Static Functions in Linux Device Driver

8/15/2019 Static Functions in Linux Device Driver

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Static functions in Linux device driver?Is there a reason why most function definition in device driver in linux code is definedas static? Is there a reason for this?

I was told this is for scoping and to prevent namespace pollution, could anyone

explain it in detail why static definition is used in this context?

ANS:-unctions declared static are not visi!le outside the translation unit they are

defined in "a translation unit is !asically a #c file$# If a function does not need to !e called

from outside the file, then it should !e made static so as to not pollute the glo!al

namespace# %his ma&es conflicts !etween names that are the same are less li&ely to

happen# 'xported sym!ols are usually indentified with some sort of su!system tag,

which further reduces scope for conflict#

(ften, pointers to these functions end up in structs, so they are actually called fromoutside the file they are defined in, !ut not !y their function name#

porting linux on !are !oard

)esterday I faced with an interview # In interview he as&ed me for steps to porting Linux

on new board# *y answer was,

+# cross-compile u-!oot for !oard architecture#

# cross-compile &ernel, with selecting driver for peripherals connected to !oard#

# cross-compile filesystem, and port all on !oard#.ut Interviewer is not happy with this answer#

ANS:-

/orting steps -

• Install a cross-development environment#

• Setup the !oard and ensure that the serial port is wor&ing so we can print data

through the serial port#

• 0ownload and install the Linux &ernel, most of the porting wor& will !e done at

this level#

• Add !oard specific code into the &ernel tree#

• .uild a &ernel image to run on the !oard

• %est that early &ernel print& is wor&ing

• 1et the real print& wor&ing with the serial console#

• or a new !oard, a new !oard-specific directory should !e added as well as

support for interrupt handling, &ernel timer services and mapping for memory areas#

• 'thernet drivers are usually the next drivers to focus on as they ena!le setup of

NS root file system to get access to user utilities and applications#

http://stackoverflow.com/questions/348984/static-functions-in-linux-device-driverhttp://stackoverflow.com/questions/9378379/porting-linux-on-bare-boardhttp://stackoverflow.com/questions/9378379/porting-linux-on-bare-boardhttp://stackoverflow.com/questions/348984/static-functions-in-linux-device-driver


2/4

• ilesystem can !e provided in different forms which are listed on Linuxilesystem

2ow to port a linux driver , which is compiled in #3

&ernel ,without compiling in other new version of &ernel

%han&s to every one,

%his is the 4uestion as&ed in one of the interview i faced#

I have a Linux device driver which was compiled in Linux &ernel version #3#I wouldli&e to port the same driver in a Linux /5 which has &ernel #6 without compiling in

new versions#

Is it possi!le ? If it is possi!le please let me &now how# If it is not possi!le please letme &now why not ?

now it7s not possi!le:

• usually, a 8driver8 is a !inary &ernel-module

• porting will involve code-changes to the &ernel module# if you change the code,

you need to compile it, in order to get a !inary#

• since &ernel modules run in &ernel space, it is crucial that they are ro!ust# since

parts of the &ernel-A/I change every now and then, trying to use a module compiled

for &ernel-6 with another &ernel-), might either not load !ecause of missing sym!ols

"if you are luc&y$ or lead to a &ernel panic !ecause semantics have changed#

• !tw, all this is not really related to 2.6.x vs 3.y, !ut holds true for any &ernel

version

!ut then: of course in theory it is possi!le to 8write8 a &ernel-module as !inary code inyour favourite hex-editor, without resorting to compilers and such# this would allow you to

8port8 a driver from one &ernel to another without recompilation# i guess this is not for

humans though##

No you cannot port module which is compiled for one version to other version#

%he reason is as follows

Modules are strongly tied to the data structures and function prototypes dened in a

particular kernel version; the interface seen by a module can change signicantly from

one kernel version to the next. This is especially true of development kernels of course

http://stackoverflow.com/questions/17830427/how-to-port-a-linux-driver-which-is-compiled-in-2-6-kernel-without-compilinghttp://stackoverflow.com/questions/17830427/how-to-port-a-linux-driver-which-is-compiled-in-2-6-kernel-without-compilinghttp://stackoverflow.com/questions/17830427/how-to-port-a-linux-driver-which-is-compiled-in-2-6-kernel-without-compilinghttp://stackoverflow.com/questions/17830427/how-to-port-a-linux-driver-which-is-compiled-in-2-6-kernel-without-compiling


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The kernel does not !ust assume that a given module has been built against the proper

kernel version. "ne of the steps in the build process is to link your module against a le

#called vermagic.o$ from the current kernel tree; this ob!ect contains a fair amount of

information about the kernel the module %as built for including the target kernel version

compiler version and the settings of a number of important conguration variables.

