linux-kernel - Re: [PATCH 2/2] arm64: regard FDT_SW

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Date:   Thu, 1 Sep 2016 12:21:13 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     zijun_hu <zijun_hu@...o.com>
Cc:     catalin.marinas@....com, will.deacon@....com, robh+dt@...nel.org,
        frowand.list@...il.com, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        zijun_hu@....com
Subject: Re: [PATCH 2/2] arm64: regard FDT_SW_MAGIC as good fdt magic

On Thu, Sep 01, 2016 at 06:58:29PM +0800, zijun_hu wrote:
> From: zijun_hu <zijun_hu@....com>
> 
> regard FDT_SW_MAGIC as good fdt magic during mapping fdt area
> see fdt_check_header() for details

It looks like we should only see FDT_SW_MAGIC for a FDT that was in the
process of being created, but was not finished. So I'm somewhat confused
as to why fdt_check_header() would allow this.

Neither ePAPR nor the new devicetree spec define FDT_SW_MAGIC. They both
only define 0xd00dfeed as a valid magic value. In libfdt, FDT_SW_MAGIC
is an internal constant, and it looks like fdt_check_header() simply
accepts this for convenience within libfdt.

Given all of that, it looks like the kernel should *not* accept
FDT_SW_MAGIC in any case.

Why do you think this is necessary? Have you seen a problem in practice?

> --- a/scripts/dtc/libfdt/fdt.h
> +++ b/scripts/dtc/libfdt/fdt.h
> @@ -92,7 +92,8 @@ struct fdt_property {
>  
>  #endif /* !__ASSEMBLY */
>  
> -#define FDT_MAGIC	0xd00dfeed	/* 4: version, 4: total size */
> +/* 4: version, 4: total size */
> +#define FDT_MAGIC	((fdt32_t)0xd00dfeed)
>  #define FDT_TAGSIZE	sizeof(fdt32_t)
>  
>  #define FDT_BEGIN_NODE	0x1		/* Start node: full name */
> diff --git a/scripts/dtc/libfdt/libfdt.h b/scripts/dtc/libfdt/libfdt.h
> index 59ca33976e56..6998f9249183 100644
> --- a/scripts/dtc/libfdt/libfdt.h
> +++ b/scripts/dtc/libfdt/libfdt.h
> @@ -54,6 +54,8 @@
>  #include "libfdt_env.h"
>  #include "fdt.h"
>  
> +#define FDT_SW_MAGIC           (~FDT_MAGIC)
> +
>  #define FDT_FIRST_SUPPORTED_VERSION	0x10
>  #define FDT_LAST_SUPPORTED_VERSION	0x11
>  
> diff --git a/scripts/dtc/libfdt/libfdt_internal.h b/scripts/dtc/libfdt/libfdt_internal.h
> index 02cfa6fb612d..f4efde0119f2 100644
> --- a/scripts/dtc/libfdt/libfdt_internal.h
> +++ b/scripts/dtc/libfdt/libfdt_internal.h
> @@ -90,6 +90,4 @@ static inline struct fdt_reserve_entry *_fdt_mem_rsv_w(void *fdt, int n)
>  	return (void *)(uintptr_t)_fdt_mem_rsv(fdt, n);
>  }
>  
> -#define FDT_SW_MAGIC		(~FDT_MAGIC)
> -
>  #endif /* _LIBFDT_INTERNAL_H */

Regardless of the above, changes to libfdt must happen in the upstream
libfdt codebase first.

Thanks,
Mark.