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1 .. _stable_api_nonsense:
3 The Linux Kernel Driver Interface
4 ==================================
6 (all of your questions answered and then some)
8 Greg Kroah-Hartman <greg@kroah.com>
10 This is being written to try to explain why Linux **does not have a binary
11 kernel interface, nor does it have a stable kernel interface**.
13 .. note::
15 Please realize that this article describes the **in kernel** interfaces, not
16 the kernel to userspace interfaces.
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.
26 Executive Summary
27 -----------------
28 You think you want a stable kernel interface, but you really do not, and
29 you don't even know it. What you want is a stable running driver, and
30 you get that only if your driver is in the main kernel tree. You also
31 get lots of other good benefits if your driver is in the main kernel
32 tree, all of which has made Linux into such a strong, stable, and mature
33 operating system which is the reason you are using it in the first
34 place.
37 Intro
38 -----
40 It's only the odd person who wants to write a kernel driver that needs
41 to worry about the in-kernel interfaces changing. For the majority of
42 the world, they neither see this interface, nor do they care about it at
43 all.
45 First off, I'm not going to address **any** legal issues about closed
46 source, hidden source, binary blobs, source wrappers, or any other term
47 that describes kernel drivers that do not have their source code
48 released under the GPL. Please consult a lawyer if you have any legal
49 questions, I'm a programmer and hence, I'm just going to be describing
50 the technical issues here (not to make light of the legal issues, they
51 are real, and you do need to be aware of them at all times.)
53 So, there are two main topics here, binary kernel interfaces and stable
54 kernel source interfaces. They both depend on each other, but we will
55 discuss the binary stuff first to get it out of the way.
58 Binary Kernel Interface
59 -----------------------
60 Assuming that we had a stable kernel source interface for the kernel, a
61 binary interface would naturally happen too, right? Wrong. Please
62 consider the following facts about the Linux kernel:
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.
71 - Depending on what kernel build options you select, a wide range of
72 different things can be assumed by the kernel:
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.
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.
84 Now a number of these issues can be addressed by simply compiling your
85 module for the exact specific kernel configuration, using the same exact
86 C compiler that the kernel was built with. This is sufficient if you
87 want to provide a module for a specific release version of a specific
88 Linux distribution. But multiply that single build by the number of
89 different Linux distributions and the number of different supported
90 releases of the Linux distribution and you quickly have a nightmare of
91 different build options on different releases. Also realize that each
92 Linux distribution release contains a number of different kernels, all
93 tuned to different hardware types (different processor types and
94 different options), so for even a single release you will need to create
95 multiple versions of your module.
97 Trust me, you will go insane over time if you try to support this kind
98 of release, I learned this the hard way a long time ago...
101 Stable Kernel Source Interfaces
102 -------------------------------
104 This is a much more "volatile" topic if you talk to people who try to
105 keep a Linux kernel driver that is not in the main kernel tree up to
106 date over time.
108 Linux kernel development is continuous and at a rapid pace, never
109 stopping to slow down. As such, the kernel developers find bugs in
110 current interfaces, or figure out a better way to do things. If they do
111 that, they then fix the current interfaces to work better. When they do
112 so, function names may change, structures may grow or shrink, and
113 function parameters may be reworked. If this happens, all of the
114 instances of where this interface is used within the kernel are fixed up
115 at the same time, ensuring that everything continues to work properly.
117 As a specific examples of this, the in-kernel USB interfaces have
118 undergone at least three different reworks over the lifetime of this
119 subsystem. These reworks were done to address a number of different
120 issues:
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.
131 This is in stark contrast to a number of closed source operating systems
132 which have had to maintain their older USB interfaces over time. This
133 provides the ability for new developers to accidentally use the old
134 interfaces and do things in improper ways, causing the stability of the
135 operating system to suffer.
137 In both of these instances, all developers agreed that these were
138 important changes that needed to be made, and they were made, with
139 relatively little pain. If Linux had to ensure that it will preserve a
140 stable source interface, a new interface would have been created, and
141 the older, broken one would have had to be maintained over time, leading
142 to extra work for the USB developers. Since all Linux USB developers do
143 their work on their own time, asking programmers to do extra work for no
144 gain, for free, is not a possibility.
146 Security issues are also very important for Linux. When a
147 security issue is found, it is fixed in a very short amount of time. A
148 number of times this has caused internal kernel interfaces to be
149 reworked to prevent the security problem from occurring. When this
150 happens, all drivers that use the interfaces were also fixed at the
151 same time, ensuring that the security problem was fixed and could not
152 come back at some future time accidentally. If the internal interfaces
153 were not allowed to change, fixing this kind of security problem and
154 insuring that it could not happen again would not be possible.
156 Kernel interfaces are cleaned up over time. If there is no one using a
157 current interface, it is deleted. This ensures that the kernel remains
158 as small as possible, and that all potential interfaces are tested as
159 well as they can be (unused interfaces are pretty much impossible to
160 test for validity.)
163 What to do
164 ----------
166 So, if you have a Linux kernel driver that is not in the main kernel
167 tree, what are you, a developer, supposed to do? Releasing a binary
168 driver for every different kernel version for every distribution is a
169 nightmare, and trying to keep up with an ever changing kernel interface
170 is also a rough job.
172 Simple, get your kernel driver into the main kernel tree (remember we are
173 talking about drivers released under a GPL-compatible license here, if your
174 code doesn't fall under this category, good luck, you are on your own here,
175 you leech). If your driver is in the tree, and a kernel interface changes,
176 it will be fixed up by the person who did the kernel change in the first
177 place. This ensures that your driver is always buildable, and works over
178 time, with very little effort on your part.
180 The very good side effects of having your driver in the main kernel tree
181 are:
183 - The quality of the driver will rise as the maintenance costs (to the
184 original developer) will decrease.
185 - Other developers will add features to your driver.
186 - Other people will find and fix bugs in your driver.
187 - Other people will find tuning opportunities in your driver.
188 - Other people will update the driver for you when external interface
189 changes require it.
190 - The driver automatically gets shipped in all Linux distributions
191 without having to ask the distros to add it.
193 As Linux supports a larger number of different devices "out of the box"
194 than any other operating system, and it supports these devices on more
195 different processor architectures than any other operating system, this
196 proven type of development model must be doing something right :)
200 ------
202 Thanks to Randy Dunlap, Andrew Morton, David Brownell, Hanna Linder,
203 Robert Love, and Nishanth Aravamudan for their review and comments on
204 early drafts of this paper.