最近在为了读取一个寄存器的值动心思时，突然发现，一个好用的工具”r”,源码附上：
#include <fcntl.h>
#include <inttypes.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>
#if __LP64__
#define strtoptr strtoull
#else
#define strtoptr strtoul
#endif
static int usage()
{
fprintf(stderr,"r [-b|-s] <address> [<value>]\n");
return -1;
}
int r_main(int argc, char *argv[])
{
if(argc < 2) return usage();
int width = 4;
if(!strcmp(argv[1], "-b")) {
width = 1;
argc--;
argv++;
} else if(!strcmp(argv[1], "-s")) {
width = 2;
argc--;
argv++;
}
if(argc < 2) return usage();
uintptr_t addr = strtoptr(argv[1], 0, 16);
uintptr_t endaddr = 0;
char* end = strchr(argv[1], '-');
if (end)
endaddr = strtoptr(end + 1, 0, 16);
if (!endaddr)
endaddr = addr + width - 1;
if (endaddr <= addr) {
fprintf(stderr, "end address <= start address\n");
return -1;
}
bool set = false;
uint32_t value = 0;
if(argc > 2) {
set = true;
value = strtoul(argv[2], 0, 16);
}
int fd = open("/dev/mem", O_RDWR | O_SYNC);
if(fd < 0) {
fprintf(stderr,"cannot open /dev/mem\n");
return -1;
}
off64_t mmap_start = addr & ~(PAGE_SIZE - 1);
size_t mmap_size = endaddr - mmap_start + 1;
mmap_size = (mmap_size + PAGE_SIZE - 1) & ~(PAGE_SIZE - 1);
void* page = mmap64(0, mmap_size, PROT_READ | PROT_WRITE,
MAP_SHARED, fd, mmap_start);
if(page == MAP_FAILED){
fprintf(stderr,"cannot mmap region\n");
return -1;
}
while (addr <= endaddr) {
switch(width){
case 4: {
uint32_t* x = (uint32_t*) (((uintptr_t) page) + (addr & 4095));
if(set) *x = value;
fprintf(stderr,"%08"PRIxPTR": %08x\n", addr, *x);
break;
}
case 2: {
uint16_t* x = (uint16_t*) (((uintptr_t) page) + (addr & 4095));
if(set) *x = value;
fprintf(stderr,"%08"PRIxPTR": %04x\n", addr, *x);
break;
}
case 1: {
uint8_t* x = (uint8_t*) (((uintptr_t) page) + (addr & 4095));
if(set) *x = value;
fprintf(stderr,"%08"PRIxPTR": %02x\n", addr, *x);
break;
}
}
addr += width;
}
return 0;
}简单好用不是！
秉承研究一下的思想，看了一下/dev/mem和mmap读写linux内存的通用C代码及原理。
附上一个mem这个字符驱动的源码：
/*
* linux/drivers/char/mem.c
*
* Copyright (C) 1991, 1992 Linus Torvalds
*
* Added devfs support.
