Home > Device Driver > Device Driver Mmap Example

Device Driver Mmap Example

Contents

I was trying to do this using OpenCV while the analog video decoder/multiplexer in question was the ADV7180. Because we are just implementing a window onto physical memory here, the remapping step is simple -- we need only locate and return a pointer to the struct page for the The VFS has kindly prepared the process's vm_area_struct for us, so we examine this to see what was requested. Figure 13-2. useful reference

These regions can also be seen in /proc/iomem: 000a0000-000bffff : Video RAM area f4000000-f4ffffff : Matrox Graphics, Inc. A return value of 0 signals success; any other value signals an error. In Linux, a page of physical addresses is marked as "reserved'' in the memory map to indicate that it is not available for memory management. unsigned long off = vma->vm_pgoff << PAGE_SHIFT; unsigned long physical = simple_region_start + off; unsigned long vsize = vma->vm_end - vma->vm_start; unsigned long psize = simple_region_size - off; if (vsize > find more

Remap_pfn_range Example

This mechanism is implemented in Linux by means of virtual memory areas, which are typically referred to as areas or VMAs. An example of a CPU family that doesn't use page tables is the PowerPC. The kernel needs a higher-level mechanism to handle the way a process sees its memory.

The mmap Device Operation Memory mapping is one of the most interesting features of modern Unix systems. The method returns the struct page pointer for the physical page, after, perhaps, having read it in from secondary storage. pte_present(pte_t pte) This macro returns a boolean value that indicates whether the data page is currently in memory. Linux Kernel Dma Example On some systems, however, the situation is more complicated.

Instead, you should use remap_pfn_range to remap I/O memory areas into user space. ⇦ prev ⇱ home next ⇨ Poster of Linux kernelThe best gift for a Linux geek Mmap Kernel Buffer To User Space If, for some reason, a normal page cannot be returned (e.g., the requested address is beyond the device's memory region), NOPAGE_SIGBUS can be returned to signal the error; that is what Linux Device Drivers, 2nd Edition By Alessandro Rubini & Jonathan Corbet 2nd Edition June 2001 0-59600-008-1, Order Number: 0081 586 pages, $39.95 Chapter 13 mmap and DMA Contents: http://www.makelinux.net/ldd3/chp-15-sect-2 size The dimension, in bytes, of the area being remapped.

Greeting The User Display 10^n as power Unique is Cheap Could an accidental fire in a modern city without another disaster at the same time burn down the entire city? 2x2-determinantal Implement Mmap Device Driver unsigned pgd_val(pgd_t pgd)unsigned pmd_val(pmd_t pmd)unsigned pte_val(pte_t pte) These three macros are used to retrieve the unsigned value from the typed data item. Therefore we cannot just compare with a maximum pageframe number. When kiobufs are available, sbull actually registers two devices.

Mmap Kernel Buffer To User Space

In the interest of writing portable drivers, however, you should use the variant of remap_pfn_range that is suited to your particular situation. Can a half-elf take human traits? Remap_pfn_range Example Growth of the mapped region, on the other hand, isn't really meaningful for the driver until the program invoking mremap accesses the new virtual addresses. Linux Mmap Example starting at PAGE_OFFSET).

If you have a set of pages, in a vmalloc allocated area, each page may have a different virtual address in the direct mapping. see here You need to configure the kernel tree (e.g. Edit the first line of the makefile to adjust to your kernel source tree. To maximize allocation performance, the Linux kernel maintains a list of free pages for each allocation order, and only the reference count of the first page in a cluster is incremented Linux Dma Driver Example

I also expect that the userspace program will access my buffer sequentially, resulting in a performance hit when my nopage() function is invoked every time a page boundary is crossed. The address parameter will contain the virtual address that caused the fault, rounded down to the beginning of the page. But isn't all kernel memory already non-swappable i.e. this page This question is about the interaction of get_free_pages and remap_pfn_range. –EML Sep 20 '15 at 15:47 add a comment| 1 Answer 1 active oldest votes up vote 3 down vote After

A private page can be shared across processes until one process writes to it. Remap_page_range With just one object file as in # this example this is not needed. The nopage method normally returns a pointer to a struct page.

The simplest way to prevent extension of the mapping is to implement a simple nopage method that always causes a bus signal to be sent to the faulting process.

What's the verb to describe a good guy becoming a bad guy? int (*sync)(struct vm_area_struct *vma, unsigned long, size_t, unsigned int flags); This method is called by the msync system call to save a dirty memory region to the storage medium. For example, if you did my_buf = vmalloc_user(MY_BUF_SIZE); then the page you use would be something like vmalloc_to_page(my_buf + (vmf->pgoff << PAGE_SHIFT)); But you could easily create an array and allocate /dev/mem Mmap These fields may be used by device drivers in their mmap implementation.

If your driver does not increment the count when adding a page to the area, the usage count becomes 0 prematurely, and the integrity of the system is compromised. The flags of the most interest to device driver writers are VM_IO and VM_RESERVED. 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 http://connectwithcanopy.com/device-driver/developing-embedded-linux-device-drivers-for-a-system-on-chip-device.php The use of three levels in a processor-independent implementation allows Linux to support both two-level and three-level processors without clobbering the code with a lot of #ifdef statements.

If you have a logical address, the macro __pa() (defined in ) will return its associated physical address. What we didn't see in that chapter was a second, char device (called sbullr), which provides raw access to the RAM-disk device. more stack exchange communities company blog Tour Start here for a quick overview of the site Help Center Detailed answers to any questions you might have Meta Discuss the workings and With 32 bits, it is possible to address 4 GB of memory.

RT Linux Linux Address Types MMAP with example ► 2009 (18) ► December (2) ► September (1) ► August (5) ► June (1) ► May (2) ► April (4) ► March Each field in /proc/*/maps (except the image name) corresponds to a field in struct vm_area_struct, and is described in the following list. An contiguous address area in the kernel segment is also contigous in physical memory. This is unlike calls such as ioctl and poll, where the kernel does not do much before calling the method.

Page Table A page-aligned array of items, each of which is called a Page Table Entry. Because there is no struct page to return a pointer to, nopage cannot be used in these situations; you must use remap_pfn_range instead. The middle mapping is at a0000, which is the standard location for video RAM in the 640 KB ISA hole. Allocate memory in user space The uClibc implements malloc using mmap().

How are banners kept upright? linux-kernel linux-device-driver share|improve this question asked May 25 '12 at 19:42 ravi 172128 Not sure if this helps but as far as I know, Perf subsystem in the kernel Putting the Parts together The example below shows a device driver, that allocates two memory area: one with vmalloc(), the other with kmalloc(). file: user/blkfin-test/mmap_test/simple.cscm failed with exit code 1: file does not exist in git And use “mknod simple c 254 0” to create the ”/dev/simple” device for applications to open.

We need to stop the VM system from removing our pages from main memory. sys_mlock() how this can be done. Mapping User-Space Buffers and Raw I/O Unix systems have long provided a "raw'' interface to some devices -- block devices in particular -- which performs I/O directly from a user-space buffer Memory returned from kmalloc has a logical address.

Then it is just a matter of calling remap_page_range to create the necessary page tables. For low-memory pages, it just returns the logical address of the page; for high-memory pages, kmapcreates a special mapping.