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Remap_pfn_range Example


The function builds page tables for the virtual address range between virt_addr and virt_addr+size. The system call is declared as follows (as described in the mmap(2) manual page): mmap (caddr_t addr, size_t len, int prot, int flags, int fd, off_t offset) On the other hand, In particular, the open and close methods from sbull are used without modification. The following lines will do the trick for a driver mapping a region of simple_region_size bytes, beginning at physical address simple_region_start (which should be page aligned).

sys_mlock() how this can be done. All logical addresses are kernel virtual addresses; memory allocated by vmalloc also has a virtual address (but no direct physical mapping). Nonuniform memory access (NUMA) systems and those with widely discontiguous physical memory may have more than one memory map array, so code that is meant to be portable should avoid direct Irrespective of the mechanisms used by the CPU, the Linux software implementation is based on three-level page tables, and the following symbols are used to access them. http://www.xml.com/ldd/chapter/book/ch13.html

Remap_pfn_range Example

Whenever the program reads or writes in the assigned address range, it is actually accessing the device. Because it is page oriented, it can implement mmap on its memory. Keeping page tables in memory simplifies the kernel code because pgd_offset and friends never fail; on the other hand, even a process with a "resident storage size'' of zero keeps its In device drivers, the proper value for type will invariably be VM_FAULT_MINOR.

They exist as a way for drivers to do any additional processing that they may require. The following list summarizes the implementation of the three levels in Linux, and Figure 13-2 depicts them. Now we'll look at providing those operations in a simple way; a more detailed example will follow later on. Linux Kernel Dma Example The step is usually performed by splitting the address into bitfields.

This works only without holding any locks for pages we are sure that they do not move in memory. One other complication has to do with caching: usually, references to device memory should not be cached by the processor. I was not able to achieve a decent enough frame rate with this. The code just shown checks the current value of the pointer as a safety measure, should something change in future kernels.

This function must also take care to increment the usage count for the page it returns by calling the get_page macro: get_page(struct page *pageptr); This step is necessary to keep the Implement Mmap Device Driver It appears we can work around this by marking the page(s) reserved using SetPageReserved so that it gets locked in memory. Logical addresses use the hardware's native pointer size, and thus may be unable to address all of physical memory on heavily equipped 32-bit systems. Bookmark the permalink.

Mmap Kernel Buffer To User Space

A bigger constraint exists when ISA devices are used on some non-x86 platforms, because their hardware view of ISA may not be contiguous. http://stackoverflow.com/questions/21115530/linux-driver-mmap-kernel-buffer-to-userspace-without-using-nopage morgana% ls -l /dev > /dev/scullp morgana% ./mapper /dev/scullp 0 140 mapped "/dev/scullp" from 0 to 140 total 77 -rwxr-xr-x 1 root root 26689 Mar 2 2000 MAKEDEV crw-rw-rw- 1 root Remap_pfn_range Example Most of the work is then performed by nopage. Linux Mmap Example This is because remap_page_range() won't allow you to map non-reserved pages (check remap_pte_range()).

As a driver writer, you may well want to prevent this sort of behavior; mapping the zero page onto the end of your region is not an explicitly bad thing to Mapping non-kernel Virtual Addresses into User Space Mapping addresses e.g. struct page *pte_page(pte_t pte) This function returns a pointer to the struct page entry for the page in this page-table entry. Load exectuable fs/binfmt_elf_fdpic.c:load_elf_fdpic_binary() Allocate stack: current->mm->start_brk = do_mmap(NULL, 0, stack_size, PROT_READ | PROT_WRITE | PROT_EXEC, MAP_PRIVATE | MAP_ANONYMOUS | MAP_GROWSDOWN | MAP_SPLIT_PAGES, 0); flags = MAP_PRIVATE | MAP_DENYWRITE | MAP_SPLIT_PAGES; if Linux Dma Driver Example

A true virtual address, remember, is an address returned by a function like vmalloc or kmap -- that is, a virtual address mapped in the kernel page tables. In my case, I know exactly what addresses will need to be mapped to the given VMA locations for the entirety of the buffer at the moment mmap() is called, so When a VMA is unmapped, the kernel will decrement the usage count for every page in the area. The page fault handler calculates the right physical page on first access of the application to the page. (method 1 is used for vmalloc'd memory, offset 0..LEN) The second way works

The code implementing memory mapping uses some of the concepts introduced in Section 15.1. Remap_page_range If you are using nopage, there is usually very little work to be done when mmap is called; our version looks like this: static int simple_nopage_mmap(struct file *filp, struct vm_area_struct *vma) What's the verb to describe a good guy becoming a bad guy?

The one complication has to do with caching: usually, references to device memory should not be cached by the processor.

Thus, many kernel functions are limited to low memory only; high memory tends to be reserved for user-space process pages. Nonetheless, the function does everything that most hardware drivers need it to do, because it can remap high PCI buffers and ISA memory. If you're doing fancier stuff around mapping non-page-backed device memory, then you might need vm_insert_pfn() but really you probably don't want to worry about that. /dev/mem Mmap Among other things, the VM_IO flag will prevent the region from being included in process core dumps.

The 2.0 kernel, however, did not perform this tracking, so portable code will still want to be able to maintain the usage count. We need to stop the VM system from removing our pages from main memory. Confusingly, for device mappings, the major and minor numbers refer to the disk partition holding the device special file that was opened by the user, and not the device itself. As a result, the PTE (page table entries) do not know if the pfn belongs to user space or kernel space (even though they are allocated via kernel driver).

If the nopage method is left NULL, kernel code that handles page faults maps the zero page to the faulting virtual address. In the X server example, using mmap allows quick and easy access to the video card's memory. The Linux kernel has traditionally not provided a raw interface, for a number of reasons. In particular, we provide open and close operations for our VMA.

The code in this section is taken from scullv, which is the module that works like scullp but allocates its storage through vmalloc. Thus kernel functions that deal with memory are increasingly using pointers to struct page instead. The limitations of remap_page_range can be seen by running mapper, one of the sample programs in misc-progs in the files provided on the O'Reilly FTP site. Instead, you must use vmalloc_to_page.

The Linux kernel manages three levels of page tables in order to map virtual addresses to physical addresses. Let's look at the exact meaning of the function's arguments: vma

The virtual memory area into which the page range is being mapped. The driver writer should therefore have at least a minimal understanding of VMAs in order to use them. Memory management for the x86 changed in version 2.1.1, and VMALLOC_VMADDR is now defined as the identity function, as it has always been for the other platforms.

It exists as a separate function because kernel versions prior to 2.4 lacked that field, requiring that other means be used to get that pointer. mmap.c#include #include #include #include #include #include #include #include #ifdef MODVERSIONS# include #endif#include /* character device structures */static dev_t mmap_dev;static struct cdev mmap_cdev;/* methods of the character device */static int mmap_open(struct inode