* Jan-11-1998, C. Scott Ananian <cananian@alumni.princeton.edu>
* Shared /dev/zero mmapping support, Feb 2000, Kanoj Sarcar <kanoj@sgi.com>
*/
#include <linux/mm.h>
#include <linux/miscdevice.h>
#include <linux/slab.h>
#include <linux/vmalloc.h>
#include <linux/mman.h>
#include <linux/random.h>
#include <linux/init.h>
#include <linux/raw.h>
#include <linux/tty.h>
#include <linux/capability.h>
#include <linux/ptrace.h>
#include <linux/device.h>
#include <linux/highmem.h>
#include <linux/crash_dump.h>
#include <linux/backing-dev.h>
#include <linux/bootmem.h>
#include <linux/splice.h>
#include <linux/pfn.h>
#include <linux/export.h>
#include <asm/uaccess.h>
#include <asm/io.h>
#ifdef CONFIG_IA64
# include <linux/efi.h>
#endif
static inline unsigned long size_inside_page(unsigned long start,
unsigned long size)
{
unsigned long sz;
sz = PAGE_SIZE - (start & (PAGE_SIZE - 1));
return min(sz, size);
}
#ifndef ARCH_HAS_VALID_PHYS_ADDR_RANGE
static inline int valid_phys_addr_range(unsigned long addr, size_t count)
{
return addr + count <= __pa(high_memory);
}
static inline int valid_mmap_phys_addr_range(unsigned long pfn, size_t size)
{
return 1;
}
#endif
#if defined(CONFIG_DEVMEM) || defined(CONFIG_DEVKMEM)
#ifdef CONFIG_STRICT_DEVMEM
static inline int range_is_allowed(unsigned long pfn, unsigned long size)
{
u64 from = ((u64)pfn) << PAGE_SHIFT;
u64 to = from + size;
u64 cursor = from;
while (cursor < to) {
if (!devmem_is_allowed(pfn)) {
printk(KERN_INFO
"Program %s tried to access /dev/mem between %Lx->%Lx.\n",
current->comm, from, to);
return 0;
}
cursor += PAGE_SIZE;
pfn++;
}
return 1;
}
#else
static inline int range_is_allowed(unsigned long pfn, unsigned long size)
{
return 1;
}
#endif
#endif
#ifdef CONFIG_DEVMEM
void __weak unxlate_dev_mem_ptr(unsigned long phys, void *addr)
{
}
/*
* This funcion reads the *physical* memory. The f_pos points directly to the
* memory location.
*/
static ssize_t read_mem(struct file *file, char __user *buf,
size_t count, loff_t *ppos)
{
unsigned long p = *ppos;
ssize_t read, sz;
char *ptr;
if (!valid_phys_addr_range(p, count))
return -EFAULT;
read = 0;
#ifdef __ARCH_HAS_NO_PAGE_ZERO_MAPPED
/* we don't have page 0 mapped on sparc and m68k.. */
if (p < PAGE_SIZE) {
sz = size_inside_page(p, count);
if (sz > 0) {
if (clear_user(buf, sz))
return -EFAULT;
buf += sz;
p += sz;
count -= sz;
read += sz;
}
}
#endif
while (count > 0) {
unsigned long remaining;
sz = size_inside_page(p, count);
if (!range_is_allowed(p >> PAGE_SHIFT, count))
return -EPERM;
/*
* On ia64 if a page has been mapped somewhere as uncached, then
* it must also be accessed uncached by the kernel or data
* corruption may occur.
*/
ptr = xlate_dev_mem_ptr(p);
if (!ptr)
return -EFAULT;
remaining = copy_to_user(buf, ptr, sz);
unxlate_dev_mem_ptr(p, ptr);
if (remaining)
return -EFAULT;
buf += sz;
p += sz;
count -= sz;
read += sz;
}
*ppos += read;
return read;
}
static ssize_t write_mem(struct file *file, const char __user *buf,
size_t count, loff_t *ppos)
{
unsigned long p = *ppos;
ssize_t written, sz;
unsigned long copied;
void *ptr;
if (!valid_phys_addr_range(p, count))
return -EFAULT;
written = 0;
#ifdef __ARCH_HAS_NO_PAGE_ZERO_MAPPED
/* we don't have page 0 mapped on sparc and m68k.. */
if (p < PAGE_SIZE) {
sz = size_inside_page(p, count);
/* Hmm. Do something? */
buf += sz;
p += sz;
count -= sz;
written += sz;
}
#endif
while (count > 0) {
sz = size_inside_page(p, count);
if (!range_is_allowed(p >> PAGE_SHIFT, sz))
return -EPERM;
/*
* On ia64 if a page has been mapped somewhere as uncached, then
* it must also be accessed uncached by the kernel or data
* corruption may occur.
