Network Drivers
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The document discusses Linux network drivers and provides information about: - The Linux network subsystem and protocol stack, typically using TCP/IP. - Network interface card (NIC) drivers which provide a uniform interface for the network layer to access physical network cards. - Key data structures like struct sk_buff and struct net_device that network drivers interact with for packet handling and device operations. - Functions for network device registration, open/close, interrupt handling, and flow control. - Examples of simple network drivers and how to write one for a Realtek NIC.
Network Drivers
- 1. © 2010-16 SysPlay Workshops <workshop@sysplay.in> All Rights Reserved. Network Drivers
- 2. 2© 2010-16 SysPlay Workshops <workshop@sysplay.in> All Rights Reserved. What to Expect? Understanding Network Subsystem Network Drivers: A different category Writing Network Drivers
- 3. 3© 2010-16 SysPlay Workshops <workshop@sysplay.in> All Rights Reserved. Network Subsystem (Network) Protocol Stack Typically, the TCP/IP Stack Interfaces with the User Space through network interfaces, instead of device files Unlike other drivers, does not provide any /sys or /dev entries Interface Examples: eth0, wlan1, … Resides in the <kernel_source>/net folder Network Interface Card (NIC) Drivers Driver for the Physical Network Cards Provides uniform hardware independent interface for the network protocol layer to access the card Typically, resides in the <kernel_source>/drivers/net folder
- 4. 4© 2010-16 SysPlay Workshops <workshop@sysplay.in> All Rights Reserved. Related Data Structures Writing a NIC or Network Driver involves Interacting with the underlying Card over its I/O Bus Providing the standard APIs to the Protocol Stack This needs three kind of Data Structures Core of Protocol Stack struct sk_buff NIC & Protocol Stack Interface struct net_device NIC I/O bus, e.g. PCI, USB, … I/O bus specific data structure
- 5. 5© 2010-16 SysPlay Workshops <workshop@sysplay.in> All Rights Reserved. struct sk_buff Network Packet Descriptor Header: <linux/skbuff.h> Driver relevant Fields head – points to the start of the packet tail – points to the end of the packet data – points to the start of the pkt payload end – points to the end of the packet payload len – amount of data that the packet contains
- 6. 6© 2010-16 SysPlay Workshops <workshop@sysplay.in> All Rights Reserved. Socket Buffer APIs Header: <linux/skbuff.h> SK Buffer Storage struct sk_buff *dev_alloc_skb(len); dev_kfree_skb(skb); SK Buffer Operations void skb_reserve(struct sk_buff *, int len); struct sk_buff *skb_clone(struct sk_buff *, gfp_t); unsigned char *skb_put(struct sk_buff *, int len);
- 7. 7© 2010-16 SysPlay Workshops <workshop@sysplay.in> All Rights Reserved. The Big Picture (with a PCI NIC) Network Stack sk_buff net_device eth0 system calls ifconfig ping tcpdump ... PCI Driver handler Tx dpv Network Vertical PCI Horizontal NIC Driver PCI Core PCI Ctrl Drv PCI Host Ctrl DMA Ctrl System RAM Tx Buf Rx Buf PHY NIC PCI Cfg Space Reg Space sk_buff... CPU User Space Kernel Space Hardware Space poll Soft IRQ TxDesc RxDesc
- 8. 8© 2010-16 SysPlay Workshops <workshop@sysplay.in> All Rights Reserved. struct net_device Header: <linux/netdevice.h> Driver relevant Operation Fields Activation: open, stop, ioctl Data Transfer: start_xmit, poll (NAPI: new API) WatchDog: tx_timeout, int watchdog_timeo Statistics: get_stats, get_wireless_stats Typically uses struct net_device_stats, populated earlier Configuration: struct *ethtool_ops, change_mtu Structure defined in Header: <linux/ethtool.h> Bus Specific: mem_start, mem_end Latest kernels have all the operations moved under struct net_dev_ops And typically, a prefix ndo_ added and some name changes
- 9. 9© 2010-16 SysPlay Workshops <workshop@sysplay.in> All Rights Reserved. Net Device Registration Header: <linux/netdevice.h> Net Device Storage struct net_device *alloc_etherdev(sizeof_priv); struct net_device *alloc_ieee80211dev(sizeof_priv); struct net_device *alloc_irdadev(sizeof_priv); struct net_device *alloc_netdev(sizeof_priv, name, setup_fn); void free_netdev(struct net_device *); Registering the Net Device int register_netdev(struct net_device *); void unregister_netdev(struct net_device *);
- 10. 10© 2010-16 SysPlay Workshops <workshop@sysplay.in> All Rights Reserved. Network Device Open & Close A typical Network Device Open Allocates ring buffers and associated sk_buffs Initializes the Device Gets the MAC, … from device EEPROM Requests firmware download, if needed int request_firmware(fw, name, device); Register the interrupt handler(s) A typical Network Device Close Would do the reverse in chronology reverse order
- 11. 11© 2010-16 SysPlay Workshops <workshop@sysplay.in> All Rights Reserved. Packet Receive Interrupt Handler A typical receive interrupt handler Minimally handles the packet received Sanity checks Puts back equal number of sk_buffs for re-use Passes the associated sk_buffs (& ring buffers) to the protocol layer by the NET_RX_SOFTIRQ On higher load, switch to poll mode, if supported, which then passes the associated sk_buffs (& ring buffers) to the protocol layer by the NET_RX_SOFTIRQ
- 12. 12© 2010-16 SysPlay Workshops <workshop@sysplay.in> All Rights Reserved. Flow Control related APIs For interaction with the protocol layer Header: <linux/netdevice.h> APIs void netif_start_queue(struct net_device *); void netif_stop_queue(struct net_device *); void netif_wake_queue(struct net_device *); int netif_queue_stopped(struct net_device *);
- 13. 13© 2010-16 SysPlay Workshops <workshop@sysplay.in> All Rights Reserved. Network Driver Examples Driver: snull Browse & Discuss Driver for Realtek NIC 8136 Browse & Hack
- 14. 14© 2010-16 SysPlay Workshops <workshop@sysplay.in> All Rights Reserved. What all have we learnt? Linux Network Subsystem How are Network Drivers different? Writing Network Drivers Key Data Structures & their APIs sk_buff & net_device Network Device Registration Network Device Operations Interrupt Handling for Packets received Flow Control related APIs
- 15. 15© 2010-16 SysPlay Workshops <workshop@sysplay.in> All Rights Reserved. Any Queries?