《Android Framework 之路》 N版本 Framework Camera的一些改动

###前言
Android N版本最近发布，Nougat是否好吃，不得而知，慢慢看下~
感谢AndroidXref这个网站，给开发者提供了大量的便捷~以后学习Android就靠它了。

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###N版本上Framework关于Camera的一些改动
CameraServer

N版本之前，CameraService是运行在MediaServer下的，这样存在一定的问题，因为MediaServer下同时还运行这其他的多媒体内容
M版本

/frameworks/av/media/mediaserver/main_mediaserver.cpp

int main(int argc __unused, char** argv)
{
    signal(SIGPIPE, SIG_IGN);
    char value[PROPERTY_VALUE_MAX];
    bool doLog = (property_get("ro.test_harness", value, "0") > 0) && (atoi(value) == 1);
    pid_t childPid;
    // FIXME The advantage of making the process containing media.log service the parent process of
    // the process that contains all the other real services, is that it allows us to collect more
    // detailed information such as signal numbers, stop and continue, resource usage, etc.
    // But it is also more complex.  Consider replacing this by independent processes, and using
    // binder on death notification instead.
    if (doLog && (childPid = fork()) != 0) {
        // media.log service
        //prctl(PR_SET_NAME, (unsigned long) "media.log", 0, 0, 0);
        // unfortunately ps ignores PR_SET_NAME for the main thread, so use this ugly hack
        strcpy(argv[0], "media.log");
        sp proc(ProcessState::self());
        MediaLogService::instantiate();
        ProcessState::self()->startThreadPool();
        for (;;) {
            siginfo_t info;
            int ret = waitid(P_PID, childPid, &info, WEXITED | WSTOPPED | WCONTINUED);
            if (ret == EINTR) {
                continue;
            }
            if (ret < 0) {
                break;
            }
            char buffer[32];
            const char *code;
            switch (info.si_code) {
            case CLD_EXITED:
                code = "CLD_EXITED";
                break;
            case CLD_KILLED:
                code = "CLD_KILLED";
                break;
            case CLD_DUMPED:
                code = "CLD_DUMPED";
                break;
            case CLD_STOPPED:
                code = "CLD_STOPPED";
                break;
            case CLD_TRAPPED:
                code = "CLD_TRAPPED";
                break;
            case CLD_CONTINUED:
                code = "CLD_CONTINUED";
                break;
            default:
                snprintf(buffer, sizeof(buffer), "unknown (%d)", info.si_code);
                code = buffer;
                break;
            }
            struct rusage usage;
            getrusage(RUSAGE_CHILDREN, &usage);
            ALOG(LOG_ERROR, "media.log", "pid %d status %d code %s user %ld.%03lds sys %ld.%03lds",
                    info.si_pid, info.si_status, code,
                    usage.ru_utime.tv_sec, usage.ru_utime.tv_usec / 1000,
                    usage.ru_stime.tv_sec, usage.ru_stime.tv_usec / 1000);
            sp sm = defaultServiceManager();
            sp binder = sm->getService(String16("media.log"));
            if (binder != 0) {
                Vector args;
                binder->dump(-1, args);
            }
            switch (info.si_code) {
            case CLD_EXITED:
            case CLD_KILLED:
            case CLD_DUMPED: {
                ALOG(LOG_INFO, "media.log", "exiting");
                _exit(0);
                // not reached
                }
            default:
                break;
            }
        }
    } else {
        // all other services
        if (doLog) {
            prctl(PR_SET_PDEATHSIG, SIGKILL);   // if parent media.log dies before me, kill me also
            setpgid(0, 0);                      // but if I die first, don't kill my parent
        }
        InitializeIcuOrDie();
        sp proc(ProcessState::self());
        sp sm = defaultServiceManager();
        ALOGI("ServiceManager: %p", sm.get());
        AudioFlinger::instantiate();
        MediaPlayerService::instantiate();
        ResourceManagerService::instantiate();
        CameraService::instantiate();
        AudioPolicyService::instantiate();
        SoundTriggerHwService::instantiate();
        RadioService::instantiate();
        registerExtensions();
        ProcessState::self()->startThreadPool();
        IPCThreadState::self()->joinThreadPool();
    }
}

可以看到AudioFlinger，MediaPlayerService, ResourceManagerService,CameraService,AudioPolicyService,SoundTriggerHwService,RadioService等都是运行在这个进程下面的，这样就存在一个问题，假如说其他任何一个服务因为某些原因挂掉了，整个MediaServer将会重启，而这个重启的过程中，倘若相机正在操作，就会出现相机突然挂掉的问题。

N版本
/frameworks/av/media/mediaserver/main_mediaserver.cpp

int main(int argc __unused, char **argv __unused)
{
    signal(SIGPIPE, SIG_IGN);

    sp proc(ProcessState::self());
    sp sm(defaultServiceManager());
    ALOGI("ServiceManager: %p", sm.get());
    InitializeIcuOrDie();
    MediaPlayerService::instantiate();
    ResourceManagerService::instantiate();
    registerExtensions();
    ProcessState::self()->startThreadPool();
    IPCThreadState::self()->joinThreadPool();
}

