kernel/dma.c

   1 /* $Id: dma.c,v 1.7 1994/12/28 03:35:33 root Exp root $
   2  * linux/kernel/dma.c: A DMA channel allocator. Inspired by linux/kernel/irq.c.
   3  *
   4  * Written by Hennus Bergman, 1992.
   5  *
   6  * 1994/12/26: Changes by Alex Nash to fix a minor bug in /proc/dma.
   7  *   In the previous version the reported device could end up being wrong,
   8  *   if a device requested a DMA channel that was already in use.
   9  *   [It also happened to remove the sizeof(char *) == sizeof(int)
  10  *   assumption introduced because of those /proc/dma patches. -- Hennus]
  11  */
  12
  13 #include <linux/kernel.h>
  14 #include <linux/errno.h>
  15 #include <asm/dma.h>
  16 #include <asm/system.h>
  17 #include <asm/spinlock.h>
  18
  19
  20 /* A note on resource allocation:
  21  *
  22  * All drivers needing DMA channels, should allocate and release them
  23  * through the public routines `request_dma()' and `free_dma()'.
  24  *
  25  * In order to avoid problems, all processes should allocate resources in
  26  * the same sequence and release them in the reverse order.
  27  *
  28  * So, when allocating DMAs and IRQs, first allocate the IRQ, then the DMA.
  29  * When releasing them, first release the DMA, then release the IRQ.
  30  * If you don't, you may cause allocation requests to fail unnecessarily.
  31  * This doesn't really matter now, but it will once we get real semaphores
  32  * in the kernel.
  33  */
  34
  35
  36 spinlock_t dma_spin_lock = SPIN_LOCK_UNLOCKED;
  37
  38
  39 /* Channel n is busy iff dma_chan_busy[n].lock != 0.
  40  * DMA0 used to be reserved for DRAM refresh, but apparently not any more...
  41  * DMA4 is reserved for cascading.
  42  */
  43
  44 struct dma_chan {
  45         int  lock;
  46         const char *device_id;
  47 };
  48
  49 static struct dma_chan dma_chan_busy[MAX_DMA_CHANNELS] = {
  50         { 0, 0 },
  51         { 0, 0 },
  52         { 0, 0 },
  53         { 0, 0 },
  54         { 1, "cascade" },
  55         { 0, 0 },
  56         { 0, 0 },
  57         { 0, 0 }
  58 };
  59
  60 int get_dma_list(char *buf)
  61 {
  62         int i, len = 0;
  63
  64         for (i = 0 ; i < MAX_DMA_CHANNELS ; i++) {
  65                 if (dma_chan_busy[i].lock) {
  66                     len += sprintf(buf+len, "%2d: %s\n",
  67                                    i,
  68                                    dma_chan_busy[i].device_id);
  69                 }
  70         }
  71         return len;
  72 } /* get_dma_list */
  73
  74
  75 int request_dma(unsigned int dmanr, const char * device_id)
  76 {
  77         if (dmanr >= MAX_DMA_CHANNELS)
  78                 return -EINVAL;
  79
  80         if (xchg(&dma_chan_busy[dmanr].lock, 1) != 0)
  81                 return -EBUSY;
  82
  83         dma_chan_busy[dmanr].device_id = device_id;
  84
  85         /* old flag was 0, now contains 1 to indicate busy */
  86         return 0;
  87 } /* request_dma */
  88
  89
  90 void free_dma(unsigned int dmanr)
  91 {
  92         if (dmanr >= MAX_DMA_CHANNELS) {
  93                 printk("Trying to free DMA%d\n", dmanr);
  94                 return;
  95         }
  96
  97         if (xchg(&dma_chan_busy[dmanr].lock, 0) == 0) {
  98                 printk("Trying to free free DMA%d\n", dmanr);
  99                 return;
 100         }
 101
 102 } /* free_dma */