Analysis of USB network card sending and receiving data

Preface

The network subsystem is Linux The top priority of the kernel . today , Let's start with the network card driver , Start the exploration of network subsystem .

Hardware environment

Raspberry pie 3B+, Equipped with Gigabit network card LAN7515, But it uses USB2.0 passageway （ The maximum theoretical speed is 480Mbps）, The official gave 300Mbps Ethernet transmission rate .

drive

LAN7515 The driver is lan78xx, The source code is relatively simple , Just two documents ./linux/drivers/net/usb/lan78xx.c/h, Compile to produce lan78xx.ko Driver file .

Drive analysis

1. USB Driver registration

static struct usb_driver lan78xx_driver = {
    
	.name			= DRIVER_NAME,
	.id_table		= products,
	.probe			= lan78xx_probe,
	.disconnect		= lan78xx_disconnect,
	.suspend		= lan78xx_suspend,
	.resume			= lan78xx_resume,
	.reset_resume		= lan78xx_reset_resume,
	.supports_autosuspend	= 1,
	.disable_hub_initiated_lpm = 1,
};

module_usb_driver(lan78xx_driver);

LAN7515 yes USB network card , and CPU The interface of interaction is USB, So it's a USB equipment , To register it in the kernel USB drive ——lan78xx.ko.
insmod lan78xx.ko That is to say USB Register on the bus LAN7515 Network card driver , here , Can traverse USB Devices registered on the bus , With bus match Method to determine whether there is a matching device , If there is , Then call the driven probe function .
Let's see in detail ：

/sys/bus/usb/ There are devices and drivers Catalog , These two directories are respectively in the current system USB On the bus Registered devices and Registered driver .
You can see , stay drivers There is no lan78xx drive , And we don't know devices Which in the directory USB The equipment is LAN7515 equipment .
Let's install lan78xx drive

See the execution lan78xx_probe() function , This also confirms our above statement （ Registration drive , Will traverse the device , Looking for a match , The match is successful , Trigger probe）.
Let's look at... Again /sys/bus/usb/drivers Directory under

You can see , Has produced lan78xx Catalog , Description: the driver registration is successful .

2. Driver and device binding

stay lan78xx_probe() Called in the function lan78xx_bind() Method , Officially will lan78xx Drive and 1-1.1.1:1.1 The device is bound . stay lan78xx This can also be reflected in the directory .

We can also bind and unbind manually

# ifconfig -a
lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:65536  Metric:1
          RX packets:42 errors:0 dropped:0 overruns:0 frame:0
          TX packets:42 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:4478 (4.3 KiB)  TX bytes:4478 (4.3 KiB)
# ls
bind       module     new_id     remove_id  uevent     unbind
# echo "1-1.1.1:1.0" > bind 
[37493.831496] lan78xx_probe()
[37494.126379] lan78xx 1-1.1.1:1.0 (unnamed net_device) (uninitialized): No External EEPROM. Setting MAC Speed
[37494.144756] libphy: lan78xx-mdiobus: probed
[37494.264385] lan78xx 1-1.1.1:1.0 (unnamed net_device) (uninitialized): int urb period 64
[37494.276093] lan78xx_phy_init()
# ls
1-1.1.1:1.0  module       remove_id    unbind
bind         new_id       uevent
# ifconfig -a
eth0      Link encap:Ethernet  HWaddr B8:27:EB:8A:BC:F4  
          BROADCAST MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:65536  Metric:1
          RX packets:42 errors:0 dropped:0 overruns:0 frame:0
          TX packets:42 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:4478 (4.3 KiB)  TX bytes:4478 (4.3 KiB)
# 
# 
# echo "1-1.1.1:1.0" > unbind 
# ls
bind       module     new_id     remove_id  uevent     unbind
# ifconfig -a
lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:65536  Metric:1
          RX packets:42 errors:0 dropped:0 overruns:0 frame:0
          TX packets:42 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:4478 (4.3 KiB)  TX bytes:4478 (4.3 KiB)
# 

3. Data transmission

With PC1 The data link layer sends a packet to PC2 Take the data link layer as an example

rx

tasklet_schedule(&dev->bh);	// USB  When data is received , The interrupt , Trigger the task 
lan78xx_bh()
    lan78xx_rx_bh()
        rx_submit()
            usb_fill_bulk_urb()	//  from  USB  Reading data 
            tasklet_schedule(&dev->bh);
                lan78xx_bh()
                    rx_process()
                        lan78xx_rx()
                            lan78xx_skb_return()
----- Above is the drive ------- The following is the data link layer -----------------------
                                netif_rx()	//  Send it to the data link layer 

tx

struct net_device_ops {
    
	.ndo_start_xmit		= lan78xx_start_xmit,
}
----- Above is the data link layer ------- The following is the drive -----------------------
lan78xx_start_xmit()
	skb_queue_tail()	//  Insert queue 
	tasklet_schedule(&dev->bh);	//  Trigger task scheduling  lan78xx_bh
		lan78xx_bh()
			lan78xx_tx_bh()
				skb_dequeue()	//  Take out the data 
------ The above is the drive network -------- The following is the drive  USB  part -----------
				usb_fill_bulk_urb()	//  Fill data to  USB
				usb_submit_urb()	// USB  send data 

RX call tree： At present, it's a little confused , The general process is USB Hardware interrupts , Drive to retrieve data , after rx_process() After processing , The final call netif_rx(skb), Send data to the kernel network subsystem , Further treatment skb.
TX call tree： The kernel network subsystem finally calls ndo_start_xmit() send data , The interface is finally implemented by the device driver , This example is lan78xx_start_xmit(), Finally, the data will be passed USB Send to LAN7515 network card , The network card is processed by its own hardware , Finally, the data is sent to the network cable .

Print frame header

static netdev_tx_t
lan78xx_start_xmit(struct sk_buff *skb, struct net_device *net)
{
    
printk("%s()\n", __FUNCTION__);

struct ethhdr *eth = eth_hdr(skb);
printk(KERN_INFO "DEST:" MAC_FMT "\n", MAC_ARG(eth->h_dest));
printk(KERN_INFO "SOURCE:" MAC_FMT "\n", MAC_ARG(eth->h_source));

struct iphdr *ip_header = ip_hdr(skb);
printk("daddr:%x is %d.%d.%d.%d\n", ip_header->daddr, inet_ntoa(ip_header->daddr));
printk("saddr:%x is %d.%d.%d.%d\n\n", ip_header->saddr, inet_ntoa(ip_header->saddr));
...

[44851.676380] lan78xx_start_xmit()
[44851.680906] DEST:08:00:27:38:fc:d0
[44851.685656] SOURCE:b8:27:eb:8a:bc:f4
[44851.690571] daddr:6401a8c0 is 192.168.1.100
[44851.696164] saddr:101a8c0 is 192.168.1.1

Print TX Direction , Drive the purpose in the data frame sent to the network card MAC、 Source MAC、 Purpose IP、 Source IP, The diagram above ① Data frame header at .

summary

network card 、 Network card driver 、 Kernel network subsystem , The core of what they do is to deliver data packets , among sk_buff In the whole process, it plays the role of threading , In the process of analyzing the network card drive sending and receiving , I also tried to print sk_buff Part of the information , As a beginning of exploration . Actually sk_buff It really plays a very important role in the network subsystem ,sk_buff yes socket buffer It means , This further reflects socket The importance of , Lianqi buffer All that NB,socket No more NB Do you ？ So the later research route is ：driver --> sk_buff --> socket.