1.. _stable_api_nonsense:
2
3The Linux Kernel Driver Interface
4==================================
5
6(all of your questions answered and then some)
7
8Greg Kroah-Hartman <greg@kroah.com>
9
10This is being written to try to explain why Linux **does not have a binary
11kernel interface, nor does it have a stable kernel interface**.
12
13.. note::
14
15  Please realize that this article describes the **in kernel** interfaces, not
16  the kernel to userspace interfaces.
17
18  The kernel to userspace interface is the one that application programs use,
19  the syscall interface.  That interface is **very** stable over time, and
20  will not break.  I have old programs that were built on a pre 0.9something
21  kernel that still work just fine on the latest 2.6 kernel release.
22  That interface is the one that users and application programmers can count
23  on being stable.
24
25
26Executive Summary
27-----------------
28You think you want a stable kernel interface, but you really do not, and
29you don't even know it.  What you want is a stable running driver, and
30you get that only if your driver is in the main kernel tree.  You also
31get lots of other good benefits if your driver is in the main kernel
32tree, all of which has made Linux into such a strong, stable, and mature
33operating system which is the reason you are using it in the first
34place.
35
36
37Intro
38-----
39
40It's only the odd person who wants to write a kernel driver that needs
41to worry about the in-kernel interfaces changing.  For the majority of
42the world, they neither see this interface, nor do they care about it at
43all.
44
45First off, I'm not going to address **any** legal issues about closed
46source, hidden source, binary blobs, source wrappers, or any other term
47that describes kernel drivers that do not have their source code
48released under the GPL.  Please consult a lawyer if you have any legal
49questions, I'm a programmer and hence, I'm just going to be describing
50the technical issues here (not to make light of the legal issues, they
51are real, and you do need to be aware of them at all times.)
52
53So, there are two main topics here, binary kernel interfaces and stable
54kernel source interfaces.  They both depend on each other, but we will
55discuss the binary stuff first to get it out of the way.
56
57
58Binary Kernel Interface
59-----------------------
60Assuming that we had a stable kernel source interface for the kernel, a
61binary interface would naturally happen too, right?  Wrong.  Please
62consider the following facts about the Linux kernel:
63
64  - Depending on the version of the C compiler you use, different kernel
65    data structures will contain different alignment of structures, and
66    possibly include different functions in different ways (putting
67    functions inline or not.)  The individual function organization
68    isn't that important, but the different data structure padding is
69    very important.
70
71  - Depending on what kernel build options you select, a wide range of
72    different things can be assumed by the kernel:
73
74      - different structures can contain different fields
75      - Some functions may not be implemented at all, (i.e. some locks
76	compile away to nothing for non-SMP builds.)
77      - Memory within the kernel can be aligned in different ways,
78	depending on the build options.
79
80  - Linux runs on a wide range of different processor architectures.
81    There is no way that binary drivers from one architecture will run
82    on another architecture properly.
83
84Now a number of these issues can be addressed by simply compiling your
85module for the exact specific kernel configuration, using the same exact
86C compiler that the kernel was built with.  This is sufficient if you
87want to provide a module for a specific release version of a specific
88Linux distribution.  But multiply that single build by the number of
89different Linux distributions and the number of different supported
90releases of the Linux distribution and you quickly have a nightmare of
91different build options on different releases.  Also realize that each
92Linux distribution release contains a number of different kernels, all
93tuned to different hardware types (different processor types and
94different options), so for even a single release you will need to create
95multiple versions of your module.
96
97Trust me, you will go insane over time if you try to support this kind
98of release, I learned this the hard way a long time ago...
99
100
101Stable Kernel Source Interfaces
102-------------------------------
103
104This is a much more "volatile" topic if you talk to people who try to
105keep a Linux kernel driver that is not in the main kernel tree up to
106date over time.
