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kobject: documentation: Fix erroneous example in kobject doc.

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Replace uio_mem example for kobjects with uio_map, since the uio_mem
struct no longer contains a kobject.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
This commit is contained in:
Robert P. J. Day 2010-03-11 07:59:09 -05:00 committed by Greg Kroah-Hartman
parent e59817bf08
commit 462bd295a3
1 changed files with 38 additions and 19 deletions
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57
Documentation/kobject.txt
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@ -59,37 +59,56 @@ nice to have in other objects. The C language does not allow for the
direct expression of inheritance, so other techniques - such as structure
embedding - must be used.
So, for example, the UIO code has a structure that defines the memory
region associated with a uio device:
(As an aside, for those familiar with the kernel linked list implementation,
this is analogous as to how "list_head" structs are rarely useful on
their own, but are invariably found embedded in the larger objects of
interest.)
struct uio_mem {
So, for example, the UIO code in drivers/uio/uio.c has a structure that
defines the memory region associated with a uio device:
struct uio_map {
struct kobject kobj;
unsigned long addr;
unsigned long size;
int memtype;
void __iomem *internal_addr;
};
struct uio_mem *mem;
};
If you have a struct uio_mem structure, finding its embedded kobject is
If you have a struct uio_map structure, finding its embedded kobject is
just a matter of using the kobj member. Code that works with kobjects will
often have the opposite problem, however: given a struct kobject pointer,
what is the pointer to the containing structure? You must avoid tricks
(such as assuming that the kobject is at the beginning of the structure)
and, instead, use the container_of() macro, found in <linux/kernel.h>:
container_of(pointer, type, member)
container_of(pointer, type, member)
where pointer is the pointer to the embedded kobject, type is the type of
the containing structure, and member is the name of the structure field to
which pointer points. The return value from container_of() is a pointer to
the given type. So, for example, a pointer "kp" to a struct kobject
embedded within a struct uio_mem could be converted to a pointer to the
containing uio_mem structure with:
where:
struct uio_mem *u_mem = container_of(kp, struct uio_mem, kobj);
* "pointer" is the pointer to the embedded kobject,
* "type" is the type of the containing structure, and
* "member" is the name of the structure field to which "pointer" points.
Programmers often define a simple macro for "back-casting" kobject pointers
to the containing type.
The return value from container_of() is a pointer to the corresponding
container type. So, for example, a pointer "kp" to a struct kobject
embedded *within* a struct uio_map could be converted to a pointer to the
*containing* uio_map structure with:
struct uio_map *u_map = container_of(kp, struct uio_map, kobj);
For convenience, programmers often define a simple macro for "back-casting"
kobject pointers to the containing type. Exactly this happens in the
earlier drivers/uio/uio.c, as you can see here:
struct uio_map {
struct kobject kobj;
struct uio_mem *mem;
};
#define to_map(map) container_of(map, struct uio_map, kobj)
where the macro argument "map" is a pointer to the struct kobject in
question. That macro is subsequently invoked with:
struct uio_map *map = to_map(kobj);
Initialization of kobjects