Init Call Mechanism in the Linux Kernel

By Andrew Murray

Introduction

The Linux kernel has for a long time (at least since v2.1.23) contained a clever and well-optimised mechanism for calling initialisation code in drivers. It's clever because its functionality is largely abstracted from the driver developer, and it's well-optimised because after initialisation, memory containing the initialisation code is released. This article explores how this mechanism works.

Typical Usage

We'll start by seeing how driver developers make use of this functionality; the following code comes from linux-2.6.27.6/drivers/net/smc911x.c and is the driver for a common Ethernet chipset (smc911x).

2206: static int __init smc911xinit(void)
2207: {
2208:     return platform_driver_register(&smc911x_driver);
2209: }
...
2216: module_init(smc911x_init);

The smc911xinit function can be considered as the entry point into the driver; of particular interest is the __init macro and the static declaration. The __init macro is used to describe the function as only being required during initialisation time: once initialisation is performed, the kernel will remove this function and release its memory. The module_init macro is used to tell the kernel where the initialisation entry point to the module lives, i.e. what function to call at 'start of day'. In a typical driver, you will often see many functions marked with the __init macro - these are used for initialisation - and a single module_init declaration.

Even though we are expecting the kernel to call smc911x_init at 'start of day', we have marked it as static - but that's OK (later, we will see how the function is called). This is a particular strength of the init call mechanism: it reduces the amount of public symbols and reduces the coupling between driver modules and other parts of the kernel.

The optimisation provided by the init call mechanism also provides a means for recovering memory used by the initialisation data. Such data can be 'tagged' with the __initdata macro.

With the above code in place, at an appropriate time during start-up, the kernel will call the smc911xinit function, and once it has been executed its memory will be released. You can see this in the output from kernel (e.g. dmesg); for example, an x86 machine may print the following:

Freeing unused kernel memory: 386k freed

This line is telling us that 386k of memory that previously contained initialisation code and data has now been freed.

Under the Hood

OK - So we've seen how the mechanism is used. Now, let's take a closer look and see how it works under the hood. A quick 'grep' reveals that the __init macro is defined in include/linux/init.h:

43: #define __init      __section(.init.text) __cold

The __section and __cold macros are defined in the various include/linux/compiler*.h files:

compiler.h: 182: #define __section(S)  __attribue__ ((__section__(#S)))
compiler-gcc4.h: #define __cold        __attribue__ ((cold))

And when we expand it out we get:

#define __init __attribute__((__section__(".init.text"))) __attribute__ ((cold))

Thus, when the __init macro is used, a number of GCC attributes are added to the function declaration - in the case of a different compiler, the compiler.h file will ensure the macros expand out to whatever is necessary for the relevant compiler. The cold attribute is a relatively new GCC attribute and has existed since GCC4.3 - its purpose is to mark the function as one that is rarely used, which results in the compiler optimising the function for size instead of speed. What we are really interested here is the 'section' attribute. The __init macro uses this attribute to inform the compiler to put the text for this function in a special section named ".init.text". The purpose here is to put all initialisation functions in a single ELF section such that the entire section can be removed after initialisation has been performed.

So what does module_init do? Its exact functionality depends on whether the module in question is built-in or compiled as a loadable module. For the purposes of this article, we'll be looking at the built-in modules. Back to include/linux/init.h:

259: #define module_init(x) __initcall(x);
204: #define __initcall(fn) device_initcall(fn)
199: #define device_initcall __define_initcall("6", fn, 6)
169: #define __define_initcall(level, fn, id) \
170:            static initcall_t __initcall_##fn##id __used \
171:            __attribute__ ((__section__(".initcall" level ".init"))) = fn

Yet another load of macros that result in even more GCC attributes being defined!

#define module_init(x) static initcall_t __initcall_x6 __used \
                       __attribute__ ((__section(".initcall6.init"))) = x;

And for clarity, let's expand our module_init macro as used in our ethernet driver:

static initcall_t __initcall_smc911x_init6 __used \
                  __attribute__ ((__section(".initcall6.init"))) = smc911x_init;

As you can see, module_init in the context of a built-in driver results in declaring a function pointer with a unique name to our point of entry. In addition, the macro ensures the function pointer is located in a special section of the ELF - we'll see why shortly.

So at present, we have ensured that all our initialisation code and data is stored in the .init.text section, and that each module has a function pointer for its point of entry - which has a unique name and is also stored in a special section of the resulting ELF. In addition, during link time the include/asm-generic/vmlinux.lds.h and arch/*/kernel/vmlinux.lds.S scripts ensure that some labels/symbols surround the start and end of these sections. I.e. __early_initcall_end and __initcall_end mark the start and end of the function pointers and __init_begin and __init_end mark the start and end of the .init.text section.

Finally we are in a position to see how these functions get called and how they are eventually freed. The function do_initcalls in init/main.c is called during kernel startup. This is shown below:

749: static void __init do_initcalls(void)
750: {
751:      initcall_t *call;
752:
753:      for (call = __early_initcall_end; call < __initcall_end; call++)
754:           do_one_initcall(*call);
755:

The purpose of this loop is to execute each of the init functions as set up by the module_init macros. This is achieved with a simple 'for' loop and a function pointer. Initially, the function pointer is pointed to the label at the start of our function pointer's ELF section, and is incremented (by the size of a function pointer (sizeof(initcall_t *)) until the end of the ELF section is reached. For each step, the pointer is invoked and the init function is thus executed.

Once initialisation is complete, a function found in the architecture- specific code named free_initmem is used to release the memory pages taken up by the initialisation functions and data. The exact nature of the function varies between architectures.

Summary

In a nutshell, the kernel makes clever use of macros and GCC attributes to ensure that initialisation functions and pointers to them are stored in unique sections of the ELF. Initialisation code at kernel startup then iterates through these function pointers and executes them in turn. Finally, once all init code has been executed, the entire ELF section (.init.text) is freed for re-use! The best part of this mechanism is that the provided macros completely hide its underlying complexity, thus leaving more time for driver developers to focus on the job at hand.

Further Reading

The best way to fully understand parts of the Linux kernel is to browse the source code - and that's exactly how I wrote this article. I did, however, make extensive use of the Linux Cross Reference - this site and many like it allow you to explore the source code and easily find out where functions are called and defined.

For more information on GCC attributes, read the GCC online documentation - in particular see section 5.2.7 Declaring Attributes of Functions.

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Andrew Murray is an embedded systems engineer at MPC Data Limited - one of the UK's leading systems integrator. His day to day role fulfils his passion for learning and provides him with a wide range of experiences including embedded Linux such as driver and kernel development, embedded applications development and even Windows driver development. Working on a wide range of projects has allowed Andrew to explore a wide range of technologies such as the inter-workings of PCI Express and High Definition (HD) audio.

Prior to his employment, Andrew graduated in 2007 from the University of Wales, Aberystwyth with a Masters degree in Software Engineering (MEng). His final year dissertation involved the creation of a 'black-box' for a sail plane glider that would assist in the automated marking of aerobatic gliding competitions. Making use of MEMS sensors, barometers, magnetometers and GPS along with Kalman filtering - the device was able to successfully record not only position but orientation in an aerobatic environment.

Being a member of the Institute of Engineering and Technology, curator of https://www.embedded-bits.co.uk and (currently) a 'one-time' kernel contributor, Andrew continually tries to contribute to the community more and more whenever possible.