On Thu, Feb 04, 2021 at 08:25:23AM -0600, Rob Herring wrote: > 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. This is actually a really interesting idea, because in a sense dtc already *does* apply overlays. It's just that it effectively resolves as it is parsing, rather than realizing separate overlay objects then merging as a separate step. I would actually like to change that, so that it *does* explicitly produce a chain of overlays internally. That has advantages for the checking code, because some checks make sense to apply to individual overlay fragments, but some only make sense on a fully resolved tree. As a bonus, it could handle this use case. Unlike libfdt, dtc is a much more normal userspace program and adding extra verbose debugging is no realy problem. It probably is more work in the short term, though. > The downside of this is not testing libfdt's code and possible > differences between 2 implementations. That can be mitigated by having a bunch of examples in the testsuite where we cross compare fdtoverlay's output with dtc's. -- 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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