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Based on kernel version 3.16. Page generated on 2014-08-06 21:41 EST.

1	The Linux Kernel Driver Interface
2	(all of your questions answered and then some)
3	
4	Greg Kroah-Hartman <greg@kroah.com>
5	
6	This is being written to try to explain why Linux does not have a binary
7	kernel interface, nor does it have a stable kernel interface.  Please
8	realize that this article describes the _in kernel_ interfaces, not the
9	kernel to userspace interfaces.  The kernel to userspace interface is
10	the one that application programs use, the syscall interface.  That
11	interface is _very_ stable over time, and will not break.  I have old
12	programs that were built on a pre 0.9something kernel that still work
13	just fine on the latest 2.6 kernel release.  That interface is the one
14	that users and application programmers can count on being stable.
15	
16	
17	Executive Summary
18	-----------------
19	You think you want a stable kernel interface, but you really do not, and
20	you don't even know it.  What you want is a stable running driver, and
21	you get that only if your driver is in the main kernel tree.  You also
22	get lots of other good benefits if your driver is in the main kernel
23	tree, all of which has made Linux into such a strong, stable, and mature
24	operating system which is the reason you are using it in the first
25	place.
26	
27	
28	Intro
29	-----
30	
31	It's only the odd person who wants to write a kernel driver that needs
32	to worry about the in-kernel interfaces changing.  For the majority of
33	the world, they neither see this interface, nor do they care about it at
34	all.
35	
36	First off, I'm not going to address _any_ legal issues about closed
37	source, hidden source, binary blobs, source wrappers, or any other term
38	that describes kernel drivers that do not have their source code
39	released under the GPL.  Please consult a lawyer if you have any legal
40	questions, I'm a programmer and hence, I'm just going to be describing
41	the technical issues here (not to make light of the legal issues, they
42	are real, and you do need to be aware of them at all times.)
43	
44	So, there are two main topics here, binary kernel interfaces and stable
45	kernel source interfaces.  They both depend on each other, but we will
46	discuss the binary stuff first to get it out of the way.
47	
48	
49	Binary Kernel Interface
50	-----------------------
51	Assuming that we had a stable kernel source interface for the kernel, a
52	binary interface would naturally happen too, right?  Wrong.  Please
53	consider the following facts about the Linux kernel:
54	  - Depending on the version of the C compiler you use, different kernel
55	    data structures will contain different alignment of structures, and
56	    possibly include different functions in different ways (putting
57	    functions inline or not.)  The individual function organization
58	    isn't that important, but the different data structure padding is
59	    very important.
60	  - Depending on what kernel build options you select, a wide range of
61	    different things can be assumed by the kernel:
62	      - different structures can contain different fields
63	      - Some functions may not be implemented at all, (i.e. some locks
64		compile away to nothing for non-SMP builds.)
65	      - Memory within the kernel can be aligned in different ways,
66		depending on the build options.
67	  - Linux runs on a wide range of different processor architectures.
68	    There is no way that binary drivers from one architecture will run
69	    on another architecture properly.
70	
71	Now a number of these issues can be addressed by simply compiling your
72	module for the exact specific kernel configuration, using the same exact
73	C compiler that the kernel was built with.  This is sufficient if you
74	want to provide a module for a specific release version of a specific
75	Linux distribution.  But multiply that single build by the number of
76	different Linux distributions and the number of different supported
77	releases of the Linux distribution and you quickly have a nightmare of
78	different build options on different releases.  Also realize that each
79	Linux distribution release contains a number of different kernels, all
80	tuned to different hardware types (different processor types and
81	different options), so for even a single release you will need to create
82	multiple versions of your module.
83	
84	Trust me, you will go insane over time if you try to support this kind
85	of release, I learned this the hard way a long time ago...
86	
87	
88	Stable Kernel Source Interfaces
89	-------------------------------
90	
91	This is a much more "volatile" topic if you talk to people who try to
92	keep a Linux kernel driver that is not in the main kernel tree up to
93	date over time.
