DKMS make.log for broadcom-wl-6.20.155.1 for kernel 3.8.1-1-ARCH (x86_64)
Mon  4 Mar 00:12:07 GMT 2013
make: Entering directory `/usr/src/linux-3.8.1-1-ARCH'
CFG80211 API is prefered for this kernel version
Using CFG80211 API
Building for x86_64
  LD      /var/lib/dkms/broadcom-wl/6.20.155.1/build/built-in.o
  CC [M]  /var/lib/dkms/broadcom-wl/6.20.155.1/build/src/shared/linux_osl.o
  CC [M]  /var/lib/dkms/broadcom-wl/6.20.155.1/build/src/wl/sys/wl_linux.o
  CC [M]  /var/lib/dkms/broadcom-wl/6.20.155.1/build/src/wl/sys/wl_iw.o
  CC [M]  /var/lib/dkms/broadcom-wl/6.20.155.1/build/src/wl/sys/wl_cfg80211.o
/var/lib/dkms/broadcom-wl/6.20.155.1/build/src/wl/sys/wl_cfg80211.c:1578:2: warning: initialization from incompatible pointer type [enabled by default]
/var/lib/dkms/broadcom-wl/6.20.155.1/build/src/wl/sys/wl_cfg80211.c:1578:2: warning: (near initialization for ‘wl_cfg80211_ops.set_tx_power’) [enabled by default]
/var/lib/dkms/broadcom-wl/6.20.155.1/build/src/wl/sys/wl_cfg80211.c:1579:2: warning: initialization from incompatible pointer type [enabled by default]
/var/lib/dkms/broadcom-wl/6.20.155.1/build/src/wl/sys/wl_cfg80211.c:1579:2: warning: (near initialization for ‘wl_cfg80211_ops.get_tx_power’) [enabled by default]
  LD [M]  /var/lib/dkms/broadcom-wl/6.20.155.1/build/wl.o
  Building modules, stage 2.
CFG80211 API is prefered for this kernel version
Using CFG80211 API
Building for x86_64
  MODPOST 1 modules
FATAL: modpost: GPL-incompatible module wl.ko uses GPL-only symbol '__rcu_read_unlock'
make[1]: *** [__modpost] Error 1
make: *** [modules] Error 2
make: Leaving directory `/usr/src/linux-3.8.1-1-ARCH'