Linux kernel driver support

Device Drivers¶

See the kerneldoc for the struct device_driver.

Allocation¶

Device drivers are statically allocated structures. Though there may be multiple devices in a system that a driver supports, struct device_driver represents the driver as a whole (not a particular device instance).

Initialization¶

The driver must initialize at least the name and bus fields. It should also initialize the devclass field (when it arrives), so it may obtain the proper linkage internally. It should also initialize as many of the callbacks as possible, though each is optional.

Declaration¶

As stated above, struct device_driver objects are statically allocated. Below is an example declaration of the eepro100 driver. This declaration is hypothetical only; it relies on the driver being converted completely to the new model:

Most drivers will not be able to be converted completely to the new model because the bus they belong to has a bus-specific structure with bus-specific fields that cannot be generalized.

The most common example of this are device ID structures. A driver typically defines an array of device IDs that it supports. The format of these structures and the semantics for comparing device IDs are completely bus-specific. Defining them as bus-specific entities would sacrifice type-safety, so we keep bus-specific structures around.

Bus-specific drivers should include a generic struct device_driver in the definition of the bus-specific driver. Like this:

A definition that included bus-specific fields would look like (using the eepro100 driver again):

Some may find the syntax of embedded struct initialization awkward or even a bit ugly. So far, it’s the best way we’ve found to do what we want…

Registration¶

The driver registers the structure on startup. For drivers that have no bus-specific fields (i.e. don’t have a bus-specific driver structure), they would use driver_register and pass a pointer to their struct device_driver object.

Most drivers, however, will have a bus-specific structure and will need to register with the bus using something like pci_driver_register.

It is important that drivers register their driver structure as early as possible. Registration with the core initializes several fields in the struct device_driver object, including the reference count and the lock. These fields are assumed to be valid at all times and may be used by the device model core or the bus driver.

Transition Bus Drivers¶

By defining wrapper functions, the transition to the new model can be made easier. Drivers can ignore the generic structure altogether and let the bus wrapper fill in the fields. For the callbacks, the bus can define generic callbacks that forward the call to the bus-specific callbacks of the drivers.

This solution is intended to be only temporary. In order to get class information in the driver, the drivers must be modified anyway. Since converting drivers to the new model should reduce some infrastructural complexity and code size, it is recommended that they are converted as class information is added.

Access¶

Once the object has been registered, it may access the common fields of the object, like the lock and the list of devices:

The devices field is a list of all the devices that have been bound to the driver. The LDM core provides a helper function to operate on all the devices a driver controls. This helper locks the driver on each node access, and does proper reference counting on each device as it accesses it.

sysfs¶

When a driver is registered, a sysfs directory is created in its bus’s directory. In this directory, the driver can export an interface to userspace to control operation of the driver on a global basis; e.g. toggling debugging output in the driver.

A future feature of this directory will be a ‘devices’ directory. This directory will contain symlinks to the directories of devices it supports.

Callbacks¶

The probe() entry is called in task context, with the bus’s rwsem locked and the driver partially bound to the device. Drivers commonly use container_of() to convert “dev” to a bus-specific type, both in probe() and other routines. That type often provides device resource data, such as pci_dev.resource[] or platform_device.resources, which is used in addition to dev->platform_data to initialize the driver.

This callback holds the driver-specific logic to bind the driver to a given device. That includes verifying that the device is present, that it’s a version the driver can handle, that driver data structures can be allocated and initialized, and that any hardware can be initialized. Drivers often store a pointer to their state with dev_set_drvdata(). When the driver has successfully bound itself to that device, then probe() returns zero and the driver model code will finish its part of binding the driver to that device.

A driver’s probe() may return a negative errno value to indicate that the driver did not bind to this device, in which case it should have released all resources it allocated.

Optionally, probe() may return -EPROBE_DEFER if the driver depends on resources that are not yet available (e.g., supplied by a driver that hasn’t initialized yet). The driver core will put the device onto the deferred probe list and will try to call it again later. If a driver must defer, it should return -EPROBE_DEFER as early as possible to reduce the amount of time spent on setup work that will need to be unwound and reexecuted at a later time.

-EPROBE_DEFER must not be returned if probe() has already created child devices, even if those child devices are removed again in a cleanup path. If -EPROBE_DEFER is returned after a child device has been registered, it may result in an infinite loop of .probe() calls to the same driver.

sync_state is called only once for a device. It’s called when all the consumer devices of the device have successfully probed. The list of consumers of the device is obtained by looking at the device links connecting that device to its consumer devices.