*/
ptr = xlate_dev_mem_ptr(p);
if (!ptr) {
if (written)
break;
return -EFAULT;
}
copied = copy_from_user(ptr, buf, sz);
unxlate_dev_mem_ptr(p, ptr);
if (copied) {
written += sz - copied;
if (written)
break;
return -EFAULT;
}
buf += sz;
p += sz;
count -= sz;
written += sz;
}
*ppos += written;
return written;
}
#endif /* CONFIG_DEVMEM */
#if defined(CONFIG_DEVMEM) || defined(CONFIG_DEVKMEM)
int __weak phys_mem_access_prot_allowed(struct file *file,
unsigned long pfn, unsigned long size, pgprot_t *vma_prot)
{
return 1;
}
#ifndef __HAVE_PHYS_MEM_ACCESS_PROT
/*
* Architectures vary in how they handle caching for addresses
* outside of main memory.
*
*/
#ifdef pgprot_noncached
static int uncached_access(struct file *file, unsigned long addr)
{
#if defined(CONFIG_IA64)
/*
* On ia64, we ignore O_DSYNC because we cannot tolerate memory
* attribute aliases.
*/
return !(efi_mem_attributes(addr) & EFI_MEMORY_WB);
#elif defined(CONFIG_MIPS)
{
extern int __uncached_access(struct file *file,
unsigned long addr);
return __uncached_access(file, addr);
}
#else
/*
* Accessing memory above the top the kernel knows about or through a
* file pointer
* that was marked O_DSYNC will be done non-cached.
*/
if (file->f_flags & O_DSYNC)
return 1;
return addr >= __pa(high_memory);
#endif
}
#endif
static pgprot_t phys_mem_access_prot(struct file *file, unsigned long pfn,
unsigned long size, pgprot_t vma_prot)
{
#ifdef pgprot_noncached
unsigned long offset = pfn << PAGE_SHIFT;
if (uncached_access(file, offset))
return pgprot_noncached(vma_prot);
#endif
return vma_prot;
}
#endif
#ifndef CONFIG_MMU
static unsigned long get_unmapped_area_mem(struct file *file,
unsigned long addr,
unsigned long len,
unsigned long pgoff,
unsigned long flags)
{
if (!valid_mmap_phys_addr_range(pgoff, len))
return (unsigned long) -EINVAL;
return pgoff << PAGE_SHIFT;
}
/* can't do an in-place private mapping if there's no MMU */
static inline int private_mapping_ok(struct vm_area_struct *vma)
{
return vma->vm_flags & VM_MAYSHARE;
}
#else
#define get_unmapped_area_mem NULL
static inline int private_mapping_ok(struct vm_area_struct *vma)
{
return 1;
}
#endif
static const struct vm_operations_struct mmap_mem_ops = {
#ifdef CONFIG_HAVE_IOREMAP_PROT
.access = generic_access_phys
#endif
};
static int mmap_mem(struct file *file, struct vm_area_struct *vma)
{
size_t size = vma->vm_end - vma->vm_start;
if (!valid_mmap_phys_addr_range(vma->vm_pgoff, size))
return -EINVAL;
if (!private_mapping_ok(vma))
return -ENOSYS;
if (!range_is_allowed(vma->vm_pgoff, size))
return -EPERM;
if (!phys_mem_access_prot_allowed(file, vma->vm_pgoff, size,
&vma->vm_page_prot))
return -EINVAL;
vma->vm_page_prot = phys_mem_access_prot(file, vma->vm_pgoff,
size,
vma->vm_page_prot);
vma->vm_ops = &mmap_mem_ops;
/* Remap-pfn-range will mark the range VM_IO and VM_RESERVED */
if (remap_pfn_range(vma,
vma->vm_start,
vma->vm_pgoff,
size,
vma->vm_page_prot)) {
return -EAGAIN;
}
return 0;
}
#endif /* CONFIG_DEVMEM */
#ifdef CONFIG_DEVKMEM
static int mmap_kmem(struct file *file, struct vm_area_struct *vma)
{
unsigned long pfn;
/* Turn a kernel-virtual address into a physical page frame */
pfn = __pa((u64)vma->vm_pgoff << PAGE_SHIFT) >> PAGE_SHIFT;
/*
* RED-PEN: on some architectures there is more mapped memory than
* available in mem_map which pfn_valid checks for. Perhaps should add a
* new macro here.