&hen an attempt is made to load a module this information can be tested for

compatibility %ith the running kernel. 'f things don(t match

the module is not loaded9 instead, you see something li&e:

) insmod hello.ko

*rror inserting +.,hello.ko+- / 'nvalid module format

A loo& in the system log file "varlogmessages or whatever your system is configured to

use$ will reveal the specific pro!lem that caused the module to fail to load#

;ernel interfaces often change !etween releases# If you are writing a module that is

intended to wor& with multiple versions of the &ernel "especially if it must wor& acrossmahat are coding conventions for using floating-point in

Linux device drivers?

Short answer: ;ernel code can use floating point if this use is surrounded

!y kernel0fpu0begin#$kernel0fpu0end#$# %hese function handle saving and restoring the

fpu context# Also, they call preempt0disable#$preempt0enable#$, which means no

sleeping, page faults etc# in the code !etween those functions# 1oogle the function

names for more information#

If I understand correctly, whenever a ;* is running, it is using a hardware context "or

hardware thread or register set -- whatever you want to call it$ that has !een preempted

from some application thread#

No, a &ernel module can run in user context as well "eg# when userspace calls syscallson a device provided !y the ;*$# It has, however, no relation to the float issue#

If you write your ;* in c, the compiler will correctly insure that the general-purpose

registers are properly saved and restored "much as in an application$, !ut that doesn7t

automatically happen with floating-point registers#

%hat is not !ecause of the compiler, !ut !ecause of the &ernel context-switching code#

*emory usage of a &ernel module

http://stackoverflow.com/questions/431081/what-are-coding-conventions-for-using-floating-point-in-linux-device-drivershttp://stackoverflow.com/questions/431081/what-are-coding-conventions-for-using-floating-point-in-linux-device-drivershttp://stackoverflow.com/questions/662526/memory-usage-of-a-kernel-modulehttp://stackoverflow.com/questions/431081/what-are-coding-conventions-for-using-floating-point-in-linux-device-drivershttp://stackoverflow.com/questions/431081/what-are-coding-conventions-for-using-floating-point-in-linux-device-drivershttp://stackoverflow.com/questions/662526/memory-usage-of-a-kernel-module


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>hile trying to estimate the amount of memory consumed !y a &ernel module "usually

device drivers$,I tried using the size utility which gave the sie of the static memory

areas of the #&o " #!ss, #data, #text etc$# So I was expecting the sum of these values to !e

exactly e4ual to the output given !y the lsmod command immediately after inserting the

module#

No dynamic memory allocation"&malloc or vmalloc$ is performed in the init"$ function to

ensure that it isn7t causing the difference#So why is there a mismatch?

5uriously the mismatch was found to !e a fixed amount most of the time@@

%he command outputs are listed !elow

sie chardev#&o

text data bss dec hex lename/12 /424/6 /42636 fa21c chardev.ko

lsmod

Module 5ie 7sed by Tainted- 8chardev /42944 4 :ive 4xc44d444

*ay !e the functions used !y the module are counted into the module sie ? %ry

cat ,proc,kallsyms < grep module0name

%he difference !etween the two sie is B# %ext C data C B D +B# *ay !e this is

some &ind of granularity pro!lem ? I don7t &now how the sie is calculated inside the&ernel###

2owever, &ernel code and data are allocated using dynamic memory# And &malloc usespre-allocated !loc& of memory, so it is 4uite li&ely that there is some rounding up whencode and data sections are allocated#

%ry to increment the sie of the data sections and see if the lsmod reported sie change

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Static Functions in Linux Device Driver