diff --git a/scripts/build-aml-prebuilts.sh b/scripts/build-aml-prebuilts.sh
index eef6149..c60eaa1 100755
--- a/scripts/build-aml-prebuilts.sh
+++ b/scripts/build-aml-prebuilts.sh
@@ -1,5 +1,15 @@
 #!/bin/bash -e
 
+# This is a wrapper around "m" that builds the given modules in multi-arch mode
+# for all architectures supported by Mainline modules. The make (kati) stage is
+# skipped, so the build targets in the arguments can only be Soong modules or
+# intermediate output files - make targets and normal installed paths are not
+# supported.
+#
+# This script is typically used with "sdk" or "module_export" modules, which
+# Soong will install in $OUT_DIR/soong/mainline-sdks (cf
+# PathForMainlineSdksInstall in android/paths.go).
+
 export OUT_DIR=${OUT_DIR:-out}
 
 if [ -e ${OUT_DIR}/soong/.soong.in_make ]; then
@@ -8,11 +18,16 @@
   # expected to be supplied by the .mk files, and that might cause errors in
   # "m --skip-make" below. We therefore default to a different out dir
   # location in that case.
-  AML_OUT_DIR=out-aml
+  AML_OUT_DIR=out/aml
   echo "Avoiding in-make OUT_DIR '${OUT_DIR}' - building in '${AML_OUT_DIR}' instead"
   OUT_DIR=${AML_OUT_DIR}
 fi
 
+if [ ! -e "build/envsetup.sh" ]; then
+  echo "$0 must be run from the top of the tree"
+  exit 1
+fi
+
 source build/envsetup.sh
 
 my_get_build_var() {
@@ -22,13 +37,13 @@
   OUT_DIR=${OUT_DIR}/get_build_var get_build_var "$@"
 }
 
-PLATFORM_SDK_VERSION=$(my_get_build_var PLATFORM_SDK_VERSION)
-PLATFORM_VERSION=$(my_get_build_var PLATFORM_VERSION)
-PLATFORM_VERSION_ALL_CODENAMES=$(my_get_build_var PLATFORM_VERSION_ALL_CODENAMES)
+readonly PLATFORM_SDK_VERSION="$(my_get_build_var PLATFORM_SDK_VERSION)"
+readonly PLATFORM_VERSION="$(my_get_build_var PLATFORM_VERSION)"
+PLATFORM_VERSION_ALL_CODENAMES="$(my_get_build_var PLATFORM_VERSION_ALL_CODENAMES)"
 
 # PLATFORM_VERSION_ALL_CODENAMES is a comma separated list like O,P. We need to
 # turn this into ["O","P"].
-PLATFORM_VERSION_ALL_CODENAMES=${PLATFORM_VERSION_ALL_CODENAMES/,/'","'}
+PLATFORM_VERSION_ALL_CODENAMES="${PLATFORM_VERSION_ALL_CODENAMES/,/'","'}"
 PLATFORM_VERSION_ALL_CODENAMES="[\"${PLATFORM_VERSION_ALL_CODENAMES}\"]"
 
 # Logic from build/make/core/goma.mk
@@ -46,11 +61,16 @@
   USE_GOMA=false
 fi
 
-SOONG_OUT=${OUT_DIR}/soong
+readonly SOONG_OUT=${OUT_DIR}/soong
 mkdir -p ${SOONG_OUT}
-SOONG_VARS=${SOONG_OUT}/soong.variables
+readonly SOONG_VARS=${SOONG_OUT}/soong.variables
 
-# We enable bionic linux builds as ART also needs prebuilts for it.
+# Aml_abis: true
+#   -  This flag configures Soong to compile for all architectures required for
+#      Mainline modules.
+# CrossHost: linux_bionic
+# CrossHostArch: x86_64
+#   -  Enable Bionic on host as ART needs prebuilts for it.
 cat > ${SOONG_VARS}.new << EOF
 {
     "Platform_sdk_version": ${PLATFORM_SDK_VERSION},
@@ -79,4 +99,6 @@
 # We use force building LLVM components flag (even though we actually don't
 # compile them) because we don't have bionic host prebuilts
 # for them.
-FORCE_BUILD_LLVM_COMPONENTS=true m --skip-make "$@"
+export FORCE_BUILD_LLVM_COMPONENTS=true
+
+m --skip-make "$@"