*
* RED-PEN: vmalloc is not supported right now.
*/
if (!pfn_valid(pfn))
return -EIO;
vma->vm_pgoff = pfn;
return mmap_mem(file, vma);
}
#endif
#ifdef CONFIG_CRASH_DUMP
/*
* Read memory corresponding to the old kernel.
*/
static ssize_t read_oldmem(struct file *file, char __user *buf,
size_t count, loff_t *ppos)
{
unsigned long pfn, offset;
size_t read = 0, csize;
int rc = 0;
while (count) {
pfn = *ppos / PAGE_SIZE;
if (pfn > saved_max_pfn)
return read;
offset = (unsigned long)(*ppos % PAGE_SIZE);
if (count > PAGE_SIZE - offset)
csize = PAGE_SIZE - offset;
else
csize = count;
rc = copy_oldmem_page(pfn, buf, csize, offset, 1);
if (rc < 0)
return rc;
buf += csize;
*ppos += csize;
read += csize;
count -= csize;
}
return read;
}
#endif
#ifdef CONFIG_DEVKMEM
/*
* This function reads the *virtual* memory as seen by the kernel.
*/
static ssize_t read_kmem(struct file *file, char __user *buf,
size_t count, loff_t *ppos)
{
unsigned long p = *ppos;
ssize_t low_count, read, sz;
char * kbuf; /* k-addr because vread() takes vmlist_lock rwlock */
int err = 0;
read = 0;
if (p < (unsigned long) high_memory) {
low_count = count;
if (count > (unsigned long)high_memory - p)
low_count = (unsigned long)high_memory - p;
#ifdef __ARCH_HAS_NO_PAGE_ZERO_MAPPED
/* we don't have page 0 mapped on sparc and m68k.. */
if (p < PAGE_SIZE && low_count > 0) {
sz = size_inside_page(p, low_count);
if (clear_user(buf, sz))
return -EFAULT;
buf += sz;
p += sz;
read += sz;
low_count -= sz;
count -= sz;
}
#endif
while (low_count > 0) {
sz = size_inside_page(p, low_count);
/*
* On ia64 if a page has been mapped somewhere as
* uncached, then it must also be accessed uncached
* by the kernel or data corruption may occur
*/
kbuf = xlate_dev_kmem_ptr((char *)p);
if (copy_to_user(buf, kbuf, sz))
return -EFAULT;
buf += sz;
p += sz;
read += sz;
low_count -= sz;
count -= sz;
}
}
if (count > 0) {
kbuf = (char *)__get_free_page(GFP_KERNEL);
if (!kbuf)
return -ENOMEM;
while (count > 0) {
sz = size_inside_page(p, count);
if (!is_vmalloc_or_module_addr((void *)p)) {
err = -ENXIO;
break;
}
sz = vread(kbuf, (char *)p, sz);
if (!sz)
break;
if (copy_to_user(buf, kbuf, sz)) {
err = -EFAULT;
break;
}
count -= sz;
buf += sz;
read += sz;
p += sz;
}
free_page((unsigned long)kbuf);
}
*ppos = p;
return read ? read : err;
}
static ssize_t do_write_kmem(unsigned long p, const char __user *buf,
size_t count, loff_t *ppos)
{
ssize_t written, sz;
unsigned long copied;
written = 0;
#ifdef __ARCH_HAS_NO_PAGE_ZERO_MAPPED
/* we don't have page 0 mapped on sparc and m68k.. */
if (p < PAGE_SIZE) {
sz = size_inside_page(p, count);
/* Hmm. Do something? */
buf += sz;
p += sz;
count -= sz;
written += sz;
}
#endif
while (count > 0) {
char *ptr;
sz = size_inside_page(p, count);
/*
* On ia64 if a page has been mapped somewhere as uncached, then
* it must also be accessed uncached by the kernel or data
* corruption may occur.
