1 引言
20世紀(jì)90年代后期，嵌入式系統(tǒng)在工業(yè)控制、遠(yuǎn)程監(jiān)控和數(shù)據(jù)采集等領(lǐng)域的應(yīng)用日趨廣泛，人們對(duì)嵌入式系統(tǒng)的存儲(chǔ)容量也提出了較高的要求。因此研制適用于嵌入式系統(tǒng)的大容量、高速率、高可靠性的數(shù)據(jù)存儲(chǔ)系統(tǒng)變得日益重要。本文針對(duì)一款基于ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" 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title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM" title="ARM">ARM920T芯片的開(kāi)發(fā)板，根據(jù)ATA硬盤接口規(guī)范，設(shè)計(jì)了IDE硬盤接口電路，實(shí)現(xiàn)了對(duì)IDE硬盤的讀寫，可以在Linux系統(tǒng)中對(duì)其上的文件系統(tǒng)自由訪問(wèn)，達(dá)到了高速率和高可靠性的要求。
2 ARM920T與S3C2410介紹
ARM 包括一系列微處理芯片技術(shù)。ARM920T是ARM系列微處理器的一種，它采用5階段管道化ARM9TDMI內(nèi)核，同時(shí)配備了Thumb擴(kuò)展、EmbeddedICE調(diào)試技術(shù)和Harvard總線。在生產(chǎn)工藝相同的情況下，性能可達(dá)ARM7TDMI芯片的兩倍之多。ARM920T系列主要應(yīng)用于機(jī)頂盒產(chǎn)品、掌上電腦、筆記本電腦和打印機(jī)。
S3C2410處理器是Samsung 公司基于ARM公司的ARM920T處理器核，采用0.18umSU造工藝的32位微控制器。該處理器擁有獨(dú)立的16KB指令Cache和16KB數(shù)據(jù)Cache、MMU、支持TFT的LCD控制器、NAND閃存控制器、3路UART、4路DMA、4路帶PWM的Timer、I/O口、RTC、8路10位ADC、TouchScreen接口、IIC-BuS接口、IIS-BuS接口、2個(gè)USB主機(jī)、1個(gè)USB設(shè)備、SD主機(jī)和MMC接口和2路SPI。S3C2410處理器最高可運(yùn)行在268MHz。
3 IDE接口及其規(guī)范
IDE(Integrated Drive Electronics)是從IBM PC/AT上使用的ATA接口發(fā)展而來(lái)的。IDE/ATA磁盤驅(qū)動(dòng)器與早期的ATA驅(qū)動(dòng)器相比，增加了任務(wù)文件寄存器，包括數(shù)據(jù)寄存器、狀態(tài)寄存器以及反映地址的驅(qū)動(dòng)器號(hào)、磁頭號(hào)、道號(hào)和扇區(qū)號(hào)寄存器等。ATA接口規(guī)范定義了信號(hào)電纜和電源線的電器特征、互聯(lián)信號(hào)的電器和邏輯特征，還定義了存儲(chǔ)設(shè)備中可操作的寄存器以及命令和協(xié)議。
3.1 寄存器
規(guī)范定義了兩組寄存器：命令寄存器和控制寄存器。命令寄存器用來(lái)接收命令和傳送數(shù)據(jù)，控制寄存器用來(lái)控制磁盤操作。常用的寄存器包括數(shù)據(jù)寄存器、命令寄存器、驅(qū)動(dòng)器/磁頭寄存器、柱面號(hào)寄存器、扇區(qū)號(hào)寄存器、扇區(qū)數(shù)寄存器和狀態(tài)寄存器。
3.2 數(shù)據(jù)傳輸方式
ATA接口規(guī)范定義了兩種數(shù)據(jù)傳輸方式：可編程I/O(PIO)方式和DMA方式。PIO傳送方式下，CPU對(duì)控制器的訪問(wèn)都是通過(guò)PIO進(jìn)行的，包括從控制器讀取狀態(tài)信息和錯(cuò)誤信息，以及向控制器發(fā)送命令和參數(shù)。在一次PIO數(shù)據(jù)傳輸過(guò)程中，CPU先選址，然后使讀/寫信號(hào)有效，CPU或控制器放數(shù)據(jù)到數(shù)據(jù)總線，控制器或CPU讀取數(shù)據(jù)，操作完成后，釋放總線，這樣一次數(shù)據(jù)傳輸完成。DMA方式，即直接內(nèi)存訪問(wèn)，CPU把緩沖區(qū)的地址與需要讀寫的長(zhǎng)度告訴外設(shè)，外設(shè)在準(zhǔn)備好后向CPU發(fā)出一個(gè)DMA請(qǐng)求，要求CPU暫停使用內(nèi)存，獲得同意后就直接在內(nèi)存和外設(shè)之間傳輸數(shù)據(jù)，完成后再把對(duì)內(nèi)存的訪問(wèn)權(quán)歸還給CPU。
