Kernel build error - Android Q&A, Help & Troubleshooting

Hi guys, I was following the kernel build guide when I hit an error:
Code:
CC init/version.o
LD init/built-in.o
drivers/built-in.o: In function `diag_process_apps_pkt':
/home/chris/android/kernel/android_kernel_sony_wukong/drivers/char/diag/diagfwd.c:1163: undefined reference to `msm_set_restart_mode'
drivers/built-in.o: In function `diag_dci_process_apps_pkt':
/home/chris/android/kernel/android_kernel_sony_wukong/drivers/char/diag/diag_dci.c:1719: undefined reference to `msm_set_restart_mode'
drivers/built-in.o: In function `set_battery_data':
/home/chris/android/kernel/android_kernel_sony_wukong/drivers/power/qpnp-bms.c:3980: undefined reference to `bms_batt_data'
/home/chris/android/kernel/android_kernel_sony_wukong/drivers/power/qpnp-bms.c:3980: undefined reference to `bms_batt_data_num'
drivers/built-in.o: In function `rm_high_pwr_cost_cpus':
/home/chris/android/kernel/android_kernel_sony_wukong/drivers/soc/qcom/msm_performance.c:1256: undefined reference to `get_cpu_pwr_stats'
Makefile:797: recipe for target 'vmlinux' failed
make: *** [vmlinux] Error 1
Does anyone know what to make of it? I was building the Xperia C3 stock kernel using Linaro GCC toolchain.

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Have you built it with the regular toolchain? The error suggests your sources are incomplete.

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Slight thread necro, but since it came up first when googling for this missing reference...
The undefined reference is a linker error.
The linker can't find the symbol anywhere in the objects it has built. In my case, it was caused by a missing kernel option. To solve it, I grep'ed through my C and C++ sources for the function, found a likely candidate for it, checked the Makefile in the folder, then the option that was missing from my kernel config.
The Android build environment provides a convenience "cgrep" alias/script; I also made myself an "mkgrep" one-liner script:
Code:
#!/bin/bash
( find . -type f -name "?akefile" ; find . -type f -name "*.mk" ; find . -type f -name Kbuild ; find . -type f -name "*config") | xargs grep -i -Hn -E -e "$1"