Android kernel and driver

From: http://gmier.com/books/understanding-android-system/chapter2-kernel

 

We say that android is based on Linux, and the most fundamental reason is that android uses the Linux kernel.

2.1 Android kernel features

The android kernel is based on the Linux 2.6 kernel (the latest version is 2.6.31). It is an enhanced kernel version. Apart from modifying some bugs, it provides device drivers for supporting Android platforms, including:

• Android Binder

A Driver Based on the openbinder framework, used to provide inter-process communication (IPC, inter-process communication) on the Android platform ).

The source code is in drivers/staging/Android/binder. c

• AndroidPower Management (PM)

A lightweight Android power management Driver Based on the standard Linux Power Management System has been optimized for embedded devices.

The source code is located

Kernel/power/earlysuspend. c
Kernel/power/consoleearlysuspend. c
Kernel/power/fbearlysuspend. c
Kernel/power/wakelock. c
Kernel/power/userwakelock. c

• Low Memory Manager (Low memory killer)

The out of memory mechanism is more flexible than the Linux Standard OOM (out of memory) mechanism. It can kill processes as needed to release the required memory.

The source code is in drivers/staging/Android/lowmemorykiller. c

• Anonymous shared memory (Ashmem)

Provides a large shared memory for processes and a mechanism for the kernel to reclaim and manage the memory.

The source code is located in mm/ashmem. C.

• Android pmem(Physical)

Pmem is used to provide continuous physical memory areas to the user space. DSPs and some devices can only work on continuous physical memory,

The source code is in drivers/MISC/pmem. c

• Android Logger

A lightweight Logging Device used to capture various logs of the android System

The source code is in drivers/staging/Android/logger. c

• Android alarm

A timer is provided to wake up a device from its sleep state, and a clock benchmark that runs even when the device is sleeping,

The source code is in drivers/RTC/alarm. c

• USB GadgetDriver

A Device Driver Based on the standard Linux USB Gadget driver framework. The Android USB driver is based on the gaeget framework,

The source code is in drivers/USB/gadget/

• Android Switch
• Android Ram Console

To provide debugging functions, Android allows you to write debugging log information into a device called the ram console, which is a ram-based buffer.

The source code is in drivers/staging/Android/ram_console.c.

• Android timed device

Provides timed Control for devices. Currently, vibrator and LED devices are supported.

The source code is in drivers/staging/Android/timed_output.c (timed_gpio.c ).

• Yaffs2File System

Android uses yaffs2 as the mtd nand flash file system

The source code is located in the FS/yaffs2/directory.

Yaffs2 is a fast and stable cross-platform embedded device file system for NAND and nor flash. Compared with other flash file systems, yaffs2 uses smaller memory to save its running status, therefore, it occupies a small amount of memory. The garbage collection of yaffs2 is very simple and fast, so it can achieve better performance. The performance of yaffs2 is particularly significant in the large-capacity NAND Flash, suitable for large-capacity flash storage.

2.2 Android Kernel configuration

Android is based on Linux. For a new device, we must first compile a kernel that supports Android. How can we make your kernel android? In addition to the driver mentioned above, you need to configure your kernel to support the Android platform. For details, refer to the goldfish Kernel configuration file-ARCH/ARM/configs/goldfish_defconfig.

In general, we will configure the android kernel based on a standard Kernel configuration option. You can select the android Kernel configuration option based on the specific hardware platform. refer to the following Android Kernel configuration list:

Table 2-1: Required Enabled)

Android_paranoid_network Ashmem Config_fb_mode_helpers Config_font_8x16 Config_font_8x8 Config_yaffs_short_names_in_ram Dab Earlysuspend FB Fb_cfb_copyarea Fb_cfb_fillrect Fb_cfb_imageb.pdf Fb_deferred_io Fb_tileblitting High_res_timers Inotify Inotify_user Input_evdev Input_gpio Input_misc Leds_class Leds_gpio Lock_kernel Lkogger Low_memory_killer Misc_devices New_leds No_hz Power_Supply Preempt Ramfs Rtc_class Rtc_lib Switch Switch_gpio Tmpfs Uid_stat Uid16 Usb_function Usb_function_adb User_wakelock Video_output_control Wakelock Yaffs_auto_yaffs2 Yaffs_fs Yaffs_yaffs1 Yaffs_yaffs2

Table 2-2: Required disabled)

Config_yaffs_disable_lazy_load Dnotify

Table 2-3: Recommended Enabled)

Android_pmem Android_ram_console Android_ram_lele_error_correction Schedstats Debug_preempt Debug_mutexes Debug_spinlock_sleep Debug_info Frame_pointer Cpu_freq Cpu_freq_table Cpu_freq_default_gov_ondemand Cpu_freq_gov_ondemand Crc_ccitt Embedded Input_touchscreen I2C I2c_boardinfo Log_buf_shift = 17 Serial_core Serial_core_console

After configuration, you can use toolchain to compile the kernel. It is relatively simple to compile the kernel. Take the emulator's kernel as an example:

-Git clone git: // android.kernel.org/kernel/common.git kernel-emulator
-CD kernel-emulator
-Git checkout original/Android-goldfish-2.6.29-B Android-goldfish-2.6.29
-Export arch = arm
-Export cross_compile = arm-Eabi-
-Export Path = <cross_compil_toolchain_path_dir>: $ path
-Make goldfish_defconfig
-Make

After compilation, the android emulator (goldfish) kernel is obtained.ARCH/ARM/boot/zimage