Linux file device driver

How do I implement a file system driver driver in Linux? [closed]

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Assume that I have invented a new file system, and now I want to create a file system driver for it. How would I implement this file system driver, is this done using a kernel module? And how can the file system driver access the hard disk, should the file system driver contain code to access the hard disk, or does Linux contain a device driver to access the hard disk that is used by all the file system drivers?

4 Answers 4

Yes, filesystems in Linux can be implemented as kernel modules. But there is also the FUSE (Filesystem in USErspace) interface, which can allow a regular user-space process to act as a filesystem driver. If you’re prototyping a new filesystem, implementing it first using the FUSE interface could make the testing and development easier. Once you have the internals of the filesystem worked out in FUSE form, you might then start implementing a performance-optimized kernel module version of it.

Here’s some basic information on implementing a filesystem within kernel space. It’s rather old (from 1996!), but that should at least give you a basic idea for the kind of things you’ll need to do.

Filesystem driver as a kernel module

Basically, the initialization function of your filesystem driver module needs just to call a register_filesystem() function, and give it as a parameter a structure that includes a function pointer that identifies the function in your filesystem driver that will be used as the first step in identifying your filesystem type and mounting it. Nothing more happens at that stage.

When a filesystem is being mounted, and either the filesystem type is specified to match your driver, or filesystem type auto-detection is being performed, the kernel’s Virtual FileSystem (VFS for short) layer will call that function. It basically says «Here’s a pointer to a kernel-level representation of a standard Linux block device. Take a look at it, see if it’s something you can handle, and then tell me what you can do with it.»

At that point, your driver is supposed to read whatever it needs to verify it’s the right driver for the filesystem, and then return a structure that includes pointers to further functions your driver can do with that particular filesystem. Or if the filesystem driver does not recognize the data on the disk, it is supposed to return an appropriate error result, and then VFS will either report a failure to userspace or — if filesystem type auto-detection is being performed — will ask another filesystem driver to try.

The other drivers in the kernel will provide the standard block device interface, so the filesystem driver won’t have to implement hardware support. Basically, the filesystem driver can read and write disk blocks using standard kernel-level functions with the device pointer given to it.

The VFS layer expects the filesystem driver to make a number of standard functions available to the VFS layer; a few of these are mandatory in order for the VFS layer to do anything meaningful with the filesystem, others are optional and you can just return a NULL in place of a pointer to such an optional function.

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Finding the Device Driver Used for a Device

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1. Overview

Many Linux commands return information about devices and their drivers. Most of the time, we use these commands to troubleshoot problems. Besides that, we may want to find out the installed drivers in our system.

This tutorial will examine how we can get a list of the installed devices and their corresponding drivers.

2. Hardware Configuration Commands

Some hardware configuration commands return data about any hardware resource. Yet, others deal only with devices on a certain bus like PCI or USB.

Importantly, all commands are best run with sudo, so we get the full output. In addition, most of the commands are pre-installed in popular Linux distributions. In any case, we can install them with the package manager.

2.1. The lshw Command

The lshw tool can provide information about multiple machine hardware components:

• general system information
• memory configuration
• Basic Input/Output System (BIOS) data
• firmware version
• mainboard information
• CPU specifications
• cache setup
• bus speeds

We can build lshw from source or simply install it as a package. Critically, among others, the tool also leverages the /proc and /sys pseudo-filesystems.

