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SubjectRe: [PATCH V4 0/3] scripts: dtc: Build fdtoverlay
On Wed, Jan 20, 2021 at 10:47:40AM +0530, Viresh Kumar wrote:
> +David.
>
> On 19-01-21, 11:12, Frank Rowand wrote:
> > On 1/12/21 2:28 AM, Viresh Kumar wrote:
> > > We will start building overlays for platforms soon in the kernel and
> > > would need fdtoverlay tool going forward. Lets start fetching and
> > > building it.
> > >
> > > While at it, also remove fdtdump.c file, which isn't used by the kernel.
> > >
> > > V4:
> > > - Don't fetch and build fdtdump.c
> > > - Remove fdtdump.c
> > >
> > > Viresh Kumar (3):
> > > scripts: dtc: Add fdtoverlay.c to DTC_SOURCE
> > > scripts: dtc: Build fdtoverlay tool
> > > scripts: dtc: Remove the unused fdtdump.c file
> > >
> > > scripts/dtc/Makefile | 6 +-
> > > scripts/dtc/fdtdump.c | 163 -------------------------------
> > > scripts/dtc/update-dtc-source.sh | 6 +-
> > > 3 files changed, 8 insertions(+), 167 deletions(-)
> > > delete mode 100644 scripts/dtc/fdtdump.c
> > >
> >
> > My first inclination was to accept fdtoverlay, as is, from the upstream
> > project.
> >
> > But my experiences debugging use of fdtoverlay against the existing
> > unittest overlay files has me very wary of accepting fdtoverlay in
> > it's current form.
> >
> > As an exmple, adding an overlay that fails to reply results in the
> > following build messages:
> >
> > linux--5.11-rc> make zImage
> > make[1]: Entering directory '/local/frowand_nobackup/src/git_linus/build/dragon_linus_5.11-rc'
> > GEN Makefile
> > CALL /local/frowand_nobackup/src/git_linus/linux--5.11-rc/scripts/checksyscalls.sh
> > CALL /local/frowand_nobackup/src/git_linus/linux--5.11-rc/scripts/atomic/check-atomics.sh
> > CHK include/generated/compile.h
> > FDTOVERLAY drivers/of/unittest-data/static_test.dtb
> >
> > Failed to apply 'drivers/of/unittest-data/overlay.dtb': FDT_ERR_NOTFOUND
> > make[4]: *** [/local/frowand_nobackup/src/git_linus/linux--5.11-rc/drivers/of/unittest-data/Makefile:96: drivers/of/unittest-data/static_test.dtb] Error 1
> > make[3]: *** [/local/frowand_nobackup/src/git_linus/linux--5.11-rc/scripts/Makefile.build:496: drivers/of/unittest-data] Error 2
> > make[2]: *** [/local/frowand_nobackup/src/git_linus/linux--5.11-rc/scripts/Makefile.build:496: drivers/of] Error 2
> > make[1]: *** [/local/frowand_nobackup/src/git_linus/linux--5.11-rc/Makefile:1805: drivers] Error 2
> > make[1]: Leaving directory '/local/frowand_nobackup/src/git_linus/build/dragon_linus_5.11-rc'
> > make: *** [Makefile:185: __sub-make] Error 2
> >
> >
> > The specific error message (copied from above) is:
> >
> > Failed to apply 'drivers/of/unittest-data/overlay.dtb': FDT_ERR_NOTFOUND
> >
> > which is cryptic and does not even point to the location in the overlay that
> > is problematic. If you look at the source of fdtoverlay / libfdt, you will
> > find that FDT_ERR_NOTFOUND may be generated in one of many places.
> >
> > I do _not_ want to do a full review of fdtoverlay, but I think that it is
> > reasonable to request enhancing fdtoverlay in the parent project to generate
> > usable error messages before enabling fdtoverlay in the Linux kernel tree.

That's... actually much harder than it sounds. fdtoverlay is
basically a trivial wrapper around the fdt_overlay_apply() function in
libfdt. Matching the conventions of the rest of the library, really
it's only way to report errors is a single error code.

Returning richer errors is not an easy problem in a C library,
especially one designed to be usable in embedded systems, without an
allocator or much else available.

Of course it would be possible to write a friendly command line tool
specifically for applying overlays, which could give better errors.
fdtoverlay as it stands isn't really that - it was pretty much written
just to invoke fdt_overlay_apply() in testcases.

> > fdtoverlay in it's current form adds a potential maintenance burden to me
> > (as the overlay maintainer). I now have the experience of how difficult it
> > was to debug the use of fdtoverlay in the context of the proposed patch to
> > use it with the devicetree unittest overlay .dtb files.
> >
> > -Frank
>

--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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