206 lines
9.3 KiB
Plaintext
206 lines
9.3 KiB
Plaintext
.. _stable_api_nonsense:
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The Linux Kernel Driver Interface
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==================================
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(all of your questions answered and then some)
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Greg Kroah-Hartman <greg@kroah.com>
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This is being written to try to explain why Linux **does not have a binary
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kernel interface, nor does it have a stable kernel interface**.
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.. note::
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Please realize that this article describes the **in kernel** interfaces, not
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the kernel to userspace interfaces.
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The kernel to userspace interface is the one that application programs use,
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the syscall interface. That interface is **very** stable over time, and
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will not break. I have old programs that were built on a pre 0.9something
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kernel that still work just fine on the latest 2.6 kernel release.
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That interface is the one that users and application programmers can count
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on being stable.
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Executive Summary
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-----------------
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You think you want a stable kernel interface, but you really do not, and
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you don't even know it. What you want is a stable running driver, and
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you get that only if your driver is in the main kernel tree. You also
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get lots of other good benefits if your driver is in the main kernel
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tree, all of which has made Linux into such a strong, stable, and mature
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operating system which is the reason you are using it in the first
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place.
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Intro
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-----
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It's only the odd person who wants to write a kernel driver that needs
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to worry about the in-kernel interfaces changing. For the majority of
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the world, they neither see this interface, nor do they care about it at
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all.
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First off, I'm not going to address **any** legal issues about closed
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source, hidden source, binary blobs, source wrappers, or any other term
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that describes kernel drivers that do not have their source code
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released under the GPL. Please consult a lawyer if you have any legal
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questions, I'm a programmer and hence, I'm just going to be describing
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the technical issues here (not to make light of the legal issues, they
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are real, and you do need to be aware of them at all times.)
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So, there are two main topics here, binary kernel interfaces and stable
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kernel source interfaces. They both depend on each other, but we will
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discuss the binary stuff first to get it out of the way.
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Binary Kernel Interface
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-----------------------
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Assuming that we had a stable kernel source interface for the kernel, a
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binary interface would naturally happen too, right? Wrong. Please
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consider the following facts about the Linux kernel:
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- Depending on the version of the C compiler you use, different kernel
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data structures will contain different alignment of structures, and
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possibly include different functions in different ways (putting
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functions inline or not.) The individual function organization
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isn't that important, but the different data structure padding is
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very important.
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- Depending on what kernel build options you select, a wide range of
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different things can be assumed by the kernel:
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- different structures can contain different fields
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- Some functions may not be implemented at all, (i.e. some locks
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compile away to nothing for non-SMP builds.)
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- Memory within the kernel can be aligned in different ways,
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depending on the build options.
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- Linux runs on a wide range of different processor architectures.
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There is no way that binary drivers from one architecture will run
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on another architecture properly.
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Now a number of these issues can be addressed by simply compiling your
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module for the exact specific kernel configuration, using the same exact
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C compiler that the kernel was built with. This is sufficient if you
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want to provide a module for a specific release version of a specific
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Linux distribution. But multiply that single build by the number of
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different Linux distributions and the number of different supported
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releases of the Linux distribution and you quickly have a nightmare of
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different build options on different releases. Also realize that each
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Linux distribution release contains a number of different kernels, all
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tuned to different hardware types (different processor types and
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different options), so for even a single release you will need to create
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multiple versions of your module.
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Trust me, you will go insane over time if you try to support this kind
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of release, I learned this the hard way a long time ago...
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Stable Kernel Source Interfaces
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-------------------------------
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This is a much more "volatile" topic if you talk to people who try to
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keep a Linux kernel driver that is not in the main kernel tree up to
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date over time.
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Linux kernel development is continuous and at a rapid pace, never
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stopping to slow down. As such, the kernel developers find bugs in
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current interfaces, or figure out a better way to do things. If they do
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that, they then fix the current interfaces to work better. When they do
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so, function names may change, structures may grow or shrink, and
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function parameters may be reworked. If this happens, all of the
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instances of where this interface is used within the kernel are fixed up
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at the same time, ensuring that everything continues to work properly.
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As a specific examples of this, the in-kernel USB interfaces have
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undergone at least three different reworks over the lifetime of this
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subsystem. These reworks were done to address a number of different
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issues:
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- A change from a synchronous model of data streams to an asynchronous
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one. This reduced the complexity of a number of drivers and
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increased the throughput of all USB drivers such that we are now
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running almost all USB devices at their maximum speed possible.
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- A change was made in the way data packets were allocated from the
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USB core by USB drivers so that all drivers now needed to provide
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more information to the USB core to fix a number of documented
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deadlocks.
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This is in stark contrast to a number of closed source operating systems
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which have had to maintain their older USB interfaces over time. This
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provides the ability for new developers to accidentally use the old
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interfaces and do things in improper ways, causing the stability of the
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operating system to suffer.
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In both of these instances, all developers agreed that these were
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important changes that needed to be made, and they were made, with
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relatively little pain. If Linux had to ensure that it will preserve a
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stable source interface, a new interface would have been created, and
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the older, broken one would have had to be maintained over time, leading
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to extra work for the USB developers. Since all Linux USB developers do
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their work on their own time, asking programmers to do extra work for no
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gain, for free, is not a possibility.
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Security issues are also very important for Linux. When a
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security issue is found, it is fixed in a very short amount of time. A
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number of times this has caused internal kernel interfaces to be
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reworked to prevent the security problem from occurring. When this
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happens, all drivers that use the interfaces were also fixed at the
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same time, ensuring that the security problem was fixed and could not
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come back at some future time accidentally. If the internal interfaces
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were not allowed to change, fixing this kind of security problem and
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insuring that it could not happen again would not be possible.
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Kernel interfaces are cleaned up over time. If there is no one using a
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current interface, it is deleted. This ensures that the kernel remains
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as small as possible, and that all potential interfaces are tested as
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well as they can be (unused interfaces are pretty much impossible to
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test for validity.)
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What to do
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----------
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So, if you have a Linux kernel driver that is not in the main kernel
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tree, what are you, a developer, supposed to do? Releasing a binary
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driver for every different kernel version for every distribution is a
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nightmare, and trying to keep up with an ever changing kernel interface
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is also a rough job.
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Simple, get your kernel driver into the main kernel tree (remember we
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are talking about GPL released drivers here, if your code doesn't fall
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under this category, good luck, you are on your own here, you leech
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<insert link to leech comment from Andrew and Linus here>.) If your
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driver is in the tree, and a kernel interface changes, it will be fixed
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up by the person who did the kernel change in the first place. This
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ensures that your driver is always buildable, and works over time, with
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very little effort on your part.
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The very good side effects of having your driver in the main kernel tree
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are:
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- The quality of the driver will rise as the maintenance costs (to the
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original developer) will decrease.
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- Other developers will add features to your driver.
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- Other people will find and fix bugs in your driver.
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- Other people will find tuning opportunities in your driver.
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- Other people will update the driver for you when external interface
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changes require it.
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- The driver automatically gets shipped in all Linux distributions
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without having to ask the distros to add it.
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As Linux supports a larger number of different devices "out of the box"
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than any other operating system, and it supports these devices on more
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different processor architectures than any other operating system, this
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proven type of development model must be doing something right :)
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------
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Thanks to Randy Dunlap, Andrew Morton, David Brownell, Hanna Linder,
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Robert Love, and Nishanth Aravamudan for their review and comments on
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early drafts of this paper.
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