4 硬件實(shí)現(xiàn)
如圖1所示，S3C2410與硬盤之間接口電路分為3個(gè)部分：片選信號(hào)、數(shù)據(jù)信號(hào)和控制信號(hào)。硬盤上寄存器分為兩組，分別由IDE_CS0和IDE_CS1選中，DA0~DA2則用于組內(nèi)寄存器尋址；數(shù)據(jù)線DD0~DD15因存在輸入/輸出方向問(wèn)題，故用nOE(讀信號(hào))接buffer(74LVTH162245)的DIR引腳來(lái)控制緩沖器方向；控制信號(hào)部分因該CPU與硬盤之間DMA時(shí)序不一致，故采用一塊EPM7032AETC44-7芯片用于調(diào)整其時(shí)序。PIO模式下，不需要DMARQ和nDMACK信號(hào)，DMA模式下，這兩個(gè)信號(hào)才起作用。
5 軟件實(shí)現(xiàn)
硬盤驅(qū)動(dòng)程序?qū)崿F(xiàn)分為初始化、打開(kāi)設(shè)備、設(shè)備I/O操作和釋放設(shè)備等幾部分。
5.1 硬盤初始化
與X86不同，在ARM 體系結(jié)構(gòu)中，對(duì)內(nèi)存和外設(shè)的訪問(wèn)使用統(tǒng)一的指令，所以要對(duì)外設(shè)地址進(jìn)行內(nèi)存映射。也就是說(shuō)，通過(guò)一張表將I/O地址映射到內(nèi)存空間中來(lái)，這部分工作是在系統(tǒng)初始化期間完成的。
在IDE子系統(tǒng)初始化期間，Linux系統(tǒng)一旦發(fā)現(xiàn)一個(gè)IDE控制器，就設(shè)置它的ide_hwif_t結(jié)構(gòu)來(lái)反映這個(gè)控制器和與之相連的磁盤；向Linux的VFS登記每一個(gè)控制器，并分別把它加到blk_dev和blkdevs向量表中；請(qǐng)求控制適當(dāng)?shù)腎RQ中斷(主IDE控制器是14，次IDE控制器是15)和I/O空間(主控制器0x1f0，次控制器0x170)：為每一個(gè)找到的IDE控制器在gendisk列表中增加一個(gè)條目。
IDE硬盤的初始化工作由idedisk_init完成：
(1)在數(shù)組ide_hwifs中找出已登記得各IDE控制器控制的所有IDE硬盤(一個(gè)控制器最多控制兩個(gè)硬盤)，每個(gè)IDE硬盤對(duì)應(yīng)一個(gè)ide_drive_t結(jié)構(gòu)。
(2)對(duì)找到的每個(gè)IDE硬盤，調(diào)用函數(shù)ide_register_subdriver填寫各IDE硬盤結(jié)構(gòu)中的相關(guān)信息域，主要是填寫其驅(qū)動(dòng)程序結(jié)構(gòu)ide_driver_t。硬盤驅(qū)動(dòng)中的函數(shù)do_rw_disk通過(guò)向磁盤控制寄存器寫參數(shù)啟動(dòng)磁盤操作，完成真正的數(shù)據(jù)讀寫。
(3)對(duì)找到的每個(gè)IDE硬盤，調(diào)用函數(shù)idedisk_setup進(jìn)一步設(shè)置其ide_drive_t結(jié)構(gòu)，包括設(shè)置該結(jié)構(gòu)的settings域、doorlocking域、cyl、head、sect域、id域等。
5.2 打開(kāi)設(shè)備
打開(kāi)塊設(shè)備的操作與打開(kāi)普通文件的操作基本相同。
(1)在當(dāng)前進(jìn)程的文件描述符表中為打開(kāi)文件找一個(gè)空位，申請(qǐng)一塊內(nèi)存，用于建立新文件的打開(kāi)文件對(duì)象，即結(jié)構(gòu)file。
(2)解析設(shè)備特殊文件名，獲得其VFSinode和dentry結(jié)構(gòu)，根據(jù)dentry結(jié)構(gòu)填寫file結(jié)構(gòu)，尤其是將file結(jié)構(gòu)的f_op域設(shè)為其VFSinode中的缺省文件操作。
(3)執(zhí)行該文件操作集中的open操作，即blkdev_open，它根據(jù)設(shè)備特殊文件的主次設(shè)備號(hào)從blkdevs向量表中取出已經(jīng)注冊(cè)的文件操作集(file_operations)fops，用該結(jié)構(gòu)代替file結(jié)構(gòu)中的f_op域。
