ISSN: 2250-3005 - ijcer

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International Journal Of Computational Engineering Research (ijceronline.com) Vol. 2 Issue. 8mode. When CS8900A is configured to Memory Mode operation, its internal registers and frame buffer aremappedto a serial 4KB host memory block, the host can directly access CS8900A's internal registers and framebuffer through the block.5. Design And The Implementation Principle Of Network DriverLinux network system can complete data transfer between all levels through the socket buffersk_buff, data structure sk_buff is each protocol data processing object. sk_buff is the media of exchange databetween driver and network. When the driver sends data to the network, data source and data length must beobtained. Data must be saved in sk_buff also after the driver received data from thenetwork. Thus upper layerprotocol can process it.For the actual development of Ethernet driver, corresponding template program inthe kernel source tree can be consulted, focused on understanding the implementation principle of networkdriver and program structural framework. Then the code is rewritten to develop specific hardware, andachieve the appropriate operation function. Through transplanting and preparing the embeddedCS8900A network card driver on embedded development board (S3C2410 processor) to show theimplementation principles of network driver.A. Initialization FunctionInitialization of network equipment is completed mainly by initialization function which is referred byinit function pointer in device data structure. After the kernel load the network driver module, initializationprocess will be called. First it is necessary to detect whether network physical device exist, which iscompleted by detecting the physical device hardware characteristic. Then the resource equipment needed isconfigured, such as interrupts. Next the device„device‟ data structure is constructed. The relevant variable in thedevice is initialized by detected data, and finally thedevice is registered to the Linux kernel and applies memoryspace.In this instance, the initialization function is “_init cs8900a_s3c2410(void)”.In the network device driver, the device „device‟ data structure is dev_cs89x0, which is defined as follows.#ifdef MODULE static struct net_device dev_cs89x0={‟‟‟‟,0, 0, 0, 0,0, 00, 0, 0, NULL,NULL};The function „cs89x0_probe‟ detect the existence of the network physicaldevice, but device initialization is completed by two functions „cs89x0_probe‟ and„cs89x0_probe1‟together. In „cs89x0_probe1‟, the function pointers of „device‟ data structure are filled.After completion offilling pointer, to register by„register_netdev‟ (struct net_device * dev) function. Two functionregister_netdev‟ and „unregister_netdev‟ are defined in file „net_init.c ‟.Since there is „init‟ function, thereshould also be„cleanup‟ function, because they are essential function of each driver. The „cleanup‟ functionrun when module is unloaded, wh ich complete mainly the work of resource release. Such as cancel deviceregistration, free memory, release the port, etc. All in all, it is some action contrary to init.The function ofCancellation of network device registration is „unregister_netdevice‟ defined in file / net / core / dev.c, thisfunction is called in „unregister_netdev‟.B. Open FunctionWhen system response „ifconfig‟ command, a network interface will be opened (closed). The „ifconfig‟command given address to interface by calling ioctl‟ (SIOCSIFADDR). Response of SIOCSIFADDR isaccomplished by the kernel, and device-independent. Then, the „ifconfig‟ command set IFF_UP bit of dev-> flagto open the device by calling „ioctl‟ (SIOCSIFFLAGS). The device's open method is called through the abovecalled. In the network device driver, Open function is called when network device is activated, that devicestatus becomes from down to up. So a lot of initialization work can be done here. In open function, operation onthe register uses twofunctions: „readreg‟ and „writereg‟. „readreg‟ function is used to read the register contents, „writereg‟ function isused to write registers, the code is as follows:inline int readreg(struct net_ device *dev,int portno){outw(portno,dev->base_addr+ADD_PORT);return inw(dev->base_addr+DATA_PORT);}inline void writereg(struct net_ device*dev,int portno,int value){ outw(portno,dev->base_addr+ADD_PORT); outw(value,dev->base_addr+DATA_PORT);}C. Close FunctionClose function (net_close) releases resources to reduce system burden. It is called when the devicestatus becomes from up to down. In addition, if the driver is loaded as a module, the macroMOD_DEC_USE_COUNT need to call in close also, to reduce the frequency of equipment cited in order touninstall the driver.||Issn 2250-3005(online)|| ||December|| 2012 Page 281
