Linux Internals Linux Internals & &

Device DriversDevice Drivers

BySateesh Kumar G

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Topics Topics Introduction to Kernel Introduction to Kernel Features of Linux KernelFeatures of Linux Kernel Understanding Linux KernelUnderstanding Linux Kernel Introduction to Device DriversIntroduction to Device Drivers Building and Running ModuleBuilding and Running Module Character DriversCharacter Drivers Concurrency and Race ConditionsConcurrency and Race Conditions Advance Char Driver OperationAdvance Char Driver Operation Blocking IOBlocking IO Async NotificationsAsync Notifications InterruptsInterrupts

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Topics cont..Topics cont.. Bottom halves and SoftirqsBottom halves and Softirqs Tasklets and Work QueuesTasklets and Work Queues Memory ManagementMemory Management Page Allocator Page Allocator Address Translation and Page TablesAddress Translation and Page Tables Portmap and memory map IOsPortmap and memory map IOs Block Device DriverBlock Device Driver PCI DriversPCI Drivers Network DriversNetwork Drivers

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1. Monolithic kernel. ( NOT DERIVED FROM UNIX, REWRITTEN FROM SCRATCH )

2. Statically compiled + support for dynamic extension through LKMs

3. Limited support for kernel threading. Kernel threads do not support user processes.

4. Multithreaded application support. Linux defines its own version of LWPs ( Light weight processes ) using a nonstandard system call clone, while all other unix versions use

kernel threads to implement LWPs.

5. 2.4.x version of the kernel is non-preemptive while running in privileged mode. The 2.6 +version allows pre-emption as an optional feature.

6. Multiprocessor support.

7. Support for several filesystems.

8. Unix STREAMS I/O is not part of the kernel. Separately maintained.

9. FREE / FAST / POWERFUL / COMPACT / RELIABLE / SUPPORTED

10. Version numbering X.Y.Z ( X => version. Y=even => stable else development. Z = stable / dev release ).

Eg: Linux 3.8.2

Kernel website : www.kernel.org

Linux features in a nutshell

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Monolithic kernel Micro KernelMonolithic kernel Micro Kernel

Characteristics that differ between the Linux kernel and other Unix variants:Characteristics that differ between the Linux kernel and other Unix variants:

Dynamic loading of kernel moduleDynamic loading of kernel module

PreemptivePreemptive

Symmetric multiprocessor (SMP) support.Symmetric multiprocessor (SMP) support.

Linux does not differentiate between threads and normal processes.Linux does not differentiate between threads and normal processes.

Linux provides an object-oriented device model with device classes,hotpluggable events and Linux provides an object-oriented device model with device classes,hotpluggable events and

user-space device file system(sysfs).user-space device file system(sysfs).

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Components:1. Process Management

2. Memory Management

3. File Systems

4. Device Management

5. Networking

Understanding The Linux Kernel

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Fundamental ConceptsDifference between a "Program" and a "Process"

A Program is a complete set of executable instructions on the hard disk.

A Process is created when the program is loaded into memory. A process space has DATA, STACK and INSTRUCTIONS. It has an ID ( Identification ) and exists as long as it is required to run.

The same program can be run (loaded) twice. There will be two processes (but only one program).

The kernel is responsible for :

a. creating and destroying processes.b. Inter Process Communications.

c. processes to communicate with the outside world (Input/Output ).

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A Process's View Of the Kernel

Kernel = Black Box"Provider Of Services"

My CPUMy Memory

My CPUMy Memory

My CPUMy Memory

CPU, Memory, Devices, NICs,...

Process-2Process-1 Process-3

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The Internal View Of the KernelProcess-2Process-1 Process-3

The Task Queue

Task-1 Task-2 Task-3 Task-1 Task-3 Task-1 Task-2 Task-3 Task-1

Process-2 over

Process-1 overProcess-3 over

Task - 1 Task - 2 Task - 3

Seen fromoutside

Seen from inside

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The Various Process States....

Waiting

Running

Ready

The task is active and running in the non-privileged User Mode.This state can only be exited via an interrupt or a system call.( System calls are a special case of interrupts ). In either case, the processor is switched to the privileged mode and the appropriate interrupt routine is activated.

The process is waiting for an external event. Only after this has occurred, will it continue its execution.

Two kinds of "Waiting Processes" Interruptible - processes can be interrupted by signals uninterruptible - waiting ONLY for hardware conditions

The process is competing for the processor, which is however occupied with another process at the present time. Its ready to execute and will be executed at the first available moment."runnable"

"blocked"

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The Various Process States

Running Interrupt

Interrupt Service Routine

Waiting

Running System Call

Return from System Call

Ready

Scheduler

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Difference between User Space & Kernel Space

Multi-user Operating Systems.

One user -> can execute several programs simultaneously.

Programs require access to hardware / resources.

Need for protection of these “resources”. O.S needs to provide this protection.

This can ONLY be achieved if the CPU cooperates with the O.S to enforce access.

CPU’s provide support by running at different “levels”.

Unix / Linux use two levels ( though CPUs map provide more…)

Under Linux, the kernel code executes at the highest level, whereas user code executes at the lowest level.

We usually refer to Kernel Space and User Space since in addition to processor privilege levels, each mode has its own memory mapping as well.

Linux transfers execution from user space to kernel space whenever an application issues a system call or whenever an application is suspended by a hardware interrupt. Triggering a 0x80 interrupt switches processor to higher level.

Kernel code executing a system call is working in the context of a process. It will be able to access data in the process's address space. Interrupt handling code however is asynchronous with respect to processes.

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Useful web sites:

1. www.kernel.org

2. www.lwn.net

3. www.tldp.org

4. www.freshmeat.net

5. www.kernelnewbies.org

6. www.linuxdevices.com

Also you can refer

Linux-Journal

Linux-Gazette

Embedded Linux Journal.© Sateesh KG