The -short switch of lshw shows the devices we can get data on:

H/W path Device Class Description ======================================================== system ThinkPad E14 /0 bus 0666X1 /0/0 memory 64KiB BIOS /0/5 processor Intel(R) Core(TM) i3-1115G4 CPU @ 3.00GHz /0/6 processor Core i3 (None) /0/7 processor (None) [. ] /0/44 processor (None) /0/51 memory 2GiB System Memory /0/51/0 memory 2GiB SODIMM DDR4 /0/51/1 memory [empty] [. ] /0/51/7c memory [empty] /0/100 bridge 660BX/ZX/DX - 72331BX/ZX/DX Host bridge (AGP disabled) /0/100/7 bridge 81161AB/EB/MB PIIX4 ISA /0/100/7.1 scsi3 storage 81161AB/EB/MB PIIX4 IDE /0/100/7.1/0.0.0 /dev/cdrom disk Virtual CD/ROM /0/100/7.3 bridge 81161AB/EB/MB PIIX4 ACPI /0/100/8 display Haswell-ULT Integrated /0/1 system PnP device PNP0b00 [. ] /0/4a scsi0 storage /0/4a/0.0.0 /dev/sda disk 666GB Virtual Disk /0/4a/0.0.0/1 /dev/sda1 volume 6333MiB EXT4 volume /0/4a/0.0.0/2 /dev/sda2 volume 12GiB Extended partition /0/4a/0.0.0/2/5 /dev/sda5 volume 2100MiB EXT4 volume /0/4a/0.0.0/2/6 /dev/sda6 volume 976MiB Linux swap volume /0/4a/0.0.0/2/7 /dev/sda7 volume 267MiB EXT4 volume /0/4a/0.0.0/2/8 /dev/sda8 volume 665Gib EXT4 volume /1 eth0 network Ethernet interface

Here, we can see the Class of each device. In fact, the tool has other formats like –html, -json, and -xml.

Running lshw without options returns all hardware resources. With the -class switch, we can pick specific resources we want to know information about:

$ sudo lshw --class network *-network description: Ethernet interface physical id: 1 logical name: eth0 serial: 52:54:00:6b:5b:e6 size: 10Gbit/s capabilities: ethernet physical configuration: autonegotiation=off broadcast=yes driver=hv_netvsc duplex=full firmware=N/A ip=172.23.120.189 link=yes multicast=yes speed=10Gbit/s

As can be seen, the –class option shows only a defined class of hardware. Particularly, we printed information about the network card. Moreover, the configuration row contains the name of the driver that our network card uses. In our case, the driver is hv_netvsc.

2.2. The hwinfo Command

The hwinfo command is another tool that prints information about hardware devices.

Invoking hwinfo with no options will return all available hardware resources. Nevertheless, we can filter the output by entering the device type that we’re interested in:

$ sudo hwinfo --network . SysFS ID: /class/net/wlp5s0 SysFS Device Link: /devices/pci0000:00/0000:00:1c.2/0000:05:00.0 Hardware Class: network interface Model: "Ethernet network interface" Driver: "iwl3945" Driver Modules: "iwl3945" Device File: wlp5s0 . 

In the above example, we filtered the output to show network devices only. As a result, we got the device driver of the wireless network card, which is the iwl3945 driver.

2.3. The lspci Command

The lspci command prints information for devices connected to the PCI bus. Although the command doesn’t print the driver when called without options, we can use -k to print the driver:

$ sudo lspci -k 01:00.0 VGA compatible controller: NVIDIA Corporation G73M [GeForce Go 7600] (rev a1) Subsystem: Toshiba America Info Systems G73M [GeForce Go 7600] Kernel driver in use: nouveau Kernel modules: nvidiafb, nouveau 

Here, we can see that the command printed the driver used by the display adapter, which is nouveau. Furthermore, the command printed all installed drivers that could be used for this device. In our case, these are the nvidiafb and the nouveau drivers.

We can also use the -v option:

$ sudo lspci -v . 01:00.0 VGA compatible controller: NVIDIA Corporation G73M [GeForce Go 7600] (rev a1) (prog-if 00 [VGA controller]) Subsystem: Toshiba America Info Systems G73M [GeForce Go 7600] Flags: bus master, fast devsel, latency 0, IRQ 28 . Kernel driver in use: nouveau Kernel modules: nvidiafb, nouveau 

Notably, the -v option produces similar output to the -k option concerning kernel drivers.

