Embedded linux network device driver development

Embedded Linux Network
Device Driver Development

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September 7, 2017
Embedded Linux Network Device Driver
Development
2

References
 www.free-electrons.com
 Essential Linux Device Drivers
 Linux Device Drivers
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Objectives
 Understanding the structure of ethernet and WIFI device drivers
 Developing Linux network device drivers
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Prerequisites
 Solid C programming
 Knowledge of Linux commands is a plus
 Knowledge of networking
 Embedded Linux Kernel and Driver Development training or equivalent
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Notes
 Ask any time.
 Turn your cell silent.
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Outline
 Introduction
 Socket Buffers
 Network Devices
 Communication with Network Protocol
 Communication with PHY
 Buffer Management and Concurrency Issues
 Network Throughput
 Other Network Driver Considerations
 Ethernet Driver vs WIFI Driver
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Outline
 Introduction
 Socket Buffers
 Network Devices
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OSI Model and Linux Kernel
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User Space
Kernel Network
Stack
Device Driver

Network Device Model
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Outline
 Introduction
 Socket Buffers
 Network Devices
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Socket Buffer (sk_buff)
 struct sk_buff (include/linux/sk_buff.h) represents a network packet
 Support data encapsulation/decapsulation through protocol layers
 In addition to data, sk_buff maintains:
 head, the start of the packet
 data, the start of the packet payload
 tail, the end of the packet payload
 end, the end of the packet
 len, the amount of data of the packet
 These fields are updated when the packet goes through the protocol
layers
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SKB Operations
Allocation
 By dev_alloc_skb() function
 Can be done in ISR
 On Ethernet, allocated size = packet
length + 2
 To word align IP header
 IP header = 14 bytes
Reservation
 By skb_reserve() function
 Skipping NET_IP_ALIGN padding
bytes
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SKB Operations cont’d
Data Copying
 From DMA buffer to SKB during
reception for example
 OR SKB to DMA buffer during
transmission (may be)
Pointers Update
 After copying payload
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Outline
 Introduction
 Socket Buffers
 Network Devices
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Network Device
 struct net_device (include/linux/netdevice.h) represents a network
interface
 Allocation takes place with alloc_netdev() function
 Size of private data is passed as argument
 Pointer to these private data can be read in net_device->priv
 Use alloc_etherdev() for ethernet interfaces
 Use alloc_ieee80211() for WIFI interfaces
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Network Device cont’d
 Registration with register_netdev() function
 Unregistration with unregister_netdev() function
 Liberation with free_netdev() function
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Network Device Operations
 Defined by struct net_device_ops (include/linux/netdevice.h)
 Set the netdev_ops field in the struct net_device structure to point to the
struct net_device_ops structure
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Operation Description
ndo_open() Called when network interface uped
ndo_close() Called when network interface downed
ndo_start_xmit() Start packet transmission
ndo_get_stats() Get statistics
ndo_do_ioctl() Implement device specific operations
ndo_set_rx_mode() Select promiscuous, multicast, etc
ndo_set_mac_address() Set MAC address
ndo_set_multicast_list() Set multicast filters

Network Device Operations cont’d
 Sample operations are:
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ndo_open() Called when network interface uped
ndo_close() Called when network interface downed
ndo_start_xmit() Start packet transmission
ndo_get_stats() Get statistics
ndo_do_ioctl() Implement device specific operations
ndo_set_rx_mode() Select promiscuous, multicast, etc
ndo_set_mac_address() Set MAC address
ndo_set_multicast_list() Set multicast filters
ndo_tx_timeout() Reset unresponsive network interface

Outline
 Introduction
 Socket Buffers
 Network Devices
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Flow Control Utility Functions
 include/linux/netdevice.h
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netif_start_queue() Tells kernel that driver is ready to send packets
netif_stop_queue() Tells kernel to stop sending packets (@cleanup or congestion)
netif_queue_stopped() Tells whether queue is stopped or not
netif_wake_queue() Wakes up a queue after a netif_stop_queue()
netif_rx() Gives SKB to kernel

