https://docs.kernel.org/index.html

Kernel headers should be copied into /usr/include to be used by libraries and compiled programs. These should be the headers glibc has been compiled against, and should be kept independent of the ones in the kernel source tree. New packages are compiled against these headers, otherwise, if packages are compiled against headers that do not match the headers glibc was compiled against, segfaults or other strange problems may appear. /usr/include headers should be what matches the library object files. By copying the headers instead of symlink them to /usr/src/<linux>/include, we make sure that they remain available if we upgrade our kernel.

====== Compiling the kernel ======

Useful in embedded systems, as only the strongly recommended options are set to Y:

  make help          # provides a lot of information about kernel config options

  make allnoconfig   # useful in embedded systems, setting the strongly recommended options to Y
  make clean         # remove most generated files but keep config
  make mrproper      # remove all generated files
  make defconfig     # gives us a new config
  make LSMOD=... localmodconfig   # uses the list of modules to create a configuration
  make distclean     # also remove the editor backup and patch reject files
  
  make oldcondif     # uses a provided .config as base
  
  scripts/diffconfig .config.old .config   # shows difference between both kernel configurations
  
  interesting option: CONFIG_WERROR
  
  make
  make LOADADDR=0x80008000 uImage
  make dtbs

[[https://github.com/a13xp0p0v/kconfig-hardened-check|Useful tool to check config security]]
  
====== Process State ======

  * TASK_RUNNING, either running or in a runqueue waiting to run. Only possible state for a process executing in user-space.
  * TASK_INTERRUPTIBLE, process blocked, waiting for some condition to exist.
  * TASK_UNINTERRUPTIBLE, like TASK_INTERRUPTIBLE but without responding to signals.
  * TASK_TRACED
  * TASK_STOPPED

====== Process Context ======

A program running in user-space executes a system call or triggers an exception, making the kernel to run in //process context//.

There is also //interrupt context//, mostly applied to when the system is executing an interrupt handler (no process is tied to interrupt handlers). Other examples are the execution of a softirq as well as initializing or running the scheduler.

The Kernel may also run **kernel threads**, standard processes that exist only in kernel-space, like management threads that maintain the buffer and page caches. These processes are scheduled like normal processes and are allowed to sleep.

====== Concurrency and shared resources ======
  * Interrupts
  * Kernel preemption, causing the kernel to switch from the execution of one system call to another
  * Multiprocessing

We need to keep as much local state as possible and use locking for the shared resources.

===== Locks =====
  * Mutex, the main kernel locking primitive. Usable in contexts where sleeping is allowed.
  * Semaphores.
  * Spinlocks, used with code that is not allowed to sleep (interrupt handlers) or that doesn't want to sleep (critical sections). In single-core processors, it is better not to use spinlocks, as they would waist resources slowing down the lock owner (timeslice schedulers) or cause a deadlock (non-timeslice schedulers).
  * Alternatives:   Lock-free algorithms like Read Copy Update (RCU) and atomic operations.

====== Debugging ======

ftp://82.96.64.7/pub/software/scm/git/docs/v1.5.0/howto/isolate-bugs-with-bisect.txt

http://www.reactivated.net/weblog/archives/2006/01/using-git-bisect-to-find-buggy-kernel-patches/

git bisect start -- [path]

to restrict bisection to commits that touch [path].

http://www.av8n.com/computer/htm/kernel-lockup.htm
====== Porting the kernel to ARM devices ======
[[http://www.digipedia.pl/usenet/thread/19011/33747/|ARM11 MPCore example]]

====== ARM specific files ======

  arch/arm/Makefile             has macros to detect machine name
           def-configs          includes the default config file
           kernel/debug.s       assembly level debugging
           lib
           boot/Makefile        lists start address from where kernel image has to be decompressed
           tools/mach-types     machine ID
           mm/proc-[CPU_TYPE]

  include/asm-arm               set up functions for a specific CPU
          asm/arch/uncompress.h output kernel decompression msgs to UART
                   hardware.h   IO map of HW architecture
                   io.h         HW specific macros
                   irq.h        base address of diferent IRQs
                   irqs.h       IRQ numbers
                   time.h       time interrupt handlers and macros
                   

====== Contributing ======

[[http://www.youtube.com/watch?v=LLBrBBImJt4|Write and Submit your first Linux kernel Patch]]

https://kernelnewbies.org/FirstKernelPatch

https://kernelnewbies.org/Outreachyfirstpatch?action=show&redirect=OPWfirstpatch

https://tapaswenipathak.wordpress.com/2014/10/11/tools-to-cleanup-linux-kernel/

====== Testing ======
https://kernelci.org/

====== Useful links ======

https://lkml.org/

http://www.freesoftwaremagazine.com/articles/drivers_linux

http://lwn.net/Kernel/LDD3/

http://sysplay.in/blog/linux-device-drivers/2013/02/linux-device-drivers-for-your-girl-friend/