*/
ptr = xlate_dev_kmem_ptr((char *)p);
copied = copy_from_user(ptr, buf, sz);
if (copied) {
written += sz - copied;
if (written)
break;
return -EFAULT;
}
buf += sz;
p += sz;
count -= sz;
written += sz;
}
*ppos += written;
return written;
}
/*
* This function writes to the *virtual* memory as seen by the kernel.
*/
static ssize_t write_kmem(struct file *file, const char __user *buf,
size_t count, loff_t *ppos)
{
unsigned long p = *ppos;
ssize_t wrote = 0;
ssize_t virtr = 0;
char * kbuf; /* k-addr because vwrite() takes vmlist_lock rwlock */
int err = 0;
if (p < (unsigned long) high_memory) {
unsigned long to_write = min_t(unsigned long, count,
(unsigned long)high_memory - p);
wrote = do_write_kmem(p, buf, to_write, ppos);
if (wrote != to_write)
return wrote;
p += wrote;
buf += wrote;
count -= wrote;
}
if (count > 0) {
kbuf = (char *)__get_free_page(GFP_KERNEL);
if (!kbuf)
return wrote ? wrote : -ENOMEM;
while (count > 0) {
unsigned long sz = size_inside_page(p, count);
unsigned long n;
if (!is_vmalloc_or_module_addr((void *)p)) {
err = -ENXIO;
break;
}
n = copy_from_user(kbuf, buf, sz);
if (n) {
err = -EFAULT;
break;
}
vwrite(kbuf, (char *)p, sz);
count -= sz;
buf += sz;
virtr += sz;
p += sz;
}
free_page((unsigned long)kbuf);
}
*ppos = p;
return virtr + wrote ? : err;
}
#endif
#ifdef CONFIG_DEVPORT
static ssize_t read_port(struct file *file, char __user *buf,
size_t count, loff_t *ppos)
{
unsigned long i = *ppos;
char __user *tmp = buf;
if (!access_ok(VERIFY_WRITE, buf, count))
return -EFAULT;
while (count-- > 0 && i < 65536) {
if (__put_user(inb(i), tmp) < 0)
return -EFAULT;
i++;
tmp++;
}
*ppos = i;
return tmp-buf;
}
static ssize_t write_port(struct file *file, const char __user *buf,
size_t count, loff_t *ppos)
{
unsigned long i = *ppos;
const char __user * tmp = buf;
if (!access_ok(VERIFY_READ, buf, count))
return -EFAULT;
while (count-- > 0 && i < 65536) {
char c;
if (__get_user(c, tmp)) {
if (tmp > buf)
break;
return -EFAULT;
}
outb(c, i);
i++;
tmp++;
}
*ppos = i;
return tmp-buf;
}
#endif
static ssize_t read_null(struct file *file, char __user *buf,
size_t count, loff_t *ppos)
{
return 0;
}
static ssize_t write_null(struct file *file, const char __user *buf,
size_t count, loff_t *ppos)
{
return count;
}
static int pipe_to_null(struct pipe_inode_info *info, struct pipe_buffer *buf,
struct splice_desc *sd)
{
return sd->len;
}
static ssize_t splice_write_null(struct pipe_inode_info *pipe, struct file *out,
loff_t *ppos, size_t len, unsigned int flags)
{
return splice_from_pipe(pipe, out, ppos, len, flags, pipe_to_null);
}
static ssize_t read_zero(struct file *file, char __user *buf,
size_t count, loff_t *ppos)
{
size_t written;
if (!count)
return 0;
if (!access_ok(VERIFY_WRITE, buf, count))
return -EFAULT;
written = 0;
while (count) {
unsigned long unwritten;
size_t chunk = count;
if (chunk > PAGE_SIZE)
chunk = PAGE_SIZE; /* Just for latency reasons */
unwritten = __clear_user(buf, chunk);
written += chunk - unwritten;
if (unwritten)
break;
if (signal_pending(current))
return written ? written : -ERESTARTSYS;
buf += chunk;
count -= chunk;
cond_resched();
}
return written ? written : -EFAULT;
}
static int mmap_zero(struct file *file, struct vm_area_struct *vma)
{
#ifndef CONFIG_MMU
return -ENOSYS;
#endif
if (vma->vm_flags & VM_SHARED)
return shmem_zero_setup(vma);
return 0;
}
static ssize_t write_full(struct file *file, const char __user *buf,
size_t count, loff_t *ppos)
{
return -ENOSPC;
}
/*
* Special lseek() function for /dev/null and /dev/zero. Most notably, you
* can fopen() both devices with "a" now. This was previously impossible.