(4)執(zhí)行中新文件操作集中的open操作，即bl帶頭kdev_open，它根據(jù)VFS
inode中的i_rdev域查找數(shù)組ide_hwifsp[]，從中找出該IDE硬盤所對(duì)應(yīng)的ide_drive_t結(jié)構(gòu)；如果ide_drive_t結(jié)構(gòu)中注冊(cè)有驅(qū)動(dòng)程序，執(zhí)行驅(qū)動(dòng)程序集中的open操作。
(5)將打開(kāi)文件對(duì)象插入到當(dāng)前進(jìn)程的文件描述符表中，返回文件描述符，即打開(kāi)文件對(duì)象在進(jìn)程文件描述符表中的索引。
5.3 設(shè)備I/O操作
讀寫塊設(shè)備時(shí)要用到塊緩沖區(qū)(bufer)，對(duì)bufer的管理采用BuferCache機(jī)制。它管理bufer的創(chuàng)建、撤銷、回收、查找、更新等，同時(shí)還要與系統(tǒng)中的其它部分(如文件系統(tǒng)、內(nèi)存管理等)交互。Linux將Buffer
Cache從塊設(shè)備驅(qū)動(dòng)程序中獨(dú)立出來(lái)，作為對(duì)塊設(shè)備讀寫的通用機(jī)制，所以對(duì)塊設(shè)備的讀、寫、同步等操作采用的都是由操作系統(tǒng)提供的公共函數(shù)，一般為block_read()和block_write()。
為了減少對(duì)塊設(shè)備操作的次數(shù)，讀寫塊設(shè)備時(shí)采用延遲操作，盡量將多個(gè)讀寫操作合并，所以操作請(qǐng)求不是馬上遞交給物理設(shè)備，而是提供了一種手段記錄每次的請(qǐng)求(request)，并為每類塊設(shè)備提供一個(gè)請(qǐng)求隊(duì)列用來(lái)排隊(duì)、合并、重組對(duì)該塊設(shè)備的請(qǐng)求。
當(dāng)需要從硬盤讀時(shí)，block_read()函數(shù)首先查找Buffer Cache
如果在其中能找到需要的buffer，則立刻返回：否則，生成一個(gè)讀請(qǐng)求，并將其加入相應(yīng)的請(qǐng)求隊(duì)列排隊(duì)。
當(dāng)需要向硬盤寫時(shí)，block_write()為此次寫操作生成一個(gè)buffer，而后生成一個(gè)寫請(qǐng)求，并將其加入相應(yīng)的請(qǐng)求隊(duì)列排隊(duì)。
塊設(shè)備驅(qū)動(dòng)程序提供了一個(gè)請(qǐng)求處理函數(shù)，對(duì)硬盤而言是函數(shù)do_rw_disk。在適當(dāng)?shù)臅r(shí)候，硬盤的請(qǐng)求處理函數(shù)啟動(dòng)，do_rw_disk處理在請(qǐng)求隊(duì)列上排隊(duì)的請(qǐng)求，通過(guò)向硬盤發(fā)出讀、寫命令完成對(duì)設(shè)備的真正操作。其偽代碼如下：
DO_RW_DISK(COMMAND)
{
Set_Registers()；
if(COMMAND=READ){
Set read_intr as interrupt process function
Send WIN_READ or WIN_MULTREAD
command to Command register
}
if(COMMAND=WRITE){
Send WIN_WRITE or
WIN_MULTWRITE command to Command register
Get the status of Status register
and set DRQ bit
Set write intr as interrupt process function
Senddatato buferin thedisk
}
}
5.4 釋放設(shè)備
由設(shè)備驅(qū)動(dòng)程序中的release操作完成，一般完成與打開(kāi)設(shè)備相反的動(dòng)作：釋放打開(kāi)設(shè)備特殊文件時(shí)在file結(jié)構(gòu)上所創(chuàng)建的私有結(jié)構(gòu)；如果是最后一個(gè)設(shè)備的釋放，則從硬件上關(guān)閉設(shè)備。
6 結(jié)束語(yǔ)
通過(guò)上述方法對(duì)IDE硬盤接口的設(shè)計(jì)與實(shí)現(xiàn)，我們可以在S3C2410開(kāi)發(fā)板上安全自由地對(duì)硬盤上的各種文件系統(tǒng)進(jìn)行訪問(wèn)，由于采用DMA方式，可以滿足用戶對(duì)速率的要求。