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Kernel Driver Mmap


The flags of the most interest to device driver writers are VM_IO and VM_RESERVED. A return value of 0 signals success; any other value signals an error. shreyas shreyasw View Public Profile View LQ Blog View Review Entries View HCL Entries Find More Posts by shreyasw Thread Tools Show Printable Version Email this Page Search this Low-memory pages are always mapped; high-memory pages usually are not. his comment is here

Otherwise, pfn_to_page gets the necessary struct page pointer; we can increment its reference count (with a call to get_page) and return it. Assuming pgd is our page directory pointer, we can check the entry with: if (!pgd_none(*pgd)) The lookup finally is done with pmd = pmd_offset(pgd, address) page table The page middle directory void *page_address(struct page *page); Returns the kernel virtual address of this page, if such an address exists. Newer Post Older Post Home Subscribe to: Post Comments (Atom) GuestBooker 1.8 There was an error in this gadget Blog Archive ► 2012 (3) ► October (3) ► 2011 (8) ► http://www.xml.com/ldd/chapter/book/ch13.html

Mmap Kernel Buffer To User Space

Main Menu LQ Calendar LQ Rules LQ Sitemap Site FAQ View New Posts View Latest Posts Zero Reply Threads LQ Wiki Most Wanted Jeremy's Blog Report LQ Bug Syndicate Latest The overhead of setting up a raw transfer can be significant, and the advantages of buffering data in the kernel are lost. Ultimately though, i didn't do a mmap() but decided to do the switching channel work in the driver itself using kernel timers and workqueues. here we use mem_map_reserve() macro */ #include /* needed for virt_to_phys() */ #include // virt_to_phys() /* here's the pointer to the buffer */ static char *buffer; /* how big

Notify me of new posts via email. If the VMA is reduced in size, the kernel can quietly flush out the unwanted pages without telling the driver. to the address below. Linux Dma Driver Example With just one object file as in # this example this is not needed.

The function builds page tables for the virtual address range between virt_add and virt_add+size. Linux Mmap Example They exist as a way for drivers to do any additional processing that they may require. For memory areas allocated with vmalloc() this is the case. http://stackoverflow.com/questions/21115530/linux-driver-mmap-kernel-buffer-to-userspace-without-using-nopage The following code comes from drivers/char/mem.c and shows how this task is performed in a typical module called simple (Simple Implementation Mapping Pages with Little Enthusiasm): #include int simple_mmap(struct file

Functions that operate on pages stored in a kiobuf deal directly with the page structures -- all of the virtual memory system overhead has been moved out of the way. Linux Kernel Dma Example I've edited the question to clarify. The source for the module : mmaptest.c And a simple user-space reader : mmapdriver.c Here's the cheesy Makefile : Makefile Source /* Example of how to provide userspace mmap access Figure 13-2.

Linux Mmap Example

Raw I/O to a block device must always be sector aligned, and its length must be a multiple of the sector size. http://www.linuxquestions.org/questions/linux-newbie-8/how-to-mmap-file-in-kernel-module-669208/ startend The beginning and ending virtual addresses for this memory area. Mmap Kernel Buffer To User Space Bus addresses are highly architecture dependent, of course. Remap_pfn_range Example It is highly unlikely that a driver will ever need to implement swapout; device mappings are not something that the kernel can just write to disk.

Remapping RAM with the nopage method The way to map real RAM to user space is to use vm_ops->nopage to deal with page faults one at a time. this content In this section, we add a raw I/O capability to the sbull sample block driver. The three levels of Linux page tables Page Directory (PGD) The top-level page table. The following session, for instance, shows that /dev/mem doesn't map the physical page located at address 64 KB -- instead we see a page full of zeros (the host computer in Implement Mmap Device Driver

returned by vmalloc(), must be converted to a kernel virtual address. Therefore if MAP_NR(address) is smaller than max_mapnr the address is part of the direct kernel virtual mapping: if (MAP_NR(address) < max_mapnr) Linux 2.4.x supports more physical memory than the virtual address One other complication has to do with caching: usually, references to device memory should not be cached by the processor. weblink The zero page is a copy-on-write page that reads as zero and that is used, for example, to map the BSS segment.

The kiobuf Structure Any code that works with kiobufs must include . /dev/mem Mmap Instead, it maps in the zero page. Find More Posts by sundialsvcs View Blog 09-12-2008, 03:55 AM #3 shreyasw LQ Newbie Registered: Sep 2008 Posts: 7 Original Poster Rep: Are the addresses "real" ones? (Not virtual...)

For high memory, that address exists only if the page has been mapped. #include void *kmap(struct page *page);void kunmap(struct page *page); kmap returns a kernel virtual address for any page in

Since sbullr is a char device, however, it needs read and writemethods. These limits are not a big constraint for drivers, because the program accessing the device is device dependent anyway. Just seeing the list of these functions is not enough for you to be proficient in the Linux memory management algorithms; real memory management is much more complex and must deal Remap_page_range Although you do not need to be a Linux virtual memory guru to implement mmap, a basic overview of how things work is useful.

Mapping non-kernel Virtual Addresses into User Space Mapping addresses e.g. We can just load the object # file produced by gcc # link the mmap driver module mmap_mod.o: mmap_drv.o ld -r -o mmap_mod.o mmap_drv.o # compile the mmap driver mmap_drv.o: mmap_drv.c Such addresses cannot be converted into physical or IO bus addresses with the functions described above. check over here To this end, we override the default vma->vm_ops with operations that call printk: void simple_vma_open(struct vm_area_struct *vma) { printk(KERN_NOTICE "Simple VMA open, virt %lx, phys %lx\n", vma->vm_start, vma->vm_pgoff << PAGE_SHIFT); }

Thus kernel functions that deal with memory are increasingly using pointers to struct page instead. This code excerpt only includes the part of nopage that differs from scullp: pgd_t *pgd; pmd_t *pmd; pte_t *pte; unsigned long lpage; /* * After scullv lookup, "page" is now the These limits are not a big constraint for drivers, because the program accessing the device is device dependent anyway. The block drivers need not even know they are doing raw I/O.

On error MAP_FAILED (-1) is returned, and the errno variable is set appropriately.