CODEWITHSUNDEEP
clinuxlinux-kernel
ppsun
ppsun

Reputation: 199

Where is the private data stored for each net_device in Linux kernel 2.6.32 or later?

For Linux kernel 2.4 or 2.6 earlier than 2.6.31, the net_device struct for each registered network device has a private data block associated with it, which is pointed to by the priv pointer in the net_device struct. However, for kernel version 2.6.32 or later, the priv pointer has been deprecated.

I am wondering where the network interface driver's private data can be stored now. Is anyone clear of the relatively new implementation of net_device struct? Thanks in advance.

Upvotes: 4

Views: 4085

Answers (1)

Jonathon Reinhart
Jonathon Reinhart

Reputation: 137398

This answer references version 3.14 of the Linux kernel.

The private data is stored at the end of the net_device structure.

You allocate a net_device by calling alloc_netdev, which is just a macro for alloc_netdev_mqs. The first parameter is int sizeof_priv, which specifies the amount of additional space you wish to be allocated at the end of the net_device for your private data.

You access this private data by calling the (inline) function netdev_priv. By looking at that function, you can see it just returns an aligned pointer after the end of the real struct net_device:

static inline void *netdev_priv(const struct net_device *dev)
{
    return (char *)dev + ALIGN(sizeof(struct net_device), NETDEV_ALIGN);
}

I'm going to assume that the developers did this for cache reasons. This way, the private data will fall on the same cache line as the rest of the structure, instead of having to access memory very far away from the net_device via the priv pointer.

As an example, the Intel e100 driver defines a private struct nic in e100.c, and allocates its net_device in e100_probe. You see that it passes sizeof(struct nic) to alloc_etherdev, which is a convenience function for allocating Ethernet devices:

static int e100_probe(struct pci_dev *pdev, const struct pci_device_id *ent)
{
     struct net_device *netdev;
     struct nic *nic;
     int err;

     if (!(netdev = alloc_etherdev(sizeof(struct nic))))
         return -ENOMEM;

Then to access this private data elsewhere, they call netdev_priv, as seen here in mdio_write:

static void mdio_write(struct net_device *netdev, int addr, int reg, int data)
{
    struct nic *nic = netdev_priv(netdev);

    nic->mdio_ctrl(nic, addr, mdi_write, reg, data);
}

Upvotes: 7

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