Network Buffers and Memory Management

Each device is created by filling in a struct device object and passing it to the register_netdev(struct device *) call. This links your device structure into the kernel network device tables. As the structure you pass in is used by the kernel, you must not free this until you have unloaded the device with void unregister_netdev(struct device *) calls. These calls are normally done at boot time or at module load/unload.

The kernel will not object if you create multiple devices with the same name, it will break. Therefore, if your driver is a loadable module you should use the struct device *dev_get(const char *name) call to ensure the name is not already in use. If it is in use, you should pick another name or your new driver will fail. If you discover a clash, you must not use unregister_netdev() to unregister the other device using the name!

A typical code sequence for registration is:

int register_my_device(void)
{
  int i=0;
  for(i=0;i<100;i++)
  {
    sprintf(mydevice.name,"mydev%d",i);
    if(dev_get(mydevice.name)==NULL)
    {
      if(register_netdev(&mydevice)!=0)
        return -EIO;
      return 0;
    }
  }
  printk(
"100 mydevs loaded. Unable to load more.\n");
  return -ENFILE;
}

All the generic information and methods for each network device are kept in the device structure. To create a device you need to supply the structure with most of the data discussed below. This section covers how a device should be set up.

First, the name field holds a string pointer to a device name in the formats discussed previously. The name field can also be " " (four spaces), in which case the kernel automatically assigns an ethn name to it. This special feature should not be used. After Linux 2.0, we intend to add a simple support function of the form dev_make_name("eth") for this purpose.

The next block of parameters is used to maintain the location of a device within the device address spaces of the architecture. The irq field holds the interrupt (IRQ) the device is using, and is normally set at boot time or by the initialization function. If an interrupt is not used, not currently known or not assigned, the value zero should be used. The interrupt can be set in a variety of fashions. The auto-irq facilities of the kernel can be used to probe for the device interrupt, or the interrupt can be set when loading the network module. Network drivers normally use a global int called irq for this so that users can load the module with insmod mydevice irq=5 style commands. Finally, the IRQ field can be set dynamically using the ifconfig command, which causes a call to your device that will be discussed later on.

The base_addr field is the base I/O space address where the device resides. If the device uses no I/O locations or is running on a system without an I/O space concept, this field should be set to zero. When this address is user settable, it is normally set by a global variable called io. The interface I/O address can also be set with ifconfig.

Two hardware-shared memory ranges are defined for things like ISA bus shared memory Ethernet cards. For current purposes, the rmem_start and rmem_end fields are obsolete and should be loaded with 0. The mem_start and mem_end addresses should be loaded with the start and end of the shared memory block used by this device. If no shared memory block is used, then the value 0 should be stored. Those devices that allow the user to to set the memory base use a global variable called mem, and then set the mem_end address appropriately themselves.

The dma variable holds the DMA channel in use by the device. Linux allows DMA (like interrupts) to be automatically probed. If no DMA channel is used or the DMA channel is not yet set, the value 0 is used. This option may have to change, since the latest PC boards allow ISA bus DMA channel 0 to be used by hardware boards and do not just tie it to memory refresh. If the user can set the DMA channel, the global variable dma is used.

It is important to realise that the physical information is provided for control and user viewing (as well as the driver's internal functions), and does not register these areas to prevent them being reused. Thus, the device driver must also allocate and register the I/O, DMA and interrupt lines it wishes to use, using the same kernel functions as any other device driver. [See the recent Kernel Korner articles on writing a character device driver in issues 23, 24, 25, 26 and 28, or visit the new Linux Kernel Hackers' Guide at www.redhat.com:8080/HyperNews/get/khg.html, for more information on the necessary functions—ED]

The if_port field holds the physical media type for multi-media devices such as combo Ethernet boards.