Apex中DMA的代碼.代碼主要來自LINUX.:)其實這些代碼已經都比較成熟了.

#include "dma.h"

/* 如何使用:
 *
 * 調用request_dma請求指定的DMA通道.如果返回0表現該通道可用;
 * 在使用完畢以後使用free_dma釋放該通道.
 */

 

/* dma_chan_busy[n] != 0 表示該通道不可用
 * DMA0 用作DRAM的重新整理.
 * DMA4 用作級連.
 */
static volatile unsigned int dma_chan_busy[MAX_DMA_CHANNELS] = {
 1, 0, 0, 0, 1, 0, 0, 0
};

 

/* 將*p指向的資料替換為newval.由關鍵字xchgl看，這個值是32位的值。
 * xchgl是原子操作，可以省去cli和sti,穩定和效能都有益的.
 */
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 */

#define EINVAL 1
#define EBUSY 2

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 */

#define printk
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 */