94	
95	Linux kernel development is continuous and at a rapid pace, never
96	stopping to slow down.  As such, the kernel developers find bugs in
97	current interfaces, or figure out a better way to do things.  If they do
98	that, they then fix the current interfaces to work better.  When they do
99	so, function names may change, structures may grow or shrink, and
100	function parameters may be reworked.  If this happens, all of the
101	instances of where this interface is used within the kernel are fixed up
102	at the same time, ensuring that everything continues to work properly.
103	
104	As a specific examples of this, the in-kernel USB interfaces have
105	undergone at least three different reworks over the lifetime of this
106	subsystem.  These reworks were done to address a number of different
107	issues:
108	  - A change from a synchronous model of data streams to an asynchronous
109	    one.  This reduced the complexity of a number of drivers and
110	    increased the throughput of all USB drivers such that we are now
111	    running almost all USB devices at their maximum speed possible.
112	  - A change was made in the way data packets were allocated from the
113	    USB core by USB drivers so that all drivers now needed to provide
114	    more information to the USB core to fix a number of documented
115	    deadlocks.
116	
117	This is in stark contrast to a number of closed source operating systems
118	which have had to maintain their older USB interfaces over time.  This
119	provides the ability for new developers to accidentally use the old
120	interfaces and do things in improper ways, causing the stability of the
121	operating system to suffer.
122	
123	In both of these instances, all developers agreed that these were
124	important changes that needed to be made, and they were made, with
125	relatively little pain.  If Linux had to ensure that it will preserve a
126	stable source interface, a new interface would have been created, and
127	the older, broken one would have had to be maintained over time, leading
128	to extra work for the USB developers.  Since all Linux USB developers do
129	their work on their own time, asking programmers to do extra work for no
130	gain, for free, is not a possibility.
131	
132	Security issues are also very important for Linux.  When a
133	security issue is found, it is fixed in a very short amount of time.  A
134	number of times this has caused internal kernel interfaces to be
135	reworked to prevent the security problem from occurring.  When this
136	happens, all drivers that use the interfaces were also fixed at the
137	same time, ensuring that the security problem was fixed and could not
138	come back at some future time accidentally.  If the internal interfaces
139	were not allowed to change, fixing this kind of security problem and
140	insuring that it could not happen again would not be possible.
141	
142	Kernel interfaces are cleaned up over time.  If there is no one using a
143	current interface, it is deleted.  This ensures that the kernel remains
144	as small as possible, and that all potential interfaces are tested as
145	well as they can be (unused interfaces are pretty much impossible to
146	test for validity.)
147	
148	
149	What to do
150	----------
151	
152	So, if you have a Linux kernel driver that is not in the main kernel
153	tree, what are you, a developer, supposed to do?  Releasing a binary
154	driver for every different kernel version for every distribution is a
155	nightmare, and trying to keep up with an ever changing kernel interface
156	is also a rough job.
157	
158	Simple, get your kernel driver into the main kernel tree (remember we
159	are talking about GPL released drivers here, if your code doesn't fall
160	under this category, good luck, you are on your own here, you leech
161	<insert link to leech comment from Andrew and Linus here>.)  If your
162	driver is in the tree, and a kernel interface changes, it will be fixed
163	up by the person who did the kernel change in the first place.  This
164	ensures that your driver is always buildable, and works over time, with
165	very little effort on your part.
166	
167	The very good side effects of having your driver in the main kernel tree
168	are:
169	  - The quality of the driver will rise as the maintenance costs (to the
170	    original developer) will decrease.
171	  - Other developers will add features to your driver.
172	  - Other people will find and fix bugs in your driver.
173	  - Other people will find tuning opportunities in your driver.
174	  - Other people will update the driver for you when external interface
175	    changes require it.
176	  - The driver automatically gets shipped in all Linux distributions
177	    without having to ask the distros to add it.
178	    
179	As Linux supports a larger number of different devices "out of the box"
180	than any other operating system, and it supports these devices on more
181	different processor architectures than any other operating system, this
182	proven type of development model must be doing something right :)
183	
184	
185	
186	------
187	
188	Thanks to Randy Dunlap, Andrew Morton, David Brownell, Hanna Linder,
189	Robert Love, and Nishanth Aravamudan for their review and comments on
190	early drafts of this paper.
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