* -- SRB.
*/
static loff_t null_lseek(struct file *file, loff_t offset, int orig)
{
return file->f_pos = 0;
}
#if defined(CONFIG_DEVMEM) || defined(CONFIG_DEVKMEM) || defined(CONFIG_DEVPORT)
/*
* The memory devices use the full 32/64 bits of the offset, and so we cannot
* check against negative addresses: they are ok. The return value is weird,
* though, in that case (0).
*
* also note that seeking relative to the "end of file" isn't supported:
* it has no meaning, so it returns -EINVAL.
*/
static loff_t memory_lseek(struct file *file, loff_t offset, int orig)
{
loff_t ret;
mutex_lock(&file->f_path.dentry->d_inode->i_mutex);
switch (orig) {
case SEEK_CUR:
offset += file->f_pos;
case SEEK_SET:
/* to avoid userland mistaking f_pos=-9 as -EBADF=-9 */
if ((unsigned long long)offset >= ~0xFFFULL) {
ret = -EOVERFLOW;
break;
}
file->f_pos = offset;
ret = file->f_pos;
force_successful_syscall_return();
break;
default:
ret = -EINVAL;
}
mutex_unlock(&file->f_path.dentry->d_inode->i_mutex);
return ret;
}
#endif
#if defined(CONFIG_DEVMEM) || defined(CONFIG_DEVKMEM) || defined(CONFIG_DEVPORT)
static int open_port(struct inode * inode, struct file * filp)
{
return capable(CAP_SYS_RAWIO) ? 0 : -EPERM;
}
#endif
#define zero_lseek null_lseek
#define full_lseek null_lseek
#define write_zero write_null
#define read_full read_zero
#define open_mem open_port
#define open_kmem open_mem
#define open_oldmem open_mem
#ifdef CONFIG_DEVMEM
static const struct file_operations mem_fops = {
.llseek = memory_lseek,
.read = read_mem,
.write = write_mem,
.mmap = mmap_mem,
.open = open_mem,
.get_unmapped_area = get_unmapped_area_mem,
};
#endif
#ifdef CONFIG_DEVKMEM
static const struct file_operations kmem_fops = {
.llseek = memory_lseek,
.read = read_kmem,
.write = write_kmem,
.mmap = mmap_kmem,
.open = open_kmem,
.get_unmapped_area = get_unmapped_area_mem,
};
#endif
static const struct file_operations null_fops = {
.llseek = null_lseek,
.read = read_null,
.write = write_null,
.splice_write = splice_write_null,
};
#ifdef CONFIG_DEVPORT
static const struct file_operations port_fops = {
.llseek = memory_lseek,
.read = read_port,
.write = write_port,
.open = open_port,
};
#endif
static const struct file_operations zero_fops = {
.llseek = zero_lseek,
.read = read_zero,
.write = write_zero,
.mmap = mmap_zero,
};
/*
* capabilities for /dev/zero
* - permits private mappings, "copies" are taken of the source of zeros
* - no writeback happens
*/
static struct backing_dev_info zero_bdi = {
.name = "char/mem",
.capabilities = BDI_CAP_MAP_COPY | BDI_CAP_NO_ACCT_AND_WRITEBACK,
