Android Kernel Features

List of Kernel features unique to Android

Binder

• Binder - corba-like IPC
  • used instead of SysV IPC for interprocess communication
  • The Linux version of Binder was originally derived from a project by PalmSource to implement a CORBA-like message-passing or method invocation system. Documentation on that system is at: http://www.angryredplanet.com/~hackbod/openbinder/docs/html/index.html

ashmem

• ashmem - Android shared memory
  • implementation is in mm/ashmem.c

According to the Kconfig 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."

Apparently it better-supports low memory devices, because it can discard shared memory units under memory pressure.

To use this, programs open /dev/ashmem, use mmap() on it, and can perform one or more of the following ioctls:

• ASHMEM_SET_NAME
• ASHMEM_GET_NAME
• ASHMEM_SET_SIZE
• ASHMEM_GET_SIZE
• ASHMEM_SET_PROT_MASK
• ASHMEM_GET_PROT_MASK
• ASHMEM_PIN
• ASHMEM_UNPIN
• ASHMEM_GET_PIN_STATUS
• ASHMEM_PURGE_ALL_CACHES

pmem

• PMEM - Process memory allocator

logger

• logger - system logging facility
  • This is the kernel support for the 'logcat' command
  • The kernel driver for the serial devices for logging are in the source code drivers/android/logging.c
  • See Android logger for more information

wakelocks

• wakelock - used for power management
  • Holds machine awake on a per-event basis until wakelock is released
  • See Android Power Management for detailed information

oom handling

• oom handling modifications
  • lowmem notifications

alarm

• alarm

paranoid network security

• paranoid network security
  • See Android_Security#Paranoid_network-ing

timed gpio

Generic gpio is a mechanism to allow programs to access and manipulate gpio registers from user space.

"Timed gpio" appears to be a system to allow for similar gpio manipulation, but with an added capability to automatically set a value in a gpio after a specified timeout.

On my ADP1, there is a driver at:

I'm not sure who uses this, or how it is used, but this is a directory with the following items:

# cd /sys/bus/platform/drivers/timed-gpio
# ls -l
--w-------    1 0        0            4096 Nov 13 02:11 bind
lrwxrwxrwx    1 0        0               0 Nov 13 02:11 timed-gpio -> ../../../../devices/platform/timed-gpio
--w-------    1 0        0            4096 Nov 13 02:11 uevent
--w-------    1 0        0            4096 Nov 13 02:11 unbind

Also, there is a device at:

# cd /sys/devices/platform/timed-gpio
# ls -lR
.:
lrwxrwxrwx    1 0        0               0 Nov 13 01:34 driver -> ../../../bus/platform/drivers/timed-gpio
-r--r--r--    1 0        0            4096 Nov 13 01:34 modalias
drwxr-xr-x    2 0        0               0 Nov 13 01:34 power
lrwxrwxrwx    1 0        0               0 Nov 13 01:34 subsystem -> ../../../bus/platform
-rw-r--r--    1 0        0            4096 Nov 13 01:34 uevent

./power:
-rw-r--r--    1 0        0            4096 Nov 13 01:34 wakeup

Possibly this means that one of the GPIO pins on the ADP1 is tied to a power wakeup event???

Kernel configuration options

The file Documentation/android.txt has a list of required configuration options for a kernel to support an Android system.

Resources

• Peter McDermott's excellent description of his work to port Android to the Nokia N810.
  • See http://www.linuxfordevices.com/c/a/Linux-For-Devices-Articles/Porting-Android-to-a-new-device/
  • Also, see his annotated list of modified and added kernel files, at: http://www.linuxfordevices.com/files/misc/porting-android-to-a-new-device-p3.html

• Jollen Chen's excellent presentation on system-level Android features, including an overview of kernel features unique to Android: Note: Parts of the presentation are in Chinese
  • http://www.slideshare.net/jollen/android-os-porting-introduction