1The Linux Kernel Driver Interface 2(all of your questions answered and then some) 3 4Greg Kroah-Hartman <greg@kroah.com> 5 6This is being written to try to explain why Linux does not have a binary 7kernel interface, nor does it have a stable kernel interface. Please 8realize that this article describes the _in kernel_ interfaces, not the 9kernel to userspace interfaces. The kernel to userspace interface is 10the one that application programs use, the syscall interface. That 11interface is _very_ stable over time, and will not break. I have old 12programs that were built on a pre 0.9something kernel that still work 13just fine on the latest 2.6 kernel release. That interface is the one 14that users and application programmers can count on being stable. 15 16 17Executive Summary 18----------------- 19You think you want a stable kernel interface, but you really do not, and 20you don't even know it. What you want is a stable running driver, and 21you get that only if your driver is in the main kernel tree. You also 22get lots of other good benefits if your driver is in the main kernel 23tree, all of which has made Linux into such a strong, stable, and mature 24operating system which is the reason you are using it in the first 25place. 26 27 28Intro 29----- 30 31It's only the odd person who wants to write a kernel driver that needs 32to worry about the in-kernel interfaces changing. For the majority of 33the world, they neither see this interface, nor do they care about it at 34all. 35 36First off, I'm not going to address _any_ legal issues about closed 37source, hidden source, binary blobs, source wrappers, or any other term 38that describes kernel drivers that do not have their source code 39released under the GPL. Please consult a lawyer if you have any legal 40questions, I'm a programmer and hence, I'm just going to be describing 41the technical issues here (not to make light of the legal issues, they 42are real, and you do need to be aware of them at all times.) 43 44So, there are two main topics here, binary kernel interfaces and stable 45kernel source interfaces. They both depend on each other, but we will 46discuss the binary stuff first to get it out of the way. 47 48 49Binary Kernel Interface 50----------------------- 51Assuming that we had a stable kernel source interface for the kernel, a 52binary interface would naturally happen too, right? Wrong. Please 53consider 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 71Now a number of these issues can be addressed by simply compiling your 72module for the exact specific kernel configuration, using the same exact 73C compiler that the kernel was built with. This is sufficient if you 74want to provide a module for a specific release version of a specific 75Linux distribution. But multiply that single build by the number of 76different Linux distributions and the number of different supported 77releases of the Linux distribution and you quickly have a nightmare of 78different build options on different releases. Also realize that each 79Linux distribution release contains a number of different kernels, all 80tuned to different hardware types (different processor types and 81different options), so for even a single release you will need to create 82multiple versions of your module. 83 84Trust me, you will go insane over time if you try to support this kind 85of release, I learned this the hard way a long time ago... 86 87 88Stable Kernel Source Interfaces 89------------------------------- 90 91This is a much more "volatile" topic if you talk to people who try to 92keep a Linux kernel driver that is not in the main kernel tree up to 93date over time. 94 95Linux kernel development is continuous and at a rapid pace, never 96stopping to slow down. As such, the kernel developers find bugs in 97current interfaces, or figure out a better way to do things. If they do 98that, they then fix the current interfaces to work better. When they do 99so, function names may change, structures may grow or shrink, and 100function parameters may be reworked. If this happens, all of the 101instances of where this interface is used within the kernel are fixed up 102at the same time, ensuring that everything continues to work properly. 103 104As a specific examples of this, the in-kernel USB interfaces have 105undergone at least three different reworks over the lifetime of this 106subsystem. These reworks were done to address a number of different 107issues: 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 117This is in stark contrast to a number of closed source operating systems 118which have had to maintain their older USB interfaces over time. This 119provides the ability for new developers to accidentally use the old 120interfaces and do things in improper ways, causing the stability of the 121operating system to suffer. 122 123In both of these instances, all developers agreed that these were 124important changes that needed to be made, and they were made, with 125relatively little pain. If Linux had to ensure that it will preserve a 126stable source interface, a new interface would have been created, and 127the older, broken one would have had to be maintained over time, leading 128to extra work for the USB developers. Since all Linux USB developers do 129their work on their own time, asking programmers to do extra work for no 130gain, for free, is not a possibility. 131 132Security issues are also very important for Linux. When a 133security issue is found, it is fixed in a very short amount of time. A 134number of times this has caused internal kernel interfaces to be 135reworked to prevent the security problem from occurring. When this 136happens, all drivers that use the interfaces were also fixed at the 137same time, ensuring that the security problem was fixed and could not 138come back at some future time accidentally. If the internal interfaces 139were not allowed to change, fixing this kind of security problem and 140insuring that it could not happen again would not be possible. 141 142Kernel interfaces are cleaned up over time. If there is no one using a 143current interface, it is deleted. This ensures that the kernel remains 144as small as possible, and that all potential interfaces are tested as 145well as they can be (unused interfaces are pretty much impossible to 146test for validity.) 147 148 149What to do 150---------- 151 152So, if you have a Linux kernel driver that is not in the main kernel 153tree, what are you, a developer, supposed to do? Releasing a binary 154driver for every different kernel version for every distribution is a 155nightmare, and trying to keep up with an ever changing kernel interface 156is also a rough job. 157 158Simple, get your kernel driver into the main kernel tree (remember we 159are talking about GPL released drivers here, if your code doesn't fall 160under 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 162driver is in the tree, and a kernel interface changes, it will be fixed 163up by the person who did the kernel change in the first place. This 164ensures that your driver is always buildable, and works over time, with 165very little effort on your part. 166 167The very good side effects of having your driver in the main kernel tree 168are: 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 179As Linux supports a larger number of different devices "out of the box" 180than any other operating system, and it supports these devices on more 181different processor architectures than any other operating system, this 182proven type of development model must be doing something right :) 183 184 185 186------ 187 188Thanks to Randy Dunlap, Andrew Morton, David Brownell, Hanna Linder, 189Robert Love, and Nishanth Aravamudan for their review and comments on 190early drafts of this paper. 191