uboot下自动检测sdram大小

函数如下：
/*
 * Check memory range for valid RAM. A simple memory test determines
 * the actually available RAM size between addresses `base' and
 * `base + maxsize'.
 */
long get_ram_size(long *base, long maxsize)
{
volatile long *addr;
long           save[32];
long           cnt;
long           val;
long           size;
int            i = 0;


for (cnt = (maxsize / sizeof (long)) >> 1; cnt > 0; cnt >>= 1) {
addr = base + cnt; /* pointer arith! */
sync ();
save[i++] = *addr;
sync ();
*addr = ~cnt;
}


addr = base;
sync ();
save[i] = *addr;
sync ();
*addr = 0;


sync ();
if ((val = *addr) != 0) {
/* Restore the original data before leaving the function.
*/
sync ();
*addr = save[i];
for (cnt = 1; cnt < maxsize / sizeof(long); cnt <<= 1) {
addr  = base + cnt;
sync ();
*addr = save[--i];
}
return (0);
}


for (cnt = 1; cnt < maxsize / sizeof (long); cnt <<= 1) {
addr = base + cnt; /* pointer arith! */
val = *addr;
*addr = save[--i];
if (val != ~cnt) {
size = cnt * sizeof (long);
/* Restore the original data before leaving the function.
*/
for (cnt <<= 1; cnt < maxsize / sizeof (long); cnt <<= 1) {
addr  = base + cnt;
*addr = save[--i];
}
return (size);
}
}


return (maxsize);
}

解释说明：
1.从最大地址>>1开始，依次地址右移1，写入~cnt。
2.从最小地址读出比较。
3.没比较过一个size增大一倍。

具体例子如下：假设设定最大地址为1G，实际地址只有256M。
1.第一次写入512M地址处为：~512M=FFFFFFFFDFFFFFFF，由于没有512M，所以写入的数据到了256M地址处。
2.第二次写入256M地址处为：~256M=FFFFFFFFEFFFFFFF，实际写到256M处的为当前数据。
3.第三次写入128M地址处为：~128M=FFFFFFFFF7FFFFFF
一次类推；
在反向检测时，会出现256M地址处正常，512M地址处有问题。进而可以监测出地址为256M。