Re: [PATCH 1/1] libfdt: Change last_comp_version to fit spec

[Rob Herring <robh-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>]

On Wed, Dec 2, 2020 at 12:46 AM Justin Covell <jujugoboom-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>
> On Mon, Nov 30, 2020 at 10:22:09AM -0700, Rob Herring wrote:
> > On Tue, Nov 24, 2020 at 10:50 AM Justin Covell <jujugoboom-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> > >
> >
> > Needs a commit message. For a single patch, you don't need a cover
> > letter. The explanation should be here.
> >
> > Besides matching the spec, what problem are you trying to solve?
> >
>
> Sorry about that, first time contributing. I'm trying to help with
> interoperability with other libraries that are made to read/write DTBs
> by matching the spec.

What library? Why does libfdt not work?

> > > ---
> > >  libfdt/fdt_sw.c | 2 +-
> > >  libfdt/libfdt.h | 1 +
> > >  2 files changed, 2 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/libfdt/fdt_sw.c b/libfdt/fdt_sw.c
> > > index 68b543c..4c569ee 100644
> > > --- a/libfdt/fdt_sw.c
> > > +++ b/libfdt/fdt_sw.c
> > > @@ -377,7 +377,7 @@ int fdt_finish(void *fdt)
> > >         fdt_set_totalsize(fdt, newstroffset + fdt_size_dt_strings(fdt));
> > >
> > >         /* And fix up fields that were keeping intermediate state. */
> > > -       fdt_set_last_comp_version(fdt, FDT_FIRST_SUPPORTED_VERSION);
> > > +       fdt_set_last_comp_version(fdt, FDT_LAST_COMPATIBLE_VERSION);
> > >         fdt_set_magic(fdt, FDT_MAGIC);
> > >
> > >         return 0;
> > > diff --git a/libfdt/libfdt.h b/libfdt/libfdt.h
> > > index 89adee3..abad93b 100644
> > > --- a/libfdt/libfdt.h
> > > +++ b/libfdt/libfdt.h
> > > @@ -14,6 +14,7 @@ extern "C" {
> > >  #endif
> > >
> > >  #define FDT_FIRST_SUPPORTED_VERSION    0x02
> > > +#define FDT_LAST_COMPATIBLE_VERSION 0x10
> >
> > If the above change is correct (I'm not sure it is offhand), why not
> > just bump up FDT_FIRST_SUPPORTED_VERSION value?
> >
>
> I didn't want to bump the FDT_FIRST_SUPPORTED_VERSION to maintin
> backwards compatability, and assumed that libfdt actually does support
> working with DTBs down to version 2.

Looking at this more closely, I think your change is correct. It's
actually fixing a regression. Prior to commit f1879e1a50eb ("Add
limited read-only support for older (V2 and V3) device tree to
libfdt."), libfdt would set last_comp_version to 16. Now it sets it to
2, but really 2 is what libfdt can read, not what should be in
last_comp_version.

Also, I'm not certain what happens if you tried to modify a version 2
dtb. It doesn't look like we do anything to fail gracefully.

Rob