Understanding Linux Network Internals

Organization of net_device Structures

Some of the subtler aspects of the net_device structure include the following:

• We saw in the section "Allocating net_device Structures" that when alloc_netdev is called to allocate a net_device structure, it is passed the size of the driver's private data block (whose size depends on the driver—some do not even use private data at all). alloc_netdev appends the private data to the net_device structure. Figure 8-1 showed how that parameter is passed and Figure 8-2 shows the effect on the memory allocation.

• Figure 8-2 also shows the relationship between the net_device data structure and the optional driver's private data structure. Normally, the second part is allocated together with the first one so that a single kmalloc is sufficient, but there are also cases where the driver prefers to allocate its private block by itself (see driver C in Figure 8-2).

• As shown in the example in Figure 8-2, the size of the driver's private block and its content change not only from one device type to another (e.g., Token Ring versus Ethernet) but also among devices of the same type (e.g., two different Ethernet cards).

• dev_base (introduced later in this section) and the next pointer in net_device point to the beginning of the net_device structure, not to the beginning of the allocated block. However, the size of the initial padding is saved in dev->padded, which allows the kernel to release the whole memory block when it is time to do so.

net_device data structures ...