函数 ioremap 物理地址到内核映射空间的映射函数

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ioremap函数用于将I/O内存的物理地址映射到内核虚拟地址空间，确保内核能直接访问高地址。通过get_vm_area获取虚拟地址空间，再使用remap_area_pages进行页面映射。在映射过程中涉及到了内核内存管理的分页机制。

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源代码：

/*
* Remap an arbitrary physical address space into the kernel virtual
* address space. Needed when the kernel wants to access high addresses
* directly.
*
* NOTE! We need to allow non-page-aligned mappings too: we will obviously
* have to convert them into an offset in a page-aligned mapping, but the
* caller shouldn't need to know that small detail.
*
* 'flags' are the extra L_PTE_ flags that you want to specify for this
* mapping. See include/asm-arm/proc-armv/pgtable.h for more information.
*/
void * __ioremap(unsigned long phys_addr, size_t size, unsigned long flags)
{
void * addr;
struct vm_struct * area;
unsigned long offset, last_addr;

/* Don't allow wraparound or zero size */
last_addr = phys_addr + size - 1;
if (!size || last_addr < phys_addr)
return NULL;

/*
* Mappings have to be page-aligned
*/
offset = phys_addr & ~PAGE_MASK;
phys_addr &= PAGE_MASK;
size = PAGE_ALIGN(last_addr) - phys_addr;

/*
* Ok, go for it..
*/
area = get_vm_area(size, VM_IOREMAP);
if (!area)
  return NULL;
addr = area->addr;
if (remap_area_pages(VMALLOC_VMADDR(addr), phys_addr, size, flags)) {
  vfree(addr);
  return NULL;
}
return (void *) (offset + (char *)addr);
}

功能就是将I/O内存资源的物理地址映射到核心虚地址空间(3GB－4GB)中.

1、返回映射后的虚拟地址（get_vm_area获取）

2、对获取的虚拟空间进行页面映射（remap_area_pages）

下面是相关的函数：

struct vm_struct * get_vm_area(unsigned long size, unsigned long flags)
{
unsigned long addr, next;
struct vm_struct **p, *tmp, *area;

area = (struct vm_struct *) kmalloc(sizeof(*area), GFP_KERNEL);
if (!area)
return NULL;

size += PAGE_SIZE;
if (!size) {
kfree (area);
return NULL;
}

addr = VMALLOC_START;
write_lock(&vmlist_lock);
for (p = &vmlist; (tmp = *p) ; p = &tmp->next) {
  if ((size + addr) < addr)
   goto out;
  if (size + addr <= (unsigned long) tmp->addr)
   break;
  next = tmp->size + (unsigned long) tmp->addr;
  if (next > addr)
   addr = next;
  if (addr > VMALLOC_END-size)
   goto out;
}
area->flags = flags;
area->addr = (void *)addr;
area->size = size;
area->next = *p;
*p = area;
write_unlock(&vmlist_lock);
return area;

out:
write_unlock(&vmlist_lock);
kfree(area);
return NULL;
}

static int remap_area_pages(unsigned long address, unsigned long phys_addr,
unsigned long size, unsigned long flags)
{
int error;
pgd_t * dir;
unsigned long end = address + size;

phys_addr -= address;
dir = pgd_offset_k(address);
flush_cache_all();
if (address >= end)
  BUG();
spin_lock(&init_mm.page_table_lock);
do {
  pmd_t *pmd;
  pmd = pmd_alloc(&init_mm, dir, address);
  error = -ENOMEM;
  if (!pmd)
   break;
  if (remap_area_pmd(pmd, address, end - address,
      phys_addr + address, flags))
   break;
  error = 0;
  address = (address + PGDIR_SIZE) & PGDIR_MASK;
  dir++;
} while (address && (address < end));
spin_unlock(&init_mm.page_table_lock);
flush_tlb_all();
return error;
}