This article has been written for kernel newcomers interested in learning about network device drivers. It assumes that reader has a significant exposure to C and the Linux environment.
This article is based on a network driver for the RealTek network card. I chose the RealTek chip for two reasons: First, RealTek provides technical specifications for its chips free of cost. Second; it's quite cheap. The driver presented in this article is minimal; it simply sends and receives packets and maintains some statistics.
For a full-fledged and professional-grade driver, please refer to the Linux source. Before starting driver development, we need to set up our system for it. This article was written and tested on Linux 2. It's very likely that the here you are running has the driver compiled either within the kernel itself or as a module.
It's advisable to build a kernel which does not have the RealTek driver in any form, to avert unnecessary surprises.
If you don't know how to recompile the Linux kernel, I recommend you take a look at http: From this point of discussion onwards, it How To Write Network Drivers In Linux assumed that you have a working kernel, which does not have here for RealTek You'll also need the technical specifications for the chip, which you can download from http: This is the best resource known to me for Linux device driver development, as of now.
Linux Device Drivers
As a first step, we need to detect the device of our interest. Each vendor has a unique ID assigned, and each vendor assigns a unique ID to a particular kind of device. Normally this is 0xffffffff. You only need to change this if your device has broken DMA or supports bit article source. They might be different!
After detecting the device, we need to enable the device before starting any kind of interaction or communication with the device. The code snippet shown in Table 1 can be extended for device detection and enabling the device. For time being, we temporarily suspend the thread of driver code study; instead, we look into some important topics in order to understand the Linux view of a network device. We have detected the PCI device and enabled it, but the networking stack in Linux sees interfaces as network devices.
Although this structure has many more members, for our minimal driver, these members are good enough. The following section describes the structure members:.
This is done by the device interrupt handler, as we will see later in this article. That is, a part of the CPU's address space is interpreted not as accesses to memory, but as accesses to a device.
Some architectures define devices to be at a fixed address, but most have some method of discovering devices. The PCI bus walk is a good example of such a scheme. This document does not cover how to receive such an address, but assumes you are starting with one. Physical addresses are of type unsigned long. These addresses should not be used directly. Instead, to get an address suitable for passing to the functions described below, you should call ioremap. An address suitable for accessing the device will be returned to you.
After you've finished using the device say, in your module's exit routinecall iounmap in order to return the address space to the kernel.
Most architectures allocate new address space each time you call ioremap, and they can run out unless you call iounmap. The part of the interface How To Write Network Drivers In Linux used by drivers is reading and writing memory-mapped registers on the device.
Linux provides interfaces to read and write 8-bit, bit, bit and bit quantities.
Due to a historical accident, these are named byte, word, long, and quad accesses. Both read and write accesses are supported; there is no prefetch support at this time. The functions are named readb, readw, readl, readq, writeb, writew, writel, and writeq. Some devices such as framebuffers would like to use larger transfers that are more than 8 bytes at a time. The read and write functions are defined to be ordered.
Network Drivers Having discussed The normal file operations (read, write, Network Drivers The network subsystem of the Linux kernel is designed to be. How to Install Hardware Drivers on Linux. These are hardware drivers that the manufacturers write and maintain on How to Change Your Wi-Fi Network’s Name. I do not know how to write this any simpler. I have several books on Linux device-drivers, but I need a little help to get started. Mar 30, · For example, we can open and write to a file of the hard (a VGA card or a network I think, for those who is not familiar with Linux drivers at.
Use this with care. The rmb provides a read memory barrier. The wmb provides a write memory barrier.
Writing Network Device Drivers for Linux. By Mohan Lal Jangir. Linux provides interfaces to read and write 8-bit, bit, bit and bit quantities. Mar 05, · Linux Network Interface Driver. This article explains how to write a network interface driver in Linux. There are many network. interface cards available. Linux Device Drivers, might help you follow the discussion and to see how real-world Linux network drivers is possible to write polling drivers. Introduction This article will help the reader to understand and develop a network driver for an ethernet card in Linux. As a note, the driver development was done in. Writing device drivers in Linux: In order to develop Linux device drivers, When you write device drivers.
While the basic functions are defined to be synchronous with respect to each other and ordered with just click for source to each other the buses the devices sit on may themselves have asynchronocity. In particular many authors are not comfortable by the fact that PCI bus writes are posted asynchronously. An author of the driver must issue a read from the same device to ensure that writes have occurred in the manner the author wanted it.
This kind of property cannot be hidden from driver writers in the API. This is a range of addresses different from the normal memory address space. Access to these addresses is generally not as fast as accesses to the memory mapped addresses, and it also has a potentially smaller address space. Accesses to this space are provided through a set of functions which allow 8-bit, bit and bit accesses; also known as byte, word and long.
These functions are inb, inw, inl, outb, outw and outl. Some variants How To Write Network Drivers In Linux provided for these functions. Some devices require that accesses to their ports are slowed down. There are also equivalents to memcpy. In this section, we will look at PCI configuration space. PCI devices feature a byte address space. The first 64 bytes are standardized while the rest of the bytes are device dependent.
Figure 1 shows the standard PCI configuration space. We have seen them in the section "Device Detection".
We will see how BARs are used shortly. Now it's time to revert back to driver development. We will declare a structure which holds data private to our device and that structure shall be pointed to by member priv. More info structure has the following members We will update structure members as we progress.
It's time to explain what we have done in Table 5. This is the starting memory location of device registers. Before accessing the addresses returned by the above APIs, we have to do two things. If you see "Register Descriptions" in RealTek specification, the first 6 bytes are the hardware address of the device. Also we have initialized How To Write Network Drivers In Linux pointer members but haven't defined any corresponding function.
For time being, we define dummy functions to compile the module. Now we have a dummy or template driver ready. Now execute a series of commands; " ifconfig ", " ifconfig - a ", " ifconfig rtl up ", " ifconfig " and " ifconfig rtl down ", and observe their output. These calls show you when each function is called.
Now again, we stop driver development in order to better understand the device transmission and receiving mechanism. The 4 descriptors are used round-robin. This means that for transmitting four packets, the driver will use descriptor 0, descriptor 1, descriptor 2 and descriptor 3 in round-robin order. For transmitting next packet, driver will use descriptor 0 again provided that is available.
These registers store "Transmit Start Address of Descriptors" i.
We will shortly see that this driver allocates DMAable memory for packet contents, and stores the address of that memory in TSAD registers. The receive path of RTL is designed as a ring buffer A liner memory, managed as ring memory. The device keeps on storing packets in this fashion until linear memory is exhausted.
In that case, the device starts again writing at the starting address of linear memory, thus making it a ring buffer. In this section, we discuss driver source used to make the device ready for transmission.
We defer discussion of the receiving source to further sections. This also means that, we can't use dirty descriptor for further packet transmission until previous packet is transmitted completely. Here is a list of register offsets, used in code. You can get more details about these values from the RealTek specifications. Now, we explain the code in Table This function shall How To Write Network Drivers In Linux called by kernel, whenever the device generates an interrupt.
Now, we allocate memory, where outgoing packets reside before being sent on wire. The physical address is returned in third argument, which is later used by driver. Also observe that we have allocated memory needed for all four descriptors. At first, we reset the device, so that device shall be in a predictable and known state. This is done by writing reset value described in specification in CR Command Register.
We wait until the written value is check this out back, which means device has reset.
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