Re[2]: mmap CMA area on /dev/mem

[Кирилл Луценко]

 I disabled CONFIG_x86_PAT and mmap() has started to work!  Thank you so much, Jeff!


Вторник, 21 января 2014, 17:05 UTC от Jeff Haran <Jeff.Haran at citrix.com>:
> 
> 
>From: kernelnewbies-bounces at kernelnewbies.org [mailto:kernelnewbies-bounces at kernelnewbies.org] On Behalf Of  ?????? ???????
>Sent: Tuesday, January 21, 2014 1:45 AM
>To: kernelnewbies at 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
> 
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-- 
Кирилл Луценко
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