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

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Date:   Fri, 22 Jan 2021 17:34:55 +1100
From:   David Gibson <david@...son.dropbear.id.au>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Frank Rowand <frowand.list@...il.com>,
        Pantelis Antoniou <pantelis.antoniou@...sulko.com>,
        Rob Herring <robh+dt@...nel.org>, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kbuild@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Bill Mills <bill.mills@...aro.org>, anmar.oueja@...aro.org,
        Masahiro Yamada <masahiroy@...nel.org>
Subject: Re: [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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