最近在升级u-boot，从1.1.4升级到1.3.3，原因是想用Linux的MTD层和nand驱动，方便管理坏块。

可是一个问题困扰了一整天，NAND初始化总是不正确，经过排查，定位到读ID不正确。

4020这颗芯片读ID应该很简单的，只要发送0x80000090命令，再读取EMI的NAND_ID寄存器就行了，可我怎么用了一整天都读不正确呢？于是写了一段最简单的代码来验证：

首先是ADS的代码：
    IntInit();
    InitEmi();
   InitGpio();
    
*(unsigned long*)EMI_NAND_COM = 0X00000090;
*(unsigned long*)EMI_NAND_COM |= 0X80000000;

   tempnand = (*(volatile unsigned long*)EMI_NAND_ID)>>8;

发现读取是正确的。

然后是Linux下的代码：

*(unsigned long*)INTC_IMR_V = 0XFFFFFFFF;

*(unsigned long*)INTC_IMR_V = 0X00000000;

*(unsigned long*)EMI_NAND_CONF1_V = 0x06302857;

   *(unsigned long*)EMI_NAND_CONF2_V = 0x00514353;

   *(unsigned long*)GPIO_PORTG_SEL_V |= 0x0080;

   *(unsigned long*)GPIO_PORTG_DATA_V |= 0x0080;

*(unsigned long*)GPIO_PORTG_DIR_V &= ~0x0080;

*(unsigned long*)EMI_NAND_COM_V = 0X00000090;

*(unsigned long*)EMI_NAND_COM_V |= 0X80000000;

unsigned int tempnand = (*(volatile unsigned long*)EMI_NAND_ID_V)>>8;

printk("read id! id is 0x%x,0x%x\n\n",tempnand,EMI_NAND_ID_V );

启动时发现，tempnand的值为0；

最后是u-boot的代码：

*(unsigned long*)INTC_IMR_V = 0XFFFFFFFF;

*(unsigned long*)INTC_IMR_V = 0X00000000;

*(unsigned long*)EMI_NAND_CONF1_V = 0x06302857;

   *(unsigned long*)EMI_NAND_CONF2_V = 0x00514353;

   *(unsigned long*)GPIO_PORTG_SEL_V |= 0x0080;

   *(unsigned long*)GPIO_PORTG_DATA_V |= 0x0080;

*(unsigned long*)GPIO_PORTG_DIR_V &= ~0x0080;

*(unsigned long*)EMI_NAND_COM_V = 0X00000090;

*(unsigned long*)EMI_NAND_COM_V |= 0X80000000;

unsigned int tempnand = (EMI_NAND_ID)>>8;
printk("read id! id is 0x%x\n\n", tempnand);

打印的值更诡异，竟然是0x800000。

奇怪吧，容我解释逐条解释原因：

1）Linux的代码，在加上volatile后，顺利的读出了ID，难道这就是传说中的因为没加volatile，同一个寄存器赋值两次，被优化掉了！！！？？？

2）怀疑u-boot代码也是volatile原因，在添加后，依然不成功？

3）仔细看u-boot的寄存器定义：

#define EMI_NAND_ID      (REGW(EMI_BASE+0X130))

比Linux多了个REGW，这个REGW是什么东西？？？

翻到上面，赫然看到：

#define REGW(addr) (*(volatile unsigned int *)(addr))

晕，人家都加好了，再加一遍，数据能对就怪了！

附修改后的代码：

Linux:

*(volatile unsigned long*)INTC_IMR_V = 0XFFFFFFFF;

*(volatile unsigned long*)INTC_IMR_V = 0X00000000;

*(volatile unsigned long*)EMI_NAND_CONF1_V = 0x06302857;

   *(volatile unsigned long*)EMI_NAND_CONF2_V = 0x00514353;

   *(volatile unsigned long*)GPIO_PORTG_SEL_V |= 0x0080;

   *(volatile unsigned long*)GPIO_PORTG_DATA_V |= 0x0080;

*(volatile unsigned long*)GPIO_PORTG_DIR_V &= ~0x0080;

*(volatile unsigned long*)EMI_NAND_COM_V = 0X00000090;

*(volatile unsigned long*)EMI_NAND_COM_V |= 0X80000000;

unsigned int tempnand = (*(volatile unsigned long*)EMI_NAND_ID_V)>>8;

printk("read id! id is 0x%x,0x%x\n\n",tempnand,EMI_NAND_ID_V );

u-boot:

INTC_IMR = 0XFFFFFFFF;

INTC_IMR = 0X00000000;

EMI_NAND_CONF1 = 0x06302857;

   EMI_NAND_CONF2 = 0x00514353;

   GPIO_PORTG_SEL |= 0x0080;

   GPIO_PORTG_DATA |= 0x0080;

GPIO_PORTG_DIR &= ~0x0080;

EMI_NAND_COM = 0X00000090;
EMI_NAND_COM |= 0X80000000;

unsigned int tempnand = (EMI_NAND_ID)>>8;
printk("read id! id is 0x%x\n\n", tempnand);