Based on kernel version 4.9. Page generated on 2016-12-21 14:37 EST.
1 .. _stable_api_nonsense: 2 3 The Linux Kernel Driver Interface 4 ================================== 5 6 (all of your questions answered and then some) 7 8 Greg Kroah-Hartman <firstname.lastname@example.org> 9 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**. 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 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. 35 36 37 Intro 38 ----- 39 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. 44 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.) 52 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. 56 57 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: 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 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. 96 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... 99 100 101 Stable Kernel Source Interfaces 102 ------------------------------- 103 104 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. 107 108 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. 116 117 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: 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. 130 131 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. 136 137 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. 145 146 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. 155 156 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.) 161 162 163 What to do 164 ---------- 165 166 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. 171 172 Simple, get your kernel driver into the main kernel tree (remember we 173 are talking about GPL released drivers here, if your code doesn't fall 174 under 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 176 driver is in the tree, and a kernel interface changes, it will be fixed 177 up by the person who did the kernel change in the first place. This 178 ensures that your driver is always buildable, and works over time, with 179 very little effort on your part. 180 181 The very good side effects of having your driver in the main kernel tree 182 are: 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. 193 194 As Linux supports a larger number of different devices "out of the box" 195 than any other operating system, and it supports these devices on more 196 different processor architectures than any other operating system, this 197 proven type of development model must be doing something right :) 198 199 200 201 ------ 202 203 Thanks to Randy Dunlap, Andrew Morton, David Brownell, Hanna Linder, 204 Robert Love, and Nishanth Aravamudan for their review and comments on 205 early drafts of this paper.