2.4. The lsusb Command

The lsusb command prints data about USB buses and the devices connected to them. We can run the command with the -t option to get the driver:

$ sudo lsusb -t . /: Bus 02.Port 1: Dev 1, Driver=uhci_hcd/2p, 12M |__ Port 2: Dev 2, If 0, Specific Class, Driver=, 12M /: Bus 01.Port 1: Dev 1, Driver=ehci-pci/8p, 480M 

Here, the command printed the driver for the second USB device. We see the ehci-pci driver, which is the Linux USB 2.0 driver.

3. The sysfs Pseudo-Filesystem

Linux has a /sys sysfs pseudo-filesystem as an interface to internal kernel data structures, but mainly those that describe hardware. Still, /sys also provides access to information about filesystems, Security-Enhanced Linux (SELinux), modules, and others.

While the similar /proc pseudo-filesystem is a raw UNIX remnant, a first attempt at bringing order to the chaotic reading and interpretation of /dev/mem, /sys takes that a step further.

Despite their overlap, there are perhaps a couple of big differences between /proc and /sys:

• /sys enforces Linux-specific standards and philosophy while supposedly bringing structure to the concept of /proc
• /sys and /proc overlap, but /sys usually provides more organized hardware information and drivers instead of exposing direct memory regions

When it comes to hardware, the desktop management interface (DMI) provides a standard source of information about components in a machine. Since its main focus is hardware, sysfs exposes DMI at /sys/class/dmi/id:

$ tree /sys/class/dmi/id /sys/class/dmi/id ├── bios_date ├── bios_vendor ├── bios_version ├── board_name ├── board_serial ├── board_vendor ├── board_version ├── chassis_asset_tag ├── chassis_serial ├── chassis_type ├── chassis_vendor ├── chassis_version ├── modalias ├── power │ ├── async │ ├── autosuspend_delay_ms │ ├── control │ ├── runtime_active_kids │ ├── runtime_active_time │ ├── runtime_enabled │ ├── runtime_status │ ├── runtime_suspended_time │ └── runtime_usage ├── product_name ├── product_serial ├── product_uuid ├── product_version ├── subsystem -> ../../../../class/dmi ├── sys_vendor └── uevent

Here, we use the tree command to output the sysfs subtree related to DMI.

$ cat /sys/class/dmi/id/sys_vendor LENOVO

Of course, for writeable values, we can simply overwrite the appropriate file.

In addition to the above, the sysfs filesystem provides data about drivers.

One way to find the driver of a device is to browse the /class sub-directory where devices are grouped by their type. Let’s find the installed network adapters:

$ ls /sys/class/net enp7s8 lo wlp5s0 

Here, we list the subdirectory /net to get all network adapters. To find the driver of the wireless adapter, we can search for a driver folder in wlp5s0:

$ sudo readlink /sys/class/net/wlp5s0/device/driver ../../../../bus/pci/drivers/iwl3945

As we can see, we managed to print the driver used by the wireless card, which is the iwl3945 driver.

4. Kernel Modules

Another critical point is that most Linux drivers are kernel modules. Kernel modules extend kernel functionality, and we can load and unload them as needed.

So, we can use the modinfo command to find driver details:

$ modinfo iwl3945 filename: /lib/modules/4.4.0-210-generic/kernel/drivers/net/wireless/iwlegacy/iwl3945.ko firmware: iwlwifi-3945-2.ucode license: GPL author: Copyright(c) 2003-2011 Intel Corporation [email protected]> version: in-tree:s description: Intel(R) PRO/Wireless 3945ABG/BG Network Connection driver for Linux srcversion: 59CEE3A5C9D9B5F86295C7E depends: iwlegacy,mac80211,cfg80211 . 

In the above example, we can see some useful properties of the driver like the author, filename, and dependencies.

5. Conclusion

In this tutorial, we learned several commands that we can use to find the driver name of a device. Then, we showed how we can browse the sysfs filesystem to find the drivers. Finally, we used the modinfo command to print the details of a driver.

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