Android(安卓)kernel code merged into mainline

现在是Linux 3.3的merge window，android的kernel code终于被merge到mainline中了，记得这个问题在一年前就开始讨论android与mainline kernel的分化问题，毕竟android是桌面Linux使用最广泛的distribution版本，把android code merge到mainline中对Linux长期的发展也有好处，当然Google也不希望自己的kernel与mainline kernel越来越远（那样Google的kernel tree维护者会整天merge code，很繁琐）。

现在merge进的subsystem有：

ashmem：Anonymous Shared Memory Subsystem

config ASHMEM        bool "Enable the Anonymous Shared Memory Subsystem"        default n        depends on SHMEM || TINY_SHMEM        help          The ashmem subsystem is a new shared memory allocator, similar to          POSIX SHM but with different behavior and sporting a simpler          file-based API.

从ashmem代码中可以看出，ashmem的后台仍是通用的share memory

binder：Android Binder IPC Driver

config ANDROID_BINDER_IPC        bool "Android Binder IPC Driver"        default n

Binder在android中非常重要，基本上所有的跨进程通信（service间通信）都是用的binder机制。binder本质上是一种IPC，和fifo，msgqueue，shm一样运行在kernel空间。palm使用的也是openbinder。另一种mobile平台使用的IPC机制是DBUS，DBUS比binder更为人熟知，一般Fedora的机器都安装了dbus，而且时时运行着。

[root@localhost ipc]# ps aux|grep bus

dbus 1835 0.0 0.1 11212 716 ? Ssl Jan03 0:02 dbus-daemon --system

root 26552 0.0 0.1 4260 672 pts/1 S+ 05:00 0:00 grep bus

logger：Android log driver

这个subsystem很简单，就是在/dev目录下创建了四个文件：

#define LOGGER_LOG_RADIO        "log_radio"     /* radio-related messages */#define LOGGER_LOG_EVENTS       "log_events"    /* system/hardware events */#define LOGGER_LOG_SYSTEM       "log_system"    /* system/framework messages */#define LOGGER_LOG_MAIN         "log_main"      /* everything else */

和syslog的7个等级差不多，android的logger实现也很简单，log放在ringbuffer中，ringbuffer的默认大小是256K

lowmemorykiller：

config ANDROID_LOW_MEMORY_KILLER        bool "Android Low Memory Killer"        default N        ---help---          Register processes to be killed when memory is low

lowmemorykiller是android中常用的的特性，iphone中也有相同的机制，当内存不够时系统会杀掉一些进程，我以前一直以为android用的是Linux mm的oom_killer，后来想一想应该不是，用Linux mm的oom_killer杀掉进程时系统应该反应已经非常迟钝了，注重用户响应速度的android不会等到那时才杀进程。

lowmemorykiller的实现原理很简单（200LOC），向mm注册一个shrinker，当系统memory压力比较大时，kswapd会调用shrinker清理内存，用户可以配置一些memory threshold来控制lowmemorykiller的行为。

pmem，ram_console，timed_gpio，timed_output，swich这几个subsystem牵涉到硬件层次比较多，暂未深入了解。

Linux在3.3 merge window中还merge了GNU的mpilib库：

config MPILIB

        tristate "Multiprecision maths library"        help          Multiprecision maths library from GnuPG.          It is used to implement RSA digital signature verification,          which is used by IMA/EVM digital signature extension.config MPILIB_EXTRA        bool "Multiprecision maths library - additional sources"        depends on MPILIB        help          Multiprecision maths library from GnuPG.          It is used to implement RSA digital signature verification,          which is used by IMA/EVM digital signature extension.          This code in unnecessary for RSA digital signature verification,          and can be compiled if needed.config DIGSIG        tristate "In-kernel signature checker"        depends on KEYS        select MPILIB        help          Digital signature verification. Currently only RSA is supported.          Implementation is done using GnuPG MPI library

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