What is the rule to create .config at backports package?

Hi All,

My device is running with kernel 4.19.84 and I try to build backports-4.19.161-1 for mac80211 driver.

In .config of openwrt, we already enable

CONFIG_PACKAGE_kmod-cfg80211=y
# CONFIG_PACKAGE_kmod-lib80211 is not set
# CONFIG_PACKAGE_kmod-libertas-sdio is not set
# CONFIG_PACKAGE_kmod-libertas-spi is not set
# CONFIG_PACKAGE_kmod-libertas-usb is not set
CONFIG_PACKAGE_kmod-mac80211=y

I double check the Makefile of backports, looks like it depends on config of openwrt to create .config for back ports.

  config_package=$(if $(CONFIG_PACKAGE_kmod-$(1)),m)
  config-$(call config_package,cfg80211) += CFG80211
  
  config-$(call config_package,mac80211) += MAC80211
  config-$(CONFIG_PACKAGE_MAC80211_MESH) += MAC80211_MESH

When I build backports, I get this error

ERROR: module '/xxxxxxxxxx/xxxxxxxx/xxxxxxxx/backports-4.19.161-1/net/wireless/cfg80211.ko' is missing.

I double check the .config in build_dir/xxx/xxxx/backports-4.19.161-1, anything about mac80211 are disabled.

# CPTCFG_BACKPORTED_MAC80211 is not set      
# CPTCFG_BACKPORTED_MAC80211_HAS_RC is not set   
# CPTCFG_BACKPORTED_MAC80211_RC_MINSTREL is not set
# CPTCFG_BACKPORTED_MAC80211_RC_MINSTREL_HT is not set
# CPTCFG_BACKPORTED_MAC80211_RC_MINSTREL_VHT is not set
# CPTCFG_BACKPORTED_MAC80211_RC_DEFAULT_MINSTREL is not set
# CPTCFG_BACKPORTED_MAC80211_RC_DEFAULT is not set

How do I enable mac80211 correctly in backports? Please help. Thanks.