Linux Devices And drivers VERIFIED

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Device drivers are statically allocated structures. Though there maybe multiple devices in a system that a driver supports, structdevice_driver represents the driver as a whole (not a particulardevice instance).

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

By defining wrapper functions, the transition to the new model can bemade easier. Drivers can ignore the generic structure altogether andlet the bus wrapper fill in the fields. For the callbacks, the bus candefine generic callbacks that forward the call to the bus-specificcallbacks of the drivers.

This solution is intended to be only temporary. In order to get classinformation in the driver, the drivers must be modified anyway. Sinceconverting drivers to the new model should reduce some infrastructuralcomplexity and code size, it is recommended that they are converted asclass information is added.

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

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

While obvious examples of resources that can benefit from sync_state() includeresources such as regulator, sync_state() can also be useful for complexresources like IOMMUs. For example, IOMMUs with multiple consumers (deviceswhose addresses are remapped by the IOMMU) might need to keep their mappingsfixed at (or additive to) the boot configuration until all its consumers haveprobed.

While the typical use case for sync_state() is to have the kernel cleanly takeover management of devices from the bootloader, the usage of sync_state() isnot restricted to that. Use it whenever it makes sense to take an action afterall the consumers of a device have probed:

Device drivers can export attributes via their sysfs directories.Drivers can declare attributes using a DRIVER_ATTR_RW and DRIVER_ATTR_ROmacro that works identically to the DEVICE_ATTR_RW and DEVICE_ATTR_ROmacros.

The device name is a string value of the name parameter. This string can pass the name of a module if it registers a single device. We use this string to identify a device in the /sys/devices file. Device file operations such as read, write, and save are processed by the function pointers stored within the file_operations structure. These functions are implemented by the module, and the pointer to the module structure identifying this module is also stored within the file_operations structure (more about this structure in the next section).

Our next step is writing a function for unregistering the device file. If a device file is successfully registered, the value of the device_file_major_number will not be 0. This value allows us to revoke the registration of a file using the unregister_chrdev function, which we declare in the linux/fs.h file. The major device number is the first parameter of this function, followed by a string containing the device name. The register_chrdev and the unresister_chrdev functions have similar contents.

To load the module, we have to execute the make load command from the source file folder. After this, the name of the driver is added to the /proc/modules file, while the device that the module registers is added to the /proc/devices file. The added records look like this:

In UNIX, hardware devices are accessed by the user through special devicefiles. These files are grouped into the /dev directory, and system callsopen, read, write, close, lseek, mmap etc. areredirected by the operating system to the device driver associated with thephysical device. The device driver is a kernel component (usually a module)that interacts with a hardware device.

In the UNIX world there are two categories of device files and thusdevice drivers: character and block. This division is done by the speed,volume and way of organizing the data to be transferred from the device to thesystem and vice versa. In the first category, there are slow devices, whichmanage a small amount of data, and access to data does not require frequentseek queries. Examples are devices such as keyboard, mouse, serial ports,sound card, joystick. In general, operations with these devices (read, write)are performed sequentially byte by byte. The second category includes deviceswhere data volume is large, data is organized on blocks, and search is common.Examples of devices that fall into this category are hard drives, cdroms, ramdisks, magnetic tape drives. For these devices, reading and writing is done atthe data block level.

For the two types of device drivers, the Linux kernel offers different APIs.If for character devices system calls go directly to device drivers, in case ofblock devices, the drivers do not work directly with system calls. Inthe case of block devices, communication between the user-space and the blockdevice driver is mediated by the file management subsystem and the block devicesubsystem. The role of these subsystems is to prepare the device driver'snecessary resources (buffers), to keep the recently read data in the cachebuffer, and to order the read and write operations for performance reasons.

In UNIX, the devices traditionally had a unique, fixed identifier associatedwith them. This tradition is preserved in Linux, although identifiers can bedynamically allocated (for compatibility reasons, most drivers still use staticidentifiers). The identifier consists of two parts: major and minor. The firstpart identifies the device type (IDE disk, SCSI disk, serial port, etc.)and the second one identifies the device (first disk, second serial port,etc.). Most times, the major identifies the driver, while the minor identifieseach physical device served by the driver. In general, a driver will have amajor associate and will be responsible for all minors associated with thatmajor.

Certain major identifiers are statically assigned to devices (in theDocumentation/admin-guide/devices.txt file from the kernel sources). When choosing theidentifier for a new device, you can use two methods: static (choose a numberthat does not seem to be used already) or dynamically. In /proc/devices are theloaded devices, along with the major identifier.

As mentioned above, the character device drivers receive unaltered system callsmade by users over device-type files. Consequently, implementation of a characterdevice driver means implementing the system calls specific to files: open,close, read, write, lseek, mmap, etc. These operations aredescribed in the fields of the struct file_operations structure:

Returning to device drivers, the two entities have almost always standard waysof using: the inode is used to determine the major and minor of the device onwhich the operation is performed, and the file is used to determine the flagswith which the file was opened, but also to save and access (later) privatedata.

Drivers are very similar to any other program, they can be comprised of multiple source files. Linux devices drivers can be directly compiled into the kernel or a kernel module. Kernel modules have the benefit of being able to be loaded dynamically (i.e. you don't need to compile a new kernel to use them).

You can obviously always look at the Linux source code, but that can be a daunting task at first. However, as with anything in Linux drivers are files. That means other programs access them via the VFS interface. For example, you can control the CDROM via ioctl() calls using its device file.

Linux drivers are built with the kernel, compiled in or as a module. Alternatively, drivers can be built against the kernel headers in a source tree. You can see a list of currently installed kernel modules by typing lsmod and, if installed, take a look at most devices connected through the bus by using lspci.

If the current kernel version does not have driver support for your NIC, you'll need to find drivers at the vendor and you'll need to compile a kernel module from source (against your kernel's headers).

In the above output there are two numbers separated by a comma (1 and 3). Here, 1 is the major and 3 is the minor number. The major number identifies the driver associated with the device, i.e., which driver is to be used. The minor number is used by the kernel to determine exactly which device is being referred to. For instance, a hard disk may have three partitions. Each partition will have a separate minor number but only one major number, because the same storage driver is used for all the partitions.Older kernels used to have a separate major number for each driver. But modern Linux kernels allow multiple drivers to share the same major number. For instance, /dev/full, /dev/null, /dev/random and /dev/zero use the same major number but different minor numbers. The output below illustrates this:

This function registers a major number for character devices. Arguments of this function are self-explanatory. The major argument implies the major number of interest, name is the name of the driver and appears in the /proc/devices area and, finally, fops is the pointer to the file_operations structure.Certain major numbers are reserved for special drivers; hence, one should exclude those and use dynamically allocated major numbers. To allocate a major number dynamically, provide the value zero to the first argument, i.e., major == 0. This function will dynamically allocate and return a major numb