有没有觉得少了很多，不仅CameraService被抽离出去了，还有一些内容好像也被抽离出去了。
在N版本的工程中多出来一个文件夹
/frameworks/av/camera/cameraserver/

说明CameraServer是独立于MediaServer的单独的进程，执行PS操作你讲看到独立的Camera进程。

#define LOG_TAG "cameraserver"
//#define LOG_NDEBUG 0

// from LOCAL_C_INCLUDES
#include "CameraService.h"

using namespace android;

int main(int argc __unused, char** argv __unused)
{
    signal(SIGPIPE, SIG_IGN);

    sp proc(ProcessState::self());
    sp sm = defaultServiceManager();
    ALOGI("ServiceManager: %p", sm.get());
    CameraService::instantiate();
    ProcessState::self()->startThreadPool();
    IPCThreadState::self()->joinThreadPool();
}

初始化的过程并没有变化，只是单纯的将它独立出来而已。
那么问题来了，CameraService何时启动？
看到cameraserver.rc文件，

service cameraserver /system/bin/cameraserver
    class main
    user cameraserver
    group audio camera drmrpc inet media mediadrm net_bt net_bt_admin net_bw_acct
    ioprio rt 4
    writepid /dev/cpuset/foreground/tasks

也就是说只要加载了cameraserver.rc文件，服务就开始启动了，就会往main_cameraserer.cpp的入口函数中执行，因为mk文件中定义了这个模块为cameraserver

LOCAL_PATH:= $(call my-dir)

include $(CLEAR_VARS)

LOCAL_SRC_FILES:= \
	main_cameraserver.cpp//源文件

LOCAL_SHARED_LIBRARIES := \//依赖库
	libcameraservice \
	libcutils \
	libutils \
	libbinder \
	libcamera_client

LOCAL_MODULE:= cameraserver//模块名
LOCAL_32_BIT_ONLY := true//只支持32位

LOCAL_CFLAGS += -Wall -Wextra -Werror -Wno-unused-parameter//一些编译条件，N版本在编译这一块也作出了很大的改动，有机会再研究下

LOCAL_INIT_RC := cameraserver.rc

include $(BUILD_EXECUTABLE)//执行文件

寻找，谁调用的camearserver.rc ???
首先我们要看这个文件编译出来之后放在手机的什么位置
/build/core/base_rules.mk

# Rule to install the module's companion init.rc.
my_init_rc := $(LOCAL_INIT_RC_$(my_32_64_bit_suffix))
my_init_rc_src :=
my_init_rc_installed :=
ifndef my_init_rc
my_init_rc := $(LOCAL_INIT_RC)
# Make sure we don't define the rule twice in multilib module.
LOCAL_INIT_RC :=
endif
ifdef my_init_rc
my_init_rc_src := $(LOCAL_PATH)/$(my_init_rc)
my_init_rc_installed := $(TARGET_OUT$(partition_tag)_ETC)/init/$(notdir $(my_init_rc_src))
$(my_init_rc_installed) : $(my_init_rc_src) | $(ACP)
	@echo "Install: $@"
	$(copy-file-to-new-target)

$(my_register_name) : $(my_init_rc_installed)

大致可以看出一些端倪，应该是放在xxx/etc/init目录下，这个xxx根据什么看判断，partition_tag变量

  ifdef LOCAL_IS_HOST_MODULE
    partition_tag :=
  else
  ifeq (true,$(LOCAL_PROPRIETARY_MODULE))
    partition_tag := _VENDOR
  else ifeq (true,$(LOCAL_OEM_MODULE))
    partition_tag := _OEM
  else ifeq (true,$(LOCAL_ODM_MODULE))
    partition_tag := _ODM
  else
    # The definition of should-install-to-system will be different depending
    # on which goal (e.g., sdk or just droid) is being built.
    partition_tag := $(if $(call should-install-to-system,$(my_module_tags)),,_DATA)
  endif

cameraserver应该是系统所有的，不是oem或者odm特有，存放在system/etc/init目录下。
接下面看看这个文件
/system/core/init/readme.txt

One may specify paths in the mount_all command line to have it import
.rc files at the specified paths instead of the default ones listed above.
This is primarily for supporting factory mode and other non-standard boot
modes.  The three default paths should be used for the normal boot process.

The intention of these directories is as follows
   1) /system/etc/init/ is for core system items such as
      SurfaceFlinger, MediaService, and logcatd.
   2) /vendor/etc/init/ is for SoC vendor items such as actions or
      daemons needed for core SoC functionality.
   3) /odm/etc/init/ is for device manufacturer items such as
      actions or daemons needed for motion sensor or other peripheral
      functionality.

显然是在执行“mount_all“的时候，CameraServer启动 ~ ，找到启动的位置，这一点和之前的版本是有很大差别的。

---

Aidl
N版本上对于Camera的一些接口采用了Aidl的文件格式
frameworks/av/camera/aidl/android/hardware/

其他模块想要访问这些接口，需要include libcamera_client模块才行

mk文件中有说明。