dma.c

GNU Mach 微内核源代码, 基于美国卡内基美隆大学的 Mach 研究项目

/* $Id: dma.c,v 1.1 1999/04/26 05:49:35 tb Exp $
 * linux/kernel/dma.c: A DMA channel allocator. Inspired by linux/kernel/irq.c.
 *
 * Written by Hennus Bergman, 1992.
 *
 * 1994/12/26: Changes by Alex Nash to fix a minor bug in /proc/dma.
 *   In the previous version the reported device could end up being wrong,
 *   if a device requested a DMA channel that was already in use.
 *   [It also happened to remove the sizeof(char *) == sizeof(int)
 *   assumption introduced because of those /proc/dma patches. -- Hennus]
 */

#define MACH_INCLUDE
#include <linux/kernel.h>
#include <linux/errno.h>
#include <asm/dma.h>
#include <asm/system.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].lock != 0.
 * DMA0 used to be reserved for DRAM refresh, but apparently not any more...
 * DMA4 is reserved for cascading.
 */

struct dma_chan
{
  int  lock;
  const char *device_id;
};

static struct dma_chan dma_chan_busy[MAX_DMA_CHANNELS] =
{
  { 0, 0 },
  { 0, 0 },
  { 0, 0 },
  { 0, 0 },
  { 1, "cascade" },
  { 0, 0 },
  { 0, 0 },
  { 0, 0 }
};

#ifndef MACH
int
get_dma_list (char *buf)
{
  int i, len = 0;
  
  for (i = 0 ; i < MAX_DMA_CHANNELS ; i++)
    {
      if (dma_chan_busy[i].lock)
	{
	  len += linux_sprintf (buf+len, "%2d: %s\n",
				i,
				dma_chan_busy[i].device_id);
	}
    }
  return len;
} /* get_dma_list */
#endif

int
request_dma (unsigned int dmanr, const char *device_id)
{
  if (dmanr >= MAX_DMA_CHANNELS)
    return -LINUX_EINVAL;
  
  if (xchg (&dma_chan_busy[dmanr].lock, 1) != 0)
    return -LINUX_EBUSY;
  
  dma_chan_busy[dmanr].device_id = device_id;
  
  /* 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 (xchg (&dma_chan_busy[dmanr].lock, 0) == 0)
    {
      printk ("Trying to free free DMA%d\n", dmanr);
      return;
    }	
} /* free_dma */