.linn@...inx.com,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 4/6] arm/dt: probe for platforms via the device tree

On Mon, Jan 31, 2011 at 3:27 PM, Nicolas Pitre <nicolas.pitre@...aro.org> wrote:
> On Mon, 31 Jan 2011, Grant Likely wrote:
>
>> If a dtb is passed to the kernel then the kernel needs to iterate
>> through compiled-in mdescs looking for one that matches and move the
>> dtb data to a safe location before it gets accidentally overwritten by
>> the kernel.
>>
>> This patch creates a new function, setup_machine_fdt() which is
>> analogous to the setup_machine_atags() created in the previous patch.
>> It does all the early setup needed to use a device tree machine
>> description.  It also creates arm_unflatten_device_tree() which copies
>> the dtb into an allocated buffer and unflattens it into the live-tree
>> representation.
>>
>> v2: Changed to save the dtb by copying into an allocated buffer.
>>     - Since the dtb will very likely be passed in the first 16k of ram
>>       where the interrupt vectors live, memblock_reserve() is
>>       insufficient to protect the dtb data.
>
> This is wrong.  The vector page can be allocated anywhere.  It is
> currently allocated with memblock_alloc(), so if you memblock_reserve()
> the dtb soon enough then it should be safe and the vector page or
> whatever will be allocated somewhere else.

Ah, thanks.  Did not know that.

> What was the actual problem?

I was seeing the first page in memory getting zeroed partway through
the boot process which wiped out the dtb.  I haven't yet narrowed down
to the exact point that it gets overwritten.  It is late enough that
all the early setup dt code works, but before init_machine gets
called.

I'm also seeing a problem with /proc/device-tree getting corrupted on
qemu which I cannot reproduce on Tegra hardware.  Still investigating
this one.

g.
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