On Sun, Jan 31, 2021 at 10:39 PM David Gibson <david@xxxxxxxxxxxxxxxxxxxxx> 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