};
static const struct file_operations full_fops = {
.llseek = full_lseek,
.read = read_full,
.write = write_full,
};
#ifdef CONFIG_CRASH_DUMP
static const struct file_operations oldmem_fops = {
.read = read_oldmem,
.open = open_oldmem,
.llseek = default_llseek,
};
#endif
static ssize_t kmsg_writev(struct kiocb *iocb, const struct iovec *iv,
unsigned long count, loff_t pos)
{
char *line, *p;
int i;
ssize_t ret = -EFAULT;
size_t len = iov_length(iv, count);
line = kmalloc(len + 1, GFP_KERNEL);
if (line == NULL)
return -ENOMEM;
/*
* copy all vectors into a single string, to ensure we do
* not interleave our log line with other printk calls
*/
p = line;
for (i = 0; i < count; i++) {
if (copy_from_user(p, iv[i].iov_base, iv[i].iov_len))
goto out;
p += iv[i].iov_len;
}
p[0] = '\0';
ret = printk("%s", line);
/* printk can add a prefix */
if (ret > len)
ret = len;
out:
kfree(line);
return ret;
}
static const struct file_operations kmsg_fops = {
.aio_write = kmsg_writev,
.llseek = noop_llseek,
};
static const struct memdev {
const char *name;
umode_t mode;
const struct file_operations *fops;
struct backing_dev_info *dev_info;
} devlist[] = {
#ifdef CONFIG_DEVMEM
[1] = { "mem", 0, &mem_fops, &directly_mappable_cdev_bdi },
#endif
#ifdef CONFIG_DEVKMEM
[2] = { "kmem", 0, &kmem_fops, &directly_mappable_cdev_bdi },
#endif
[3] = { "null", 0666, &null_fops, NULL },
#ifdef CONFIG_DEVPORT
[4] = { "port", 0, &port_fops, NULL },
#endif
[5] = { "zero", 0666, &zero_fops, &zero_bdi },
[7] = { "full", 0666, &full_fops, NULL },
[8] = { "random", 0666, &random_fops, NULL },
[9] = { "urandom", 0666, &urandom_fops, NULL },
[11] = { "kmsg", 0, &kmsg_fops, NULL },
#ifdef CONFIG_CRASH_DUMP
[12] = { "oldmem", 0, &oldmem_fops, NULL },
#endif
};
static int memory_open(struct inode *inode, struct file *filp)
{
int minor;
const struct memdev *dev;
minor = iminor(inode);
if (minor >= ARRAY_SIZE(devlist))
return -ENXIO;
dev = &devlist[minor];
if (!dev->fops)
return -ENXIO;
filp->f_op = dev->fops;
if (dev->dev_info)
filp->f_mapping->backing_dev_info = dev->dev_info;
/* Is /dev/mem or /dev/kmem ? */
if (dev->dev_info == &directly_mappable_cdev_bdi)
filp->f_mode |= FMODE_UNSIGNED_OFFSET;
if (dev->fops->open)
return dev->fops->open(inode, filp);
return 0;
}
static const struct file_operations memory_fops = {
.open = memory_open,
.llseek = noop_llseek,
};
static char *mem_devnode(struct device *dev, umode_t *mode)
{
if (mode && devlist[MINOR(dev->devt)].mode)
*mode = devlist[MINOR(dev->devt)].mode;
return NULL;
}
static struct class *mem_class;
