linux下的/dev/mem设备认识和使用
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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#endifstatic 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>#endifstatic 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_RANGEstatic 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_DEVMEMstatic 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;}#elsestatic inline int range_is_allowed(unsigned long pfn, unsigned long size){ return 1;}#endif#endif#ifdef CONFIG_DEVMEMvoid __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_noncachedstatic 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}#endifstatic 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_MMUstatic 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 NULLstatic inline int private_mapping_ok(struct vm_area_struct *vma){ return 1;}#endifstatic 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_DEVKMEMstatic 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_DEVPORTstatic 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;}#endifstatic 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_DEVMEMstatic 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_DEVKMEMstatic 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,};#endifstatic const struct file_operations null_fops = { .llseek = null_lseek, .read = read_null, .write = write_null, .splice_write = splice_write_null,};#ifdef CONFIG_DEVPORTstatic const struct file_operations port_fops = { .llseek = memory_lseek, .read = read_port, .write = write_port, .open = open_port,};#endifstatic 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_DUMPstatic const struct file_operations oldmem_fops = { .read = read_oldmem, .open = open_oldmem, .llseek = default_llseek,};#endifstatic 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。
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