International Journal Of Computational Engineering Research (ijceronline.com) Vol. 2 Issue. 8D. Send FunctionSending and receiving of data packets are two key processes achieving Linux network driver,good or bad of the two processes affected directly the driver's overall running quality. First of all, whenthe network device driver is loaded, device is initialized by init function pointer in the device domain callingnetwork device in itialization function. If the operation is successful, device is opened by open function pointerin the device domain calling network device open function. Next, hardware packet header information is set up bybuilding hardware packet header function pointer hard_header in the device domain. Finally, the packet sent iscompleted through protocol interface layer function dev_queue_xmit calling function pointerhard_start_xmit in the device domain. If sent successfully, sk_buff is released in hard_start_xmitmethod, and return 0 (send success). If the device can not be process temporarily, such as the hardwareis busy,and then return 1. At this time if the dev-> tbusy is set to non-zero, the system regards that the hardware is busy, itwill not re-send until dev-> tbusy is set to 0. tbusy's setting zero task is completed by interrupt generally.Hardware interrupt at the end of sent, at this time tbusy can be set to 0, then call mark_bh () to notify system to resent.In the case of sending unsuccessful, dev-> tbusy can not be set to non- zero, this system will try toresent continuously. If hard_start_xmit is not sent successful, then sk_buff cannot be release. The datawhich is sent in Sk_buff already contains the frame head of hardware requirement. Hardware frame head neednot be fill in send method, data can be sent directly to the hardware. sk_buff is locked, and it is assured that otherprograms will not access it.E. Receive FunctionReceive function is different from receive data, network interface does not provide receive function pointersimilar to net_receive_packet, because that network device receive data is achieved through interrupts. Uponreceiving the information, an interrupt is generated, the driver will apply a sk_buff (skb) in interrupt handler, andthe data read from hardware is placed to applied buffer. Next, some information is filled in sk_buff. skb-> dev =dev judges protocol type ofreceived frame, and fills in the skb-> protocol (multi- protocol support). Pointerskb-> mac.raw point to the hardware data and then discard the hardware frame head (skb_pull). Also skb->pkt_type is set, indicating the type of data link layer. If the data packet is obtained, then net_rx() isimplemented, net_rx () is a subroutines of data reception, it is called by interrupt service routine. Last the data issent to the protocol layer by calling netif_rx (). netif_rx () put data into the processing queue and then return, thereal processing will be implemented after interrupted return. Thus interrupt time is reduced. After netif_rx ()is called, the driver cannot be saved in data buffer skb. In the protocol layer, flows control of receiving datapackets is divided into two levels: first, netif_rx () function is limited the number of data frames from the physicallayer to protocol layer. Secondly, each socket has a queue, which limits the number of data frames from theprotocol layer to the socket layer. In transmission, the dev-> tx_queue_lenparameter in driver limits the length of the queue.F. Interrupt HhandlerIn the open function, the interrupt is applied, the interrupt handler is net_interrupt. The preparation ofwriting this function is to understand the interrupt process of network control chip. For the CS8900A chip, thisprocess can explain the flow chart using fig. 2.First kernel need to read ISQ (Interrupt Status Queue)value, ISQ event has the following 5 types:#define ISQ_RECEIVER_EVENT0x04#define ISQ_TRANSMITTER_EVENT 0x08#define ISQ_BUFFER_EVENT0x0c#define ISQ_RX_MISS_EVENT0x10#define ISQ_TX_COL_EVENTAfter receiving a data packet (RxEvent), to deliverfunction net_rx() to process. Interrupt handler parameter is set as follows:Static void net_interrupt(int irq,void *dev_id ,structpt_regs *regs0x12||Issn 2250-3005(online)|| ||December|| 2012 Page 282
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ISSN: 2250-3005 - ijcer

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