Re: [PATCH V4 0/3] scripts: dtc: Build fdtoverlay

[Rob Herring <robh+dt@kernel.org>]

On Sun, Jan 31, 2021 at 10:39 PM David Gibson
<david@gibson.dropbear.id.au> wrote:
>
> On Mon, Jan 25, 2021 at 09:42:21PM -0600, Frank Rowand wrote:
> > Hi David,
> >
> > On 1/22/21 12:34 AM, David Gibson wrote:
> > > 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.
> >
> > Thank you for providing that context.
> >
> > I do not know if there is a way to enable the code that is currently in libfdt
> > to both be useful as an embedded library (for example, U-boot seems to often
> > have a need to keep memory usage very small) and also be part of a tool with
> > effective warning and error messages.
>
> Yeah, I don't know either.
>
> > Before having looked at libfdt only at a cursory level while debugging the proposed
> > use of fdtoverlay in Linux, my first thought was that maybe it would be possible
> > to add warning and error messages within "#ifdef" blocks, or other ways that
> > cause the error code to _not_ be compiled as part of library version of libfdt,
> > but only be compiled as part of fdtoverlay _when built in the Linux kernel_
> > (noting that the proposed Linux patch builds the libfdt files as part of
> > the fdtoverlay compile instead of as a discrete library).  After looking at
> > the libfdt source a tiny bit more carefully, I would probably shoot down this
> > suggestion, as it makes the source code uglier and harder to understand and
> > maintain for the primary purpose of being an embedded library.
>
> Oof.  That sounds really ugly, but maybe it could be pulled off.
>
> > Do you have any thoughts on how warning and error messages could be added,
> > or if it is even possible?  Or maybe your suggestion of writing a "friendly
> > command line tool specifically for applying overlays" is the path that
> > Viresh should pursue?
>
> I think at this stage it's a matter of trying a few approaches and
> seeing what works out.

Another way would be applying overlays to dtc's live tree. This could
apply overlays from dts in addition to dtb. It could be a plug-in if
we ever get that finished up.

The downside of this is not testing libfdt's code and possible
differences between 2 implementations.

Rob