linux-kernel - Re: [PATCH 1/9] dt: Handle passed/built-in DT selection in early_init_dt

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Message-ID: <CAMuHMdXjROx-LqNkq_GhNp4M-iYvDXV6X4j7EkgQGdkUmAMvhg@mail.gmail.com>
Date:	Thu, 21 Nov 2013 18:49:08 +0100
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Grant Likely <grant.likely@...retlab.ca>
Cc:	Rob Herring <rob.herring@...xeda.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Linux-Arch <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/9] dt: Handle passed/built-in DT selection in early_init_dt_scan()

On Thu, Nov 21, 2013 at 4:53 PM, Grant Likely <grant.likely@...retlab.ca> wrote:
>> My changes don't change the current behavior much: currently
>> early_init_dt_scan() is already called with &__dtb_start in several places.
>> If this is broken, it's already broken.
>
> Yes, but it is called on platforms that already make that assumption
> that __dtb_start must be copied and therefore call
> unflatten_and_copy_devicetree(). This change makes *all* platforms do
> that. That breaks arm, arm64, c6x, microblaze, some mips platforms, and
> powerpc!

arm, arm64, and powerpc don't have builtin DTBs, hence no change.
microblaze and c6x already did "early_init_devtree(&__dtb_start)" before.

That leaves mips, which has DT handling in too many variants, which I
didn't all verify.

>> > memory. The dtb section can also potentially contain multiple .dtb
>> > blobs. In the use case that you care about you are probably only
>>
>> Multiple dtb blobs are currently handled in platform-specific code, which
>> passes the right dtb to early_init_dt_scan().
>
> The problem is that it makes the default dt completely random because
> the generic code still deferrences __dtb_start.

Only if no DT was passed by the bootloader. And only if there really
is something at __dtb_start. Before it would fail if no DTB was passed
by the bootloader.

>> > thinking about one, but it is entirely possible for device drivers to
>> > have a dtb linked in which may break this if it gets linked in a
>> > different order. The specific example I'm thinking about is I want to
>> > have the DT selftest code load an overlay to get testcase data from a
>> > dtb blob.
>> >
>> > The other concern I have here is that I don't really want this to be the
>> > default on a lot of platforms. ARM and PowerPC for instance should only
>> > get the default dtb from the boot wrapper. It needs to be configurable
>> > in some way.
>>
>> On ARM and PowerPC, the section is empty, hence &__dbt_start ==
>> &__dtb_end.
>
> There is no guarantee that that will always be so. The patch makes some
> poor assumtions, but they shouldn't be difficult to fix.

OK, how to proceed?

Thanks!

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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