The first attempt to call sync_state() is made during late_initcall_sync() to give firmware and drivers time to link devices to each other. During the first attempt at calling sync_state(), if all the consumers of the device at that point in time have already probed successfully, sync_state() is called right away. If there are no consumers of the device during the first attempt, that too is considered as “all consumers of the device have probed” and sync_state() is called right away.

If during the first attempt at calling sync_state() for a device, there are still consumers that haven’t probed successfully, the sync_state() call is postponed and reattempted in the future only when one or more consumers of the device probe successfully. If during the reattempt, the driver core finds that there are one or more consumers of the device that haven’t probed yet, then sync_state() call is postponed again.

A typical use case for sync_state() is to have the kernel cleanly take over management of devices from the bootloader. For example, if a device is left on and at a particular hardware configuration by the bootloader, the device’s driver might need to keep the device in the boot configuration until all the consumers of the device have probed. Once all the consumers of the device have probed, the device’s driver can synchronize the hardware state of the device to match the aggregated software state requested by all the consumers. Hence the name sync_state().

While obvious examples of resources that can benefit from sync_state() include resources such as regulator, sync_state() can also be useful for complex resources like IOMMUs. For example, IOMMUs with multiple consumers (devices whose addresses are remapped by the IOMMU) might need to keep their mappings fixed at (or additive to) the boot configuration until all its consumers have probed.

While the typical use case for sync_state() is to have the kernel cleanly take over management of devices from the bootloader, the usage of sync_state() is not restricted to that. Use it whenever it makes sense to take an action after all the consumers of a device have probed:

remove is called to unbind a driver from a device. This may be called if a device is physically removed from the system, if the driver module is being unloaded, during a reboot sequence, or in other cases.

It is up to the driver to determine if the device is present or not. It should free any resources allocated specifically for the device; i.e. anything in the device’s driver_data field.

If the device is still present, it should quiesce the device and place it into a supported low-power state.

suspend is called to put the device in a low power state.

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The Linux Kernel Driver Interface¶

(all of your questions answered and then some)

This is being written to try to explain why Linux does not have a binary kernel interface, nor does it have a stable kernel interface.

Please realize that this article describes the in kernel interfaces, not the kernel to userspace interfaces.

The kernel to userspace interface is the one that application programs use, the syscall interface. That interface is very stable over time, and will not break. I have old programs that were built on a pre 0.9something kernel that still work just fine on the latest 2.6 kernel release. That interface is the one that users and application programmers can count on being stable.

Executive Summary¶

You think you want a stable kernel interface, but you really do not, and you don’t even know it. What you want is a stable running driver, and you get that only if your driver is in the main kernel tree. You also get lots of other good benefits if your driver is in the main kernel tree, all of which has made Linux into such a strong, stable, and mature operating system which is the reason you are using it in the first place.

Intro¶

It’s only the odd person who wants to write a kernel driver that needs to worry about the in-kernel interfaces changing. For the majority of the world, they neither see this interface, nor do they care about it at all.

First off, I’m not going to address any legal issues about closed source, hidden source, binary blobs, source wrappers, or any other term that describes kernel drivers that do not have their source code released under the GPL. Please consult a lawyer if you have any legal questions, I’m a programmer and hence, I’m just going to be describing the technical issues here (not to make light of the legal issues, they are real, and you do need to be aware of them at all times.)

So, there are two main topics here, binary kernel interfaces and stable kernel source interfaces. They both depend on each other, but we will discuss the binary stuff first to get it out of the way.

Binary Kernel Interface¶

Assuming that we had a stable kernel source interface for the kernel, a binary interface would naturally happen too, right? Wrong. Please consider the following facts about the Linux kernel:

Depending on the version of the C compiler you use, different kernel data structures will contain different alignment of structures, and possibly include different functions in different ways (putting functions inline or not.) The individual function organization isn’t that important, but the different data structure padding is very important.

Depending on what kernel build options you select, a wide range of different things can be assumed by the kernel:

different structures can contain different fields

Some functions may not be implemented at all, (i.e. some locks compile away to nothing for non-SMP builds.)

Memory within the kernel can be aligned in different ways, depending on the build options.

