============= start
Only variables and functions that are used by another source should be
declared as global. Variables and functions that are only used by the
current source should be declared as static. Declaring a name as
static has several advantages :-
It makes it obvious that it is only used in the current source. No
need to hunt through the rest of the kernel to see if any other code
refers to the name.
The compiler will flag unused static variables, it cannot flag unused
global variables.
It greatly reduces the chance that two sources will have the same
global symbol. Otherwise the linker complains about multiply defined
symbols.
No need to define static names in header files, reducing the cross
source impact of a change.
To save space on rescue and install disks, modules may be stripped.
Static names can be removed (saving space), global ones cannot.
============= end
A note on that last point. At the moment it is not really safe to
strip modules. Some modules already define the insmod target variables
as static. Blindly stripping such a module deletes the static symbols
and insmod does not work very well.
With the addition of MODULE_PARM, it is now safe to define the insmod
variables and functions (including init_module and cleanup_module) as
static. Then do a selective strip of the module, discarding most
static names and keeping those required by insmod. A script to do this
is being tested at the moment.