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This increases the risk of external > module Makefiles attempting to write to the kernel directory. > > This commit switches the working directory to the external module > directory, allowing the removal of the $(KBUILD_EXTMOD)/ prefix from > some build artifacts. > > The command for building external modules maintains backward > compatibility, but Makefiles that rely on working in the kernel > directory may break. In such cases, $(objtree) and $(srctree) should > be used to refer to the output and source directories of the kernel. > > The appearance of the build log will change as follows: > > [Before] > > $ make -C /path/to/my/linux M=/path/to/my/externel/module > make: Entering directory '/path/to/my/linux' > CC [M] /path/to/my/externel/module/helloworld.o > MODPOST /path/to/my/externel/module/Module.symvers > CC [M] /path/to/my/externel/module/helloworld.mod.o > CC [M] /path/to/my/externel/module/.module-common.o > LD [M] /path/to/my/externel/module/helloworld.ko > make: Leaving directory '/path/to/my/linux' > > [After] > > $ make -C /path/to/my/linux M=/path/to/my/externel/module > make: Entering directory '/path/to/my/linux' > make[1]: Entering directory '/path/to/my/externel/module' > CC [M] helloworld.o > MODPOST Module.symvers > CC [M] helloworld.mod.o > CC [M] .module-common.o > LD [M] helloworld.ko > make[1]: Leaving directory '/path/to/my/externel/module' > make: Leaving directory '/path/to/my/linux' > > Printing "Entering directory" twice is cumbersome. This will be > addressed later. > > Signed-off-by: Masahiro Yamada Since this change I have been observing the following build error when building an external module ... MODPOST Module.symvers ERROR: modpost: drivers/gpu/host1x/host1x: 'host1x_device_init' exported twice. Previous export was in drivers/gpu/host1x/host1x.ko ERROR: modpost: drivers/gpu/host1x/host1x: 'host1x_device_exit' exported twice. Previous export was in drivers/gpu/host1x/host1x.ko Now host1x is an upstream driver, but I have a local copy that using to stage development changes (and avoid polluting the upstream driver). Plus I can swap between which version I am using on a live system. What I noticed is that previously the Modules.symvers for the external module had the full path of the external module for the name. However, now the name is just the relative path and in this case 'drivers/gpu/host1x/host1x'. Hence, this clashes with the in-kernel driver and we get the 'exported twice' error. I have been looking to see if there is a way to fix this because it has been a useful feature to override an upstream driver with a locally modified version. Thanks Jon -- nvpublic