Transmission
 Kernel calls ndo_start_xmit() with SKB as argument
 ndo_start_xmit():
1. Sets up DMA buffers and other HW mechanisms
2. Starts transmission
3. Can stop queueing if no more free DMA buffers available using
netif_stop_queue()
4. Returns NETDEV_TX_OK or NETDEV_TX_BUSY
 When N packets have been sent, an interrupt is raised and the driver
should:
1. Acknowledging interrupt
2. Freeing used DMA buffers
3. Freeing SKB with dev_kfree_skb_irq()
4. If the queue was stopped, start it again
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Reception: Non-NAPI
 Reception is via an interrupt that should:
1. Allocate an SKB
2. Reserve NET_IP_ALIGN padding bytes
3. Copy packet from DMA buffers to SKB
4. Update SKB pointers
5. Update the skb->protocol field with eth_type_trans(skb, netdevice)
6. Give SKB to kernel by netif_rx()
 netif_rx() use NET_RX_SOFTIRQ to offload the work of posting received data
packets to protocol layers
 Nice and simple
 @high traffic, interrupt rate is high
 Solution: switch to polled mode when interrupt rate is too high (NAPI)
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Reception: NAPI
 In network interface private structure, add a struct napi_struct
(include/linux/netdevice.h)
 @ driver initialization, register the NAPI poll operation
 dev: network interface
 &lp->napi: struct napi_struct
 r6040_poll: NAPI poll operation
 64: weight that represents the importance of network interface
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Reception: NAPI cont’d
Rx ISR
 Disables Rx interrupt and switch to
polled mode
if (napi_schedule_prep(&lp->napi)) {
/* Disable reception interrupts */
__napi_schedule(&lp->napi);
}
 Kernel calls our poll() operation
regularly
Poll Function
 static int r6040_poll(struct
napi_struct *napi, int budget)
 Receives at most budget packets and
pushes them to the network stack
using netif_receive_skb()
 If less than budget packets have been
received, switch back to interrupt
mode using napi_complete(&lp-
>napi) and reenable interrupts
 Must return the number of packets
received
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Outline
 Introduction
 Socket Buffers
 Network Devices
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Connection with PHY
 MAC and PHY are connected using a MII or RMII interface
 This interface contains two wires used for the MDIO bus
 Driver needs to communicate with the PHY to get link information (up,
down, speed, full or half duplex) and configure the MAC accordingly
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PHY in Kernel
 Kernel has a framework (drivers/net/phy/) that
 Exposes an API to communicate with PHY
 Allows to implement PHY drivers
 Implements a basic generic PHY driver that works with all PHY
 See Documentation/networking/phy.txt
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MDIO Bus Initialization
 Driver creates a MDIO bus struct mii_bus (include/linux/mii.h) to tells PHY
infrastructure how to communicate with the PHY
 mdio_read() and mdio_write are HW specific and must be implemented
by the driver
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MDIO Bus Initialization cont’d
 The ->irq[] array must be allocated and initialized
 To use polling, set the values to PHY_POLL
Lp->mii_if->irq = kmalloc(sizeof(int)*PHY_MAX_ADDR, GFP_KERNEL);
for (i = 0; i < PHY_MAX_ADDR; i++)
lp->mii_if>irq[i] = PHY_POLL;
 Finally, register the MDIO bus
 This will scan the bus for PHYs and fill the mii_if->phy_map[] array with the
result
mdiobus_register(bp->mii_if);
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Connecting with PHY
 mdiobus_register() function filled the mii_if->phy_map[] array with struct
phy_device * pointers
 Appropriate PHY (usually, only one is detected) must be selected
 Connecting to the PHY allows to register a callback that will be called
when the link changes :
 interface is usually PHY_INTERFACE_MODE_MII or
PHY_INTERFACE_MODE_RMII
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Updating MAC Capabilities
 MAC and PHY might have different capabilities
 PHY handling Gigabit speed, but not MAC
 Driver is responsible for updating phydev->advertise and phydev-
>supported to remove any PHY capability that the MAC doesn't support
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Handling Link Changes
 The callback that handle link changes should have the following prototype:
void foo_handle_link_change(struct net_device *dev)
 It must check the duplex, speed and link fields of the struct phy_device
structure, and update Ethernet controller configuration accordingly
 duplex is either DUPLEX_HALF or DUPLEX_FULL
 speed is either SPEED_10, SPEED_100, SPEED_1000, SPEED_2500 or
SPEED_10000
 link is a Boolean
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Starting and stopping the PHY
 After set up, to make the PHY driver poll regularly the PHY hardware, one
must start it with phy_start() functions
 And stop it using phy_stop(), when no longer needed
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Outline
 Introduction
 Socket Buffers
 Network Devices
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Buffers Management
 During open(), driver pre-allocates needed DMA descriptos needed for
transmission and reception
 When totally utilized, the driver tells the kernel to stop sending packets to
the driver
 SKB are allocated and freed as per need
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Concurrency
 Access protection needed in the face of multiple execution threads:
