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

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Date:	Thu, 21 Feb 2013 14:38:02 +0530
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	James Hogan <james.hogan@...tec.com>
CC:	<linux-kernel@...r.kernel.org>,
	Vineet Gupta <Vineet.Gupta1@...opsys.com>,
	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

Hi James,

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.

And if this is a general problem, then it is probably better done in DT core.

I'm sure I'm missing something here.

> Signed-off-by: James Hogan <james.hogan@...tec.com>
> ---
>  arch/metag/include/asm/prom.h |  1 +
>  arch/metag/kernel/devtree.c   | 17 +++++++++++++++++
>  arch/metag/kernel/setup.c     |  2 ++
>  3 files changed, 20 insertions(+)
>
> diff --git a/arch/metag/include/asm/prom.h b/arch/metag/include/asm/prom.h
> index d881396..ccac97e 100644
> --- a/arch/metag/include/asm/prom.h
> +++ b/arch/metag/include/asm/prom.h
> @@ -18,6 +18,7 @@
>  #define HAVE_ARCH_DEVTREE_FIXUPS
>  
>  extern struct machine_desc *setup_machine_fdt(void *dt);
> +extern void copy_fdt(void);
>  extern void metag_dt_memblock_reserve(void);
>  
>  #endif /* __ASM_METAG_PROM_H */
> diff --git a/arch/metag/kernel/devtree.c b/arch/metag/kernel/devtree.c
> index 5b6b1d85..7cd0252 100644
> --- a/arch/metag/kernel/devtree.c
> +++ b/arch/metag/kernel/devtree.c
> @@ -95,3 +95,20 @@ struct machine_desc * __init setup_machine_fdt(void *dt)
>  
>  	return mdesc_best;
>  }
> +
> +/**
> + * copy_fdt - Copy device tree into non-init memory.
> + *
> + * We must copy the flattened device tree blob into non-init memory because the
> + * unflattened device tree will reference the strings in it directly.
> + */
> +void __init copy_fdt(void)
> +{
> +	void *alloc = early_init_dt_alloc_memory_arch(
> +			be32_to_cpu(initial_boot_params->totalsize), 0x40);
> +	if (alloc) {
> +		memcpy(alloc, initial_boot_params,
> +		       be32_to_cpu(initial_boot_params->totalsize));
> +		initial_boot_params = alloc;
> +	}
> +}
> diff --git a/arch/metag/kernel/setup.c b/arch/metag/kernel/setup.c
> index dd6c5ad..8792461 100644
> --- a/arch/metag/kernel/setup.c
> +++ b/arch/metag/kernel/setup.c
> @@ -406,6 +406,8 @@ void __init setup_arch(char **cmdline_p)
>  	cpu_2_hwthread_id[smp_processor_id()] = hard_processor_id();
>  	hwthread_id_2_cpu[hard_processor_id()] = smp_processor_id();
>  
> +	/* Copy device tree blob into non-init memory before unflattening */
> +	copy_fdt();
>  	unflatten_device_tree();
>  
>  #ifdef CONFIG_SMP

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