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    SubjectRe: [PATCH 1/9] dt: Handle passed/built-in DT selection in early_init_dt_scan()
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    On Thu, 21 Nov 2013 18:49:08 +0100, Geert Uytterhoeven <geert@linux-m68k.org> wrote:
    > On Thu, Nov 21, 2013 at 4:53 PM, Grant Likely <grant.likely@secretlab.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?

    Add a global flag, bool __init of_dtb_needs_copy, and make it default to
    false. On platforms that need it, change it to true. Make
    unflatten_device_tree() check the flag and copy the tree if necessary.
    Make early_init_dt_scan() set the flag if it choses to use a default
    built-in tree. Then get rid of unflatten_and_copy_devicetree() because
    it has become redundant.

    Finally, make an explicit backup __dtb pointer. Don't assume that
    __dtb_start == __dtb_end is the correct test. We never want to have that
    behavour on arm/powerpc/microblaze, but that doesn't mean there won't be
    compiled in dtbs for other reasons. Create a __dtb_default symbol
    require platforms to do something specific (ie. select a config symbol)
    to set it to __dtb_start. It's not perfect, but at least it acknowledges
    the problem and can be refined later.

    g.


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