static int __init chr_dev_init(void)
{
int minor;
int err;
err = bdi_init(&zero_bdi);
if (err)
return err;
if (register_chrdev(MEM_MAJOR, "mem", &memory_fops))
printk("unable to get major %d for memory devs\n", MEM_MAJOR);
mem_class = class_create(THIS_MODULE, "mem");
if (IS_ERR(mem_class))
return PTR_ERR(mem_class);
mem_class->devnode = mem_devnode;
for (minor = 1; minor < ARRAY_SIZE(devlist); minor++) {
if (!devlist[minor].name)
continue;
device_create(mem_class, NULL, MKDEV(MEM_MAJOR, minor),
NULL, devlist[minor].name);
}
return tty_init();
}
fs_initcall(chr_dev_init);利用/dev/mem和mmap导出系统物理地址，免去了用户虚拟地址到内核逻辑地址的繁琐拷贝，提升效率。
注意：如果地址不是一个有效物理地址（处理器地址空间分布中该地址没用），mmap建立该物理地址与用户空间虚拟地址的映射，填TLB，CPU经过TLB翻译后去访问该不存在的物理地址访问就有可能导致CPU挂掉。
内核中定义了4个变量来表示内核一些基本的物理地址和虚拟地址，如下：
KERNELBASE 内核的起始虚拟地址，
PAGE_OFFSET 低端内存的起始虚拟地址，一般是0xc0000000
PHYSICAL_START 内核的起始物理地址，
MEMORY_START 低端内存的起始物理地址，
内核在启动过程中对于lowmem的静态映射，就是以上述的物理地址和虚拟地址的差值进行线性映射的。
所以__pa __va转换的是线性映射的内存部分，也就是lowmem。
所以kmem映射的是lowmem，如果cmdline参数中mem=512M，这就意味着通过kmem的mmap最多可以访问内核地址空间开始的512M内存。
对于超过lowmem范围，访问highmem，如果使用__pa访问，由于highmem是动态映射的，其映射关系不是线性的那么简单了，根据__pa获取的物理地址与我们想要的内核虚拟地址是不对应的。
一个简单GPIO的应用层驱动：
int main(int argc, char *argv[])
{
int mfd;
unsigned int val=0, last_val;
void *base;
char *sys_pinstaterd;
time_t t_now, t_old;
int flag_issued = 0;
#if 0
// uncomment these to make the program a daemon.
pid_t pid;
int i;
if ( (pid=fork())<0)
return -1;
else if (pid!=0)
exit(0);
setsid();
chdir("/");
umask(0);
for (i=0;i<256;i++)
close(i);
#endif
// open the memery mapped file.
mfd=open("/dev/mem", O_RDWR);
if (mfd < 0){
printf("Cannot open /dev/mem.\n");
exit(-1);
}
// Initialize the map
base = mmap( NULL, 0x130, PROT_READ | PROT_WRITE, MAP_SHARED, mfd, 0x1fe00000);
if ( base < 0){
exit(-1);
}
sys_pinstaterd = base + 0x011c;
// init the temperay variables
t_now=t_old=time(NULL);
last_val = 0;
while(1)
{
val = *( (volatile unsigned int*)sys_pinstaterd );
val = (val&0x4) ? 1:0;
printf("\tgpio 7 stat=%x.\n", val);
if (val){
// the button is pressed down !!
if ( last_val==0 ){
// starting time of press, log the time
t_old = time(NULL);
last_val=1;
printf("Button Down\n");
}else {
// already pressed down! let's count the time!
t_now = time(NULL);
if (t_now-t_old>=TIME_OUT && flag_issued==0){
// Pressed LONG ENOUGH, issue the handler script!!
flag_issued = 1;
longtu_timeout();
}
}
}else{
// No button pressed.
if (flag_issued){
flag_issued = 0;
printf("Button UP.\n");
}
}
last_val = val;
usleep(100);
}
munmap(base, 0x000);
close(mfd);
return 0;
}写在应用层的物理地址操作。快速开发的一个方法。