kernel/dma.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  * linux/kernel/dma.c: A DMA channel allocator. Inspired by linux/kernel/irq.c.
   4  *
   5  * Written by Hennus Bergman, 1992.
   6  *
   7  * 1994/12/26: Changes by Alex Nash to fix a minor bug in /proc/dma.
   8  *   In the previous version the reported device could end up being wrong,
   9  *   if a device requested a DMA channel that was already in use.
  10  *   [It also happened to remove the sizeof(char *) == sizeof(int)
  11  *   assumption introduced because of those /proc/dma patches. -- Hennus]
  12  */
  13 #include <linux/export.h>
  14 #include <linux/kernel.h>
  15 #include <linux/errno.h>
  16 #include <linux/spinlock.h>
  17 #include <linux/string.h>
  18 #include <linux/seq_file.h>
  19 #include <linux/proc_fs.h>
  20 #include <linux/init.h>
  21 #include <asm/dma.h>
  22
  23
  24
  25 /* A note on resource allocation:
  26  *
  27  * All drivers needing DMA channels, should allocate and release them
  28  * through the public routines `request_dma()' and `free_dma()'.
  29  *
  30  * In order to avoid problems, all processes should allocate resources in
  31  * the same sequence and release them in the reverse order.
  32  *
  33  * So, when allocating DMAs and IRQs, first allocate the IRQ, then the DMA.
  34  * When releasing them, first release the DMA, then release the IRQ.
  35  * If you don't, you may cause allocation requests to fail unnecessarily.
  36  * This doesn't really matter now, but it will once we get real semaphores
  37  * in the kernel.
  38  */
  39
  40
  41 DEFINE_SPINLOCK(dma_spin_lock);
  42
  43 /*
  44  *      If our port doesn't define this it has no PC like DMA
  45  */
  46
  47 #ifdef MAX_DMA_CHANNELS
  48
  49
  50 /* Channel n is busy iff dma_chan_busy[n].lock != 0.
  51  * DMA0 used to be reserved for DRAM refresh, but apparently not any more...
  52  * DMA4 is reserved for cascading.
  53  */
  54
  55 struct dma_chan {
  56         int  lock;
  57         const char *device_id;
  58 };
  59
  60 static struct dma_chan dma_chan_busy[MAX_DMA_CHANNELS] = {
  61         [4] = { 1, "cascade" },
  62 };
  63
  64
  65 /**
  66  * request_dma - request and reserve a system DMA channel
  67  * @dmanr: DMA channel number
  68  * @device_id: reserving device ID string, used in /proc/dma
  69  */
  70 int request_dma(unsigned int dmanr, const char * device_id)
  71 {
  72         if (dmanr >= MAX_DMA_CHANNELS)
  73                 return -EINVAL;
  74
  75         if (xchg(&dma_chan_busy[dmanr].lock, 1) != 0)
  76                 return -EBUSY;
  77
  78         dma_chan_busy[dmanr].device_id = device_id;
  79
  80         /* old flag was 0, now contains 1 to indicate busy */
  81         return 0;
  82 } /* request_dma */
  83
  84 /**
  85  * free_dma - free a reserved system DMA channel
  86  * @dmanr: DMA channel number
  87  */
  88 void free_dma(unsigned int dmanr)
  89 {
  90         if (dmanr >= MAX_DMA_CHANNELS) {
  91                 printk(KERN_WARNING "Trying to free DMA%d\n", dmanr);
  92                 return;
  93         }
  94
  95         if (xchg(&dma_chan_busy[dmanr].lock, 0) == 0) {
  96                 printk(KERN_WARNING "Trying to free free DMA%d\n", dmanr);
  97                 return;
  98         }
  99
 100 } /* free_dma */
 101
 102 #else
 103
 104 int request_dma(unsigned int dmanr, const char *device_id)
 105 {
 106         return -EINVAL;
 107 }
 108
 109 void free_dma(unsigned int dmanr)
 110 {
 111 }
 112
 113 #endif
 114
 115 #ifdef CONFIG_PROC_FS
 116
 117 #ifdef MAX_DMA_CHANNELS
 118 static int proc_dma_show(struct seq_file *m, void *v)
 119 {
 120         int i;
 121
 122         for (i = 0 ; i < MAX_DMA_CHANNELS ; i++) {
 123                 if (dma_chan_busy[i].lock) {
 124                         seq_printf(m, "%2d: %s\n", i,
 125                                    dma_chan_busy[i].device_id);
 126                 }
 127         }
 128         return 0;
 129 }
 130 #else
 131 static int proc_dma_show(struct seq_file *m, void *v)
 132 {
 133         seq_puts(m, "No DMA\n");
 134         return 0;
 135 }
 136 #endif /* MAX_DMA_CHANNELS */
 137
 138 static int __init proc_dma_init(void)
 139 {
 140         proc_create_single("dma", 0, NULL, proc_dma_show);
 141         return 0;
 142 }
 143
 144 __initcall(proc_dma_init);
 145 #endif
 146
 147 EXPORT_SYMBOL(request_dma);
 148 EXPORT_SYMBOL(free_dma);
 149 EXPORT_SYMBOL(dma_spin_lock);