From: kernelnewbies-bounces@kernelnewbies.org [mailto:kernelnewbies-bounces@kernelnewbies.org] On Behalf Of ?????? ??????? Sent: Tuesday, January 21, 2014 1:45 AM To: kernelnewbies@kernelnewbies.org Subject: mmap CMA area on /dev/mem Hello everyone! I need reserve 256-512 Mb of continuous physical memory and have access to this memory from the user space. I decided to use CMA for memory reserving. Here are the steps on my idea that must be performed: 1. Reservation required amount of memory by CMA during system booting. 2. Parsing of CMA patch output which looks like for example: "CMA: reserved 256 MiB at 27400000" and saving two parameters: size of CMA area = 256*1024*1024 bytes and phys address of CMA area = 0x27400000. 3. Mapping of CMA area at /dev/mem file with offset = 0x27400000 using mmap(). (Of course, CONFIG_STRICT_DEVMEM is disabled) It would let me to read data directly from phys memory from user space. But the next code make segmentation fault(there size = 1Mb): int file; void* start; file=open("/dev/mem", O_RDWR | O_SYNC); if ( (start = mmap(0, 1024*1024, PROT_READ | PROT_WRITE, MAP_SHARED, file, 0x27400000)) == MAP_FAILED ){ perror("mmap"); } for (int offs = 0; offs<50; offs++){ cout<<((char *)start)[offs]; } Output of this code: "mmap: Invalid argument". When I changed offset = 0x27400000 on 0, this code worked fine and program displayed trash. It also work for alot of offsets which I looked at /proc/iomem. According to information from /proc/iomem, phys addr of CMA area (0x27400000 on my system) always situated in System RAM. Does anyone have any ideas, how to mmap CMA area on /dev/mem? What am I doing wrong? Thanks alot for any help! You don’t say what kind of processor you are running this on, but if its x86_64 you might want to look at your kernel configuration. If CONFIG_X86_PAT is configured you will have problems mapping memory to user space. It basically implements the same restrictions as CONFIG_STRICT_DEVMEM. Jeff Haran