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

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Date:   Wed, 20 Jan 2021 10:47:40 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Frank Rowand <frowand.list@...il.com>,
        David Gibson <david@...son.dropbear.id.au>
Cc:     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

+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.
> 
> 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

-- 
viresh