Re: Having problem with mmap system call!!!

["Richard B. Johnson" <root@chaos.analogic.com>]

On Thu, 16 Sep 2004, Srinivas G. wrote:

> Hi All,
>
> I have a doubt about mmap system call.
>

mmap() works. Otherwise you wouldn't be sending any email.
It is used every time you open an application because that's
how shared libraries work.

You need to return the PHYSICAL address of your camera buffer
to user-space (probably using a driver ioctl()). Then the
user-mode code does ....


#include <stdio.h>
#include <errno.h>
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/mman.h>

#define HINT  0x20000000
#define PROT (PROT_READ|PROT_WRITE)
#define FLAGS (MAP_FIXED|MAP_SHARED)
#define SHM_FAIL (void *)-1


void *init_shmem(size_t addr, size_t len)
{
    int fd;
    void *vp;
    if((fd = open("/dev/mem", O_RDWR, 0)) < 0)
    {
        fprintf(stderr, "Can't open memory device\n");
        exit(EXIT_FAILURE);
    }
    if((vp = mmap((caddr_t) HINT, len, PROT, FLAGS, fd, addr)) == SHM_FAIL)
    {
        fprintf(stderr, "Can't access shared memory\n");
        exit(EXIT_FAILURE);
    }
    (void)close(fd);
    return vp;
}

After that, anything the camera writes to its address will
be available in user-mode at the memory-mapped address.
This DOES work. That's how I do direct DMA to user-space
all the time.

Cheers,
Dick Johnson
Penguin : Linux version 2.4.26 on an i686 machine (5570.56 BogoMips).
            Note 96.31% of all statistics are fiction.