linux-kernel - Re: [PATCH 14/17] dt: Consolidate __dtb_start declarations in <linux/of

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Message-ID: <CAMuHMdU6Lqup-MuAOH0iTg16b-SHw4D52WsoPXMsumOxQWbJrg@mail.gmail.com>
Date:	Wed, 13 Nov 2013 18:20:21 +0100
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Rob Herring <robherring2@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Vineet Gupta <vgupta@...opsys.com>,
	James Hogan <james.hogan@...tec.com>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Jonas Bonn <jonas@...thpole.se>,
	Chris Zankel <chris@...kel.net>,
	Rob Herring <rob.herring@...xeda.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>
Subject: Re: [PATCH 14/17] dt: Consolidate __dtb_start declarations in <linux/of_fdt.h>

On Wed, Nov 13, 2013 at 4:51 PM, Rob Herring <robherring2@...il.com> wrote:
> On Tue, Nov 12, 2013 at 1:42 PM, Geert Uytterhoeven
> <geert@...ux-m68k.org> wrote:
>> The different architectures used their own (and different) declarations:
>>
>>     extern struct boot_param_header __dtb_start;
>>     extern u32 __dtb_start[];
>>     extern char __dtb_start[];
>>
>> Consolidate them using the first variant in <linux/of_fdt.h>.
>> This requires adding a few "address of" operators on architectures where
>> __dtb_start was an array before.
>>
>> Signed-off-by: Geert Uytterhoeven <geert@...ux-m68k.org>
>> Cc: Vineet Gupta <vgupta@...opsys.com>
>> Cc: James Hogan <james.hogan@...tec.com>
>> Cc: Ralf Baechle <ralf@...ux-mips.org>
>> Cc: Jonas Bonn <jonas@...thpole.se>
>> CC: Chris Zankel <chris@...kel.net>
>> Cc: Rob Herring <rob.herring@...xeda.com>
>> Cc: devicetree@...r.kernel.org
>> ---
>>  arch/arc/include/asm/sections.h             |    1 -
>>  arch/arc/kernel/setup.c                     |    2 +-
>>  arch/metag/kernel/setup.c                   |    6 +-----
>>  arch/mips/include/asm/mips-boards/generic.h |    4 ----
>>  arch/mips/lantiq/prom.h                     |    2 --
>>  arch/mips/netlogic/xlp/dt.c                 |    4 ++--
>>  arch/mips/ralink/of.c                       |    2 --
>>  arch/openrisc/kernel/setup.c                |    2 +-
>>  arch/openrisc/kernel/vmlinux.h              |    2 --
>>  arch/xtensa/kernel/setup.c                  |    3 +--
>>  include/linux/of_fdt.h                      |    3 +++
>>  11 files changed, 9 insertions(+), 22 deletions(-)
>
> This is great, but I would like to see this taken one step further and
> move the selection logic into the core code. We can try to use the
> built-in dtb if early_init_dt_scan is passed a NULL or invalid dtb. This
> should work for most arches I think and still allows an arch to do a
> different selection algorithm if desired.

Yes, that makes sense.

> Are the DT related patches dependent on the rest of the series?

Not really. There may be small rejects if you apply it out-of-order,
but nothing serious.

> Something like this is what I have in mind:
>
> diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
> index 5c47910..723f854d 100644
> --- a/drivers/of/fdt.c
> +++ b/drivers/of/fdt.c
> @@ -869,14 +869,16 @@ void * __init __weak
> early_init_dt_alloc_memory_arch(u64 size, u64 align)
>
>  bool __init early_init_dt_scan(void *params)
>  {
> -       if (!params)
> -               return false;
> -
>         /* Setup flat device-tree pointer */
>         initial_boot_params = params;
>
> +       if (!initial_boot_params ||
> +           (be32_to_cpu(initial_boot_params->magic) != OF_DT_HEADER))
> +               initial_boot_params = &__dtb_start;
> +
>         /* check device tree validity */
> -       if (be32_to_cpu(initial_boot_params->magic) != OF_DT_HEADER) {
> +       if (!initial_boot_params ||

initial_boot_params cannot be NULL here, so no need to check.

> +           (be32_to_cpu(initial_boot_params->magic) != OF_DT_HEADER)) {

However, if __dtb_end == __dtb_start, you may be reading random
data here from the next section. The OF_DT_HEADER check should cover
this, but better safe than sorry?

>                 initial_boot_params = NULL;
>                 return false;
>         }

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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