107
108Linux kernel development is continuous and at a rapid pace, never
109stopping to slow down.  As such, the kernel developers find bugs in
110current interfaces, or figure out a better way to do things.  If they do
111that, they then fix the current interfaces to work better.  When they do
112so, function names may change, structures may grow or shrink, and
113function parameters may be reworked.  If this happens, all of the
114instances of where this interface is used within the kernel are fixed up
115at the same time, ensuring that everything continues to work properly.
116
117As a specific examples of this, the in-kernel USB interfaces have
118undergone at least three different reworks over the lifetime of this
119subsystem.  These reworks were done to address a number of different
120issues:
121
122  - A change from a synchronous model of data streams to an asynchronous
123    one.  This reduced the complexity of a number of drivers and
124    increased the throughput of all USB drivers such that we are now
125    running almost all USB devices at their maximum speed possible.
126  - A change was made in the way data packets were allocated from the
127    USB core by USB drivers so that all drivers now needed to provide
128    more information to the USB core to fix a number of documented
129    deadlocks.
130
131This is in stark contrast to a number of closed source operating systems
132which have had to maintain their older USB interfaces over time.  This
133provides the ability for new developers to accidentally use the old
134interfaces and do things in improper ways, causing the stability of the
135operating system to suffer.
136
137In both of these instances, all developers agreed that these were
138important changes that needed to be made, and they were made, with
139relatively little pain.  If Linux had to ensure that it will preserve a
140stable source interface, a new interface would have been created, and
141the older, broken one would have had to be maintained over time, leading
142to extra work for the USB developers.  Since all Linux USB developers do
143their work on their own time, asking programmers to do extra work for no
144gain, for free, is not a possibility.
145
146Security issues are also very important for Linux.  When a
147security issue is found, it is fixed in a very short amount of time.  A
148number of times this has caused internal kernel interfaces to be
149reworked to prevent the security problem from occurring.  When this
150happens, all drivers that use the interfaces were also fixed at the
151same time, ensuring that the security problem was fixed and could not
152come back at some future time accidentally.  If the internal interfaces
153were not allowed to change, fixing this kind of security problem and
154insuring that it could not happen again would not be possible.
155
156Kernel interfaces are cleaned up over time.  If there is no one using a
157current interface, it is deleted.  This ensures that the kernel remains
158as small as possible, and that all potential interfaces are tested as
159well as they can be (unused interfaces are pretty much impossible to
160test for validity.)
161
162
163What to do
164----------
165
166So, if you have a Linux kernel driver that is not in the main kernel
167tree, what are you, a developer, supposed to do?  Releasing a binary
168driver for every different kernel version for every distribution is a
169nightmare, and trying to keep up with an ever changing kernel interface
170is also a rough job.
171
172Simple, get your kernel driver into the main kernel tree (remember we
173are talking about GPL released drivers here, if your code doesn't fall
174under this category, good luck, you are on your own here, you leech
175<insert link to leech comment from Andrew and Linus here>.)  If your
176driver is in the tree, and a kernel interface changes, it will be fixed
177up by the person who did the kernel change in the first place.  This
178ensures that your driver is always buildable, and works over time, with
179very little effort on your part.
180
181The very good side effects of having your driver in the main kernel tree
182are:
183
184  - The quality of the driver will rise as the maintenance costs (to the
185    original developer) will decrease.
186  - Other developers will add features to your driver.
187  - Other people will find and fix bugs in your driver.
188  - Other people will find tuning opportunities in your driver.
189  - Other people will update the driver for you when external interface
190    changes require it.
191  - The driver automatically gets shipped in all Linux distributions
192    without having to ask the distros to add it.
193
194As Linux supports a larger number of different devices "out of the box"
195than any other operating system, and it supports these devices on more
196different processor architectures than any other operating system, this
197proven type of development model must be doing something right :)
198
199
200
201------
202
203Thanks to Randy Dunlap, Andrew Morton, David Brownell, Hanna Linder,
204Robert Love, and Nishanth Aravamudan for their review and comments on
205early drafts of this paper.
206