linux0.11、linux1.0、linux1.3、linux2.0、linux2.6
分别是1万多行、16万行、32万行、70万行、480万行c代码。
1.0带vfs和proc文件系统，内核的基本模型是没有多少变化。因此用于讲解kernel源码比较适合。
kernel一级源码目录：
[root@bogon linux-1.0]# ll
total 92
drwxr-xr-x. 2 root root 53 Nov 30 2001 boot
-rw-r--r--. 1 root root 6424 Mar 14 1994 CHANGES
-rw-r--r--. 1 root root 4903 Mar 13 1994 config.in
-rw-r--r--. 1 root root 6139 Mar 21 22:58 Configure
-rw-r--r--. 1 root root 18809 Dec 1 1993 COPYING
-rw-r--r--. 1 root root 13732 Mar 14 1994 CREDITS
drwxr-xr-x. 8 root root 98 Nov 30 2001 drivers
drwxr-xr-x. 12 root root 4096 Nov 30 2001 fs
drwxr-xr-x. 2 root root 39 Nov 30 2001 ibcs
drwxr-xr-x. 4 root root 30 Nov 30 2001 include
drwxr-xr-x. 2 root root 20 Nov 30 2001 init
drwxr-xr-x. 2 root root 75 Nov 30 2001 ipc
drwxr-xr-x. 2 root root 4096 Nov 30 2001 kernel
drwxr-xr-x. 2 root root 202 Nov 30 2001 lib
-rw-r--r--. 1 root root 7189 Mar 14 1994 Makefile
-rw-r--r--. 1 root root 179 Dec 1 1993 makever.sh
drwxr-xr-x. 2 root root 100 Nov 30 2001 mm
drwxr-xr-x. 4 root root 90 Nov 30 2001 net
-rw-r--r--. 1 root root 8734 Mar 14 1994 README
drwxr-xr-x. 2 root root 38 Nov 30 2001 tools
drwxr-xr-x. 2 root root 159 Nov 30 2001 zBoot
[root@bogon linux-1.0]#
/kernel/dma.c里面只做了一件事情：资源管理。
早期的cpu中dma管理器硬件资源不是那么强大，默认只有8个通道。所以是用数组来标识资源。比较新的内核源码是用链表来做资源管理。
#define MAX_DMA_CHANNELS 8
static volatile unsigned int dma_chan_busy[MAX_DMA_CHANNELS]; 每个数组元素对应一路dma，为0标识空闲，不为0标识资源已经被占用
mutex_atomic_swap(volatile unsigned int * p, unsigned int newval)函数里面内嵌汇编码，利用xchgl汇编指令（对应cpu的一个特殊电路，保证原子操作）原子的设置变量的值。如果p指向的值等于newval，则返回0设置不成功，如果*p不等于newval，则返回1。*p的值总是等于newval。
以下是dma.c的源码：
/* $Id: dma.c,v 1.5 1992/11/18 02:49:05 root Exp root $
* linux/kernel/dma.c: A DMA channel allocator. Inspired by linux/kernel/irq.c.
* Written by Hennus Bergman, 1992.
*/
#include <linux/kernel.h>
#include <linux/errno.h>
#include <asm/dma.h>
/* A note on resource allocation:
*
* All drivers needing DMA channels, should allocate and release them
* through the public routines `request_dma()' and `free_dma()'.
*
* In order to avoid problems, all processes should allocate resources in
* the same sequence and release them in the reverse order.
*
* So, when allocating DMAs and IRQs, first allocate the IRQ, then the DMA.
* When releasing them, first release the DMA, then release the IRQ.
* If you don't, you may cause allocation requests to fail unnecessarily.
* This doesn't really matter now, but it will once we get real semaphores
* in the kernel.
*/
/* Channel n is busy iff dma_chan_busy[n] != 0.
* DMA0 is reserved for DRAM refresh, I think.
* DMA4 is reserved for cascading (?).
*/
static volatile unsigned int dma_chan_busy[MAX_DMA_CHANNELS] = {
1, 0, 0, 0, 1, 0, 0, 0
};
/* Atomically swap memory location [32 bits] with `newval'.
* This avoid the cli()/sti() junk and related problems.
* [And it's faster too :-)]
* Maybe this should be in include/asm/mutex.h and be used for
* implementing kernel-semaphores as well.
*/
static __inline__ unsigned int mutex_atomic_swap(volatile unsigned int * p, unsigned int newval)
{
unsigned int semval = newval;
/* If one of the operands for the XCHG instructions is a memory ref,
* it makes the swap an uninterruptible RMW cycle.
*
* One operand must be in memory, the other in a register, otherwise
* the swap may not be atomic.
*/
asm __volatile__ ("xchgl %2, %0\n"
: /* outputs: semval */ "=r" (semval)
: /* inputs: newval, p */ "0" (semval), "m" (*p)
); /* p is a var, containing an address */
return semval;
} /* mutex_atomic_swap */
int request_dma(unsigned int dmanr)
{
if (dmanr >= MAX_DMA_CHANNELS)
return -EINVAL;
if (mutex_atomic_swap(&dma_chan_busy[dmanr], 1) != 0)
return -EBUSY;
else
/* old flag was 0, now contains 1 to indicate busy */
return 0;
} /* request_dma */
void free_dma(unsigned int dmanr)
{
if (dmanr >= MAX_DMA_CHANNELS) {
printk("Trying to free DMA%d\n", dmanr);
return;
}
if (mutex_atomic_swap(&dma_chan_busy[dmanr], 0) == 0)
printk("Trying to free free DMA%d\n", dmanr);
} /* free_dma */
