diff --git a/.travis.yml b/.travis.yml
index c873701..1a129c7 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,30 +1,31 @@
 sudo: required
 dist: trusty
+os: linux
 language: python
-python:
-  - "3.5"
-  - "3.6"
-
 cache: pip
+stage:
+  - docs
+  - core
 
-matrix:
-  fast_finish: true
+jobs:
   include:
-    - os: linux
+    - stage: docs
       env: TESTING=docs
-      language: python
       python: 3.6
+      before_install:
+        - sudo apt-get update -qq
+      install:
+        - sudo apt-get install cscope
       before_script:
         - cd docs
         - pip install sphinx
       script:
         - make check
         - make html
-    - os: linux
+    - &core-stage
+      stage: core
+      python: 3.5
       env: TESTING=Piko/RT
-      language: c
-      compiler: arm-none-eabi-gcc
-
       before_install:
         - sudo add-apt-repository -y ppa:team-gcc-arm-embedded/ppa
         - sudo apt-get update -qq
@@ -37,9 +38,12 @@ matrix:
         - sudo apt install genromfs
         - wget https://github.com/PikoRT/tools/raw/master/bin/x86_64-linux/qemu-system-arm
         - chmod 777 qemu-system-arm
-        - export PATH=$PATH:$PWD
+        - export PATH=$PWD:$PATH
       before_script:
         - arm-none-eabi-gcc --version
+        - python --version
       script:
         - make PLAT=stm32p103
         - make PLAT=stm32p103 check
+    - <<: *core-stage
+      python: 3.6
diff --git a/Makefile b/Makefile
index 41bb495..208da12 100644
--- a/Makefile
+++ b/Makefile
@@ -37,9 +37,9 @@ OBJS := $(sort $(OBJS))
 
 deps := $(OBJS:%.o=.%.o.d)
 
-.PHONY: all check clean distclean
+.PHONY: all check clean distclean check_cc
 
-all: $(CMSIS)/$(PLAT) $(NAME).lds $(NAME).bin
+all: check_cc $(CMSIS)/$(PLAT) $(NAME).lds $(NAME).bin
 
 # generic build rules
 include mk/flags.mk
@@ -48,9 +48,17 @@ include mk/cmsis.mk
 
 prebuild: $(CMSIS)/$(PLAT)
 
-check:
+check: check_cc
 	$(PYTHON) -m tests -p $(PLAT) --qemu $(QEMU_SYSTEM_ARM) --cc $(CC)
 
+check_cc:
+	@$(eval PYTHON_VERSION=$(shell echo `$(PYTHON) --version 2>&1 | grep -oE '[^ ]+$$'`))
+	@$(eval PYTHON_VERSION=$(shell echo $(PYTHON_VERSION) | awk -F "." '{print $$1$$2 0}'))
+	@if [ $(PYTHON_VERSION) -lt 350 ]; then \
+		echo "Error: Python version must >= 3.5, use PYTHON=/python/binary/path"; \
+		return 1;\
+	fi;
+
 clean:
 	find . -name "*.o" -type f -delete
 	find . -name "*.o.d" -type f -delete
diff --git a/mk/flags.mk b/mk/flags.mk
index 20a3a6d..4d138c2 100644
--- a/mk/flags.mk
+++ b/mk/flags.mk
@@ -6,8 +6,7 @@ OBJCOPY = $(CROSS_COMPILE)objcopy
 GDB = $(CROSS_COMPILE)gdb
 HOSTCC  = gcc
 WGET = wget
-# FIXME: check version >= 3.5
-PYTHON = python3
+PYTHON ?= python
 QEMU_SYSTEM_ARM ?= qemu-system-arm
 
 # FIXME: configurable via menuconfig or command line