[翻译]Android Q: Compiling Kernels in-tree

Kernel fixes

关于在Android10上如何修改kernel的Makefile，可参考Sony Kernel: Update makefiles for Android Q and clang

Android 10 build相关改动

主要的变更点如下：

对$PATH变量的限制
弃用GCC4.9
移除ccache
PHONY 目标规则强制

更多信息可以参考， AOSP: Build System Changes for Android.mk Writers

PHONY targets

更多信息参考 .PHONY rule enforcement

真实的输出文件不能被定义为PHONY目标，否则会给出编译警告，如下

warning: PHONY target "out/.../foo" looks like a real file (contains a "/")

如果输出文件不是真正的PHONY目标，请移除相关属性定义，如下

-.PHONY: $(PRODUCT_OUT)/kernel
-$(PRODUCT_OUT)/kernel: $(KERNEL_BIN) | $(ACP)
-        cp $(KERNEL_BIN) $(PRODUCT_OUT)/kernel
+$(PRODUCT_OUT)/kernel: $(KERNEL_BIN)
+    $(ACP) $(KERNEL_BIN) $(PRODUCT_OUT)/kernel

当然，如果您的最终目标不是PHONY，中间就不可以依赖PHONY target,这是make的标准配置

Output directory restrictions

如果你依赖具有绝对文件名的构建目标，这回给你带来麻烦。

例如：

KERNEL_OUT: $(PWD)/$(OUT)/obj/KERNEL_OBJ
$(KERNEL_OUT)/drivers:
    make -C $(KERNEL_SRC) -O $(KERNEL_OUT) drivers

抛出如下警告
warning: writing to readonly directory: "/home/[...]/out/[...]/KERNEL_OBJ/drivers"

这是因为，编译输出目录$(KERNEL_OUT)/drivers不是从out/target/...开始，而是一个绝对路径；编译系统无法确定这个路径是否应该被允许。
使用一个相对路径KERNEL\_OUT作为make的目标，在make命令中使用真正的绝对路径KERNEL\_OUT\_ABS：

KERNEL_OUT: $(OUT)/obj/KERNEL_OBJ
KERNEL_OUT_ABS: $(PWD)/$(OUT)/obj/KERNEL_OBJ
$(KERNEL_OUT)/drivers:
    make -C $(KERNEL_SRC) -O $(KERNEL_OUT_ABS) drivers

GCC is broken for host compilation

Even with correct PATH supplied for cc1, the prebuilt GCC 4.9 still fails at compiling the host scripts:

 HOSTCC  scripts/basic/fixdep
  [...]/kernel/scripts/basic/fixdep.c:105:23:
    fatal error: sys/types.h: No such file or directory
     #include <sys/types.h>

It’s better to use clang for HOSTCC:

# Android.mk
CLANG_HOST_TOOLCHAIN := $(PWD)/prebuilts/clang/host/linux-x86/clang-r365631/bin
CLANG_HOSTCC := $(CLANG_HOST_TOOLCHAIN)/clang
[...]
KERNEL_CROSS_COMPILE += HOSTCC="$(CLANG_HOSTCC)"
[...]
my-target:
    make $(MAKE_FLAGS) -C $(KERNEL_SRC_ABS) O=$(KERNEL_OUT_ABS) \
        ARCH=$(KERNEL_ARCH) $(KERNEL_CROSS_COMPILE)

Makefile:

-HOSTCC       = gcc
-HOSTCXX      = g++
+HOSTCC       = $(HOSTCC)
+HOSTCXX      = $(HOSTCXX)

Absolute paths for invoking build tools

调用构建工具的绝对路径

CLANG_HOST_TOOLCHAIN := $(PWD)/prebuilts/clang/host/linux-x86/clang-r365631/bin
KERNEL_HOSTCC := $(CLANG_HOST_TOOLCHAIN)/clang
KERNEL_TOOLCHAIN := $(PWD)/prebuilts/gcc/linux-x86/aarch64/aarch64-linux-android-4.9/bin
KERNEL_TOOLCHAIN_PREFIX := aarch64-linux-android-
KERNEL_TOOLCHAIN_PATH := $(KERNEL_TOOLCHAIN)/$(KERNEL_TOOLCHAIN_PREFIX)
KERNEL_CROSS_COMPILE :=
KERNEL_CROSS_COMPILE += CC="$(CLANG_CC)"
KERNEL_CROSS_COMPILE += CLANG_TRIPLE="aarch64-linux-gnu"
KERNEL_CROSS_COMPILE += HOSTCC="$(KERNEL_HOSTCC)"
KERNEL_CROSS_COMPILE += CROSS_COMPILE="$(KERNEL_TOOLCHAIN_PATH)"
KERNEL_CROSS_COMPILE += CROSS_COMPILE_ARM32="$(KERNEL_TOOLCHAIN_32BITS_PATH)"

No relying on tools being in PATH

不要依赖PATH变量中的的工具

不可以使用$PATH中的make，perl，gcc等工具。更多信息可以参考Android Q Changes: PATH restrictions.

Perl is no longer allowed as a PATH tool:

# Android.mk
KERNEL_PERL := /usr/bin/perl
[...]
KERNEL_CROSS_COMPILE += PERL=$(KERNEL_PERL)