I am attempting to add a linux kernel configuration fragment file (linux-1024MB-frags.config) into my build. Inside the fragment file I have set, CONFIG_HIGHMEM=y.
In my buildroot .config I have enabled,
BR2_LINUX_KERNEL_CONFIG_FRAGMENT_FILES="$(BR2_EXTERNAL_MYPATH)/board/common/linux-1024MB-frags.config"
When I do linux-dirclean && linux-configure I see the following output,
Merging /home/labuser/buildroot/trunk/board/common/linux-1024MB-frags.config
#
# merged configuration written to /home/labuser/buildroot/trunk/buildroot/output/build/linux-xilinx-v2017.1/.config (needs make)
#
Inspecting the new .config file shows that CONFIG_HIGHMEM is still not set.
If I set CONFIG_HIGHMEM in the main defconfig that I am using for the kernel (the one defined by BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE) and repeat the make steps I see that the CONFIG_HIGHMEM option IS set.