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 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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