 Transmit thread
 Receive thread
 Transmit-complete interrupt
 Receive interrupt
 NAPI polling
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Outline
 Introduction
 Socket Buffers
 Network Devices
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Network Benchmarking
 Netperf (free from www.netperf.org ) can set up complex TCP/UDP
connection scenarios to control C/C’s such as protocol parameters,
number of simultaneous sessions, and size of data blocks.
 Benchmarking = Comparing resulting throughput with maximum practical
bandwidth
 Factors affecting throughput:
 Driver implementation
 Protocol used
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Driver Performance
 Execution time
 Considering usage of DMA for large packets
 Offloading CPU by moving functionalities to hardware (TCP checksum)
 Taking into consideration upper layers behavior
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Protocol Performance
 TCP window size = data that can be transmitted before receiving ACK
 Fast network interfaces + small window size = TCP sitting idle waiting for ACK
 In UDP, the window size is not relevant as there is no ACK
 Packet loss in TCP and UDP
 Mapping protocol block sizes to MTU sizes
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Outline
 Introduction
 Socket Buffers
 Network Devices
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Ethtool (include/linux/ethtool.h)
 ethtool is a user space tool that query low-level information from an
Ethernet interface and to modify its configuration
 @ driver, a struct ethtool_ops structure can be declared and connected to
the struct net_device using the ethtool_ops field
 List of operations: get_settings(), set_settings(), get_drvinfo(), get_wol(),
set_wol(), get_link(), get_eeprom(), set_eeprom(), get_tso(), set_tso(),
get_flags(), set_flags(), etc.
 Some of these operations can be implemented using the PHY interface
(phy_ethtool_gset(), phy_ethtool_sset()) or using generic
operations(ethtool_op_get_link() for example)
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Statistics
 The driver is responsible for keeping statistics up to date about the
number of packets/bytes received/transmitted, the number of errors, of
collisions, etc.
 To expose these information, the driver must implement a get_stats()
operation, with the following prototype struct net_device_stats
*foo_get_stats(struct net_device *dev);
 The net_device_stats structure must be filled by the driver
 It contains fields such as rx_packets, tx_packets, rx_bytes, tx_bytes, rx_errors,
tx_errors, rx_dropped, tx_dropped, multicast, collisions, etc
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Power Management
 suspend() function should:
 Call netif_device_detach()
 Do the hardware dependent operations to suspend the devices (like disable
the clocks)
 The resume() function should:
 Do the hardware dependent operations (like enable the clocks)
 Call netif_device_attach()
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Lab: Training Setup
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Outline
 Introduction
 Socket Buffers
 Network Devices
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Ethernet vs. WIFI
Ethernet
 CSMA/CD
 Frames are not acknowledged
WIFI
 CSMA/CA
 Frames are acknowledged
 Uses WEP for enhanced security
 2 modes of operation:
 Ad-hoc
 Infrastructure
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Configuring WIFI Drivers
 The Wireless Extensions project defines a generic Linux API to configure
WLAN device drivers in a device independent manner
 It also provides a set of common tools to set and access information from
WLAN drivers
 Individual drivers must implement support for Wireless Extensions to
connect themselves with the common interface and, hence, with the tools
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Talking to WIFI Drivers
 Using iwconfig utility
 To glue your driver to iwconfig, you need to implement prescribed functions
corresponding to commands that set parameters such as ESSID and WEP keys
 Using iwpriv utility
 To use iwpriv over your driver, define private ioctls relevant to your hardware
and implement the corresponding handler functions
 Using /proc/net/wireless to get driver statistics
 For this, implement the get_wireless_stats() function in your driver, in addition
to the get_stats() function
 WLAN drivers tie these three pieces of information inside a structure
called iw_handler_def, defined in include/net/iw_handler.h
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Ethernet/WIFI Famous Device Drivers
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Device(s) Driver Location in Kernel Tree
Intersil Prism2 WLAN Compact Flash Card /drivers/net/wireless/intersil/orinoco
Intel Pro/Wireless Mini PCI (and PCIe Mini) /drivers/net/wireless/intel/ipw2x00
Atmel WLAN USB /drivers/net/wireless/atmel
Intel PRO/1000 /drivers/net/ethernet/intel/e1000

Ethernet Driver Example
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Lab: Ethernet Driver
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WIFI Driver Example
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Lab: WIFI Driver
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 To contact us:
 https://www.facebook.com/groups/EmbeddedSystemsTraining/
 www.swift-act.com
 training@swift-act.com
 (+2)0122-3600-207
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Embedded linux network device driver development

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