LXR linux/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
  41DEFINE_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
  55struct dma_chan {
  56        int  lock;
  57        const char *device_id;
  58};
  59
  60static 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 */
  70int 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 */
  88void 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
 104int request_dma(unsigned int dmanr, const char *device_id)
 105{
 106        return -EINVAL;
 107}
 108
 109void free_dma(unsigned int dmanr)
 110{
 111}
 112
 113#endif
 114
 115#ifdef CONFIG_PROC_FS
 116
 117#ifdef MAX_DMA_CHANNELS
 118static 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
 131static 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
 138static 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
 147EXPORT_SYMBOL(request_dma);
 148EXPORT_SYMBOL(free_dma);
 149EXPORT_SYMBOL(dma_spin_lock);
 150