《linux内核源码分析----dma.c》

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 
#include 
#include 


/* 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 */