Linux runs on a wide range of different processor architectures. There is no way that binary drivers from one architecture will run on another architecture properly.

Now a number of these issues can be addressed by simply compiling your module for the exact specific kernel configuration, using the same exact C compiler that the kernel was built with. This is sufficient if you want to provide a module for a specific release version of a specific Linux distribution. But multiply that single build by the number of different Linux distributions and the number of different supported releases of the Linux distribution and you quickly have a nightmare of different build options on different releases. Also realize that each Linux distribution release contains a number of different kernels, all tuned to different hardware types (different processor types and different options), so for even a single release you will need to create multiple versions of your module.

Trust me, you will go insane over time if you try to support this kind of release, I learned this the hard way a long time ago…

Stable Kernel Source Interfaces¶

This is a much more “volatile” topic if you talk to people who try to keep a Linux kernel driver that is not in the main kernel tree up to date over time.

Linux kernel development is continuous and at a rapid pace, never stopping to slow down. As such, the kernel developers find bugs in current interfaces, or figure out a better way to do things. If they do that, they then fix the current interfaces to work better. When they do so, function names may change, structures may grow or shrink, and function parameters may be reworked. If this happens, all of the instances of where this interface is used within the kernel are fixed up at the same time, ensuring that everything continues to work properly.

As a specific examples of this, the in-kernel USB interfaces have undergone at least three different reworks over the lifetime of this subsystem. These reworks were done to address a number of different issues:

A change from a synchronous model of data streams to an asynchronous one. This reduced the complexity of a number of drivers and increased the throughput of all USB drivers such that we are now running almost all USB devices at their maximum speed possible.

A change was made in the way data packets were allocated from the USB core by USB drivers so that all drivers now needed to provide more information to the USB core to fix a number of documented deadlocks.

This is in stark contrast to a number of closed source operating systems which have had to maintain their older USB interfaces over time. This provides the ability for new developers to accidentally use the old interfaces and do things in improper ways, causing the stability of the operating system to suffer.

In both of these instances, all developers agreed that these were important changes that needed to be made, and they were made, with relatively little pain. If Linux had to ensure that it will preserve a stable source interface, a new interface would have been created, and the older, broken one would have had to be maintained over time, leading to extra work for the USB developers. Since all Linux USB developers do their work on their own time, asking programmers to do extra work for no gain, for free, is not a possibility.

Security issues are also very important for Linux. When a security issue is found, it is fixed in a very short amount of time. A number of times this has caused internal kernel interfaces to be reworked to prevent the security problem from occurring. When this happens, all drivers that use the interfaces were also fixed at the same time, ensuring that the security problem was fixed and could not come back at some future time accidentally. If the internal interfaces were not allowed to change, fixing this kind of security problem and insuring that it could not happen again would not be possible.

Kernel interfaces are cleaned up over time. If there is no one using a current interface, it is deleted. This ensures that the kernel remains as small as possible, and that all potential interfaces are tested as well as they can be (unused interfaces are pretty much impossible to test for validity.)

What to do¶

So, if you have a Linux kernel driver that is not in the main kernel tree, what are you, a developer, supposed to do? Releasing a binary driver for every different kernel version for every distribution is a nightmare, and trying to keep up with an ever changing kernel interface is also a rough job.

Simple, get your kernel driver into the main kernel tree (remember we are talking about drivers released under a GPL-compatible license here, if your code doesn’t fall under this category, good luck, you are on your own here, you leech). If your driver is in the tree, and a kernel interface changes, it will be fixed up by the person who did the kernel change in the first place. This ensures that your driver is always buildable, and works over time, with very little effort on your part.

The very good side effects of having your driver in the main kernel tree are:

The quality of the driver will rise as the maintenance costs (to the original developer) will decrease.

Other developers will add features to your driver.

Other people will find and fix bugs in your driver.

Other people will find tuning opportunities in your driver.

Other people will update the driver for you when external interface changes require it.

The driver automatically gets shipped in all Linux distributions without having to ask the distros to add it.

As Linux supports a larger number of different devices “out of the box” than any other operating system, and it supports these devices on more different processor architectures than any other operating system, this proven type of development model must be doing something right 🙂

Thanks to Randy Dunlap, Andrew Morton, David Brownell, Hanna Linder, Robert Love, and Nishanth Aravamudan for their review and comments on early drafts of this paper.

© Copyright The kernel development community.

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