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Title: frame buffer mmap error [Print This Page]

Author: puffbleedoff Time: 6/18/2019 10:54
Title: frame buffer mmap error
Hello.

I am trying to test linux frame buffer on the ubuntu 18.04 installed aio-3399j board.

On the console,

# cat /dev/urandom > /dev/fb0

Testing with this command works fine. (noise displayed on the screen)

However, when I test frame buffer with c code using mmap(), it fails.
(mmap returns -1 (EINVAL))

# ./fb
The framebuffer device was opened successfully.
1280x800, 32bpp
Error: failed to map framebuffer device to memory: Invalid argument

Can anyone help me to solve this problem?

c source code:

#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <fcntl.h>
#include <linux/fb.h>
#include <sys/mman.h>
#include <sys/ioctl.h>

int main(int argc, char *argv[])
{
      int fbfd = 0;
      struct fb_var_screeninfo vinfo;
      struct fb_fix_screeninfo finfo;
      long int screensize = 0;
      char *fbp = 0;
      int x = 0, y = 0;
      long int location = 0;

      // Open the file for reading and writing
      fbfd = open("/dev/fb0", O_RDWR);
      if (fbfd == -1)
      {
            perror("Error: cannot open framebuffer device");
            exit(1);
      }
      printf("The framebuffer device was opened successfully.\n");

      // Get fixed screen information
      if (ioctl(fbfd, FBIOGET_FSCREENINFO, &finfo) == -1)
      {
            perror("Error reading fixed information");
            exit(2);
      }
      // Get variable screen information
      if (ioctl(fbfd, FBIOGET_VSCREENINFO, &vinfo) == -1)
      {
            perror("Error reading variable information");
            exit(3);
      }

      printf("%dx%d, %dbpp\n", vinfo.xres, vinfo.yres, vinfo.bits_per_pixel);

      // Figure out the size of the screen in bytes
      screensize = vinfo.xres * vinfo.yres * vinfo.bits_per_pixel / 8;

      // Map the device to memory
      fbp = (char *) mmap(0, screensize, PROT_READ | PROT_WRITE, MAP_SHARED, fbfd, 0);
      if ((int) fbp == -1)
      {
            perror("Error: failed to map framebuffer device to memory");
            exit(4);
      }
      printf("The framebuffer device was mapped to memory successfully.\n");

      x = 300;
      y = 100;       // Where we are going to put the pixel

      // Figure out where in memory to put the pixel
      for (y = 100; y < 300; y++)
            for (x = 100; x < 300; x++)
            {

                     location = (x + vinfo.xoffset) * (vinfo.bits_per_pixel / 8) + (y + vinfo.yoffset) * finfo.line_length;

                     if (vinfo.bits_per_pixel == 32)
                     {
                              *(fbp + location) = 100;       // Some blue
                              *(fbp + location + 1) = 15 + (x - 100) / 2;    // A little green
                              *(fbp + location + 2) = 200 - (y - 100) / 5; // A lot of red
                              *(fbp + location + 3) = 0;    // No transparency
                     } else
                     {    //assume 16bpp
                              int b = 10;
                              int g = (x - 100) / 6;  // A little green
                              int r = 31 - (y - 100) / 16; // A lot of red
                              unsigned short int t = r << 11 | g << 5 | b;
                              *((unsigned short int *) (fbp + location)) = t;
                     }

            }
      munmap(fbp, screensize);
      close(fbfd);
      return 0;
}

Author: penguin Time: 6/18/2019 15:49
Last edited by penguin In 6/18/2019 16:00 Editor

This is similar to your problem:

https://stackoverflow.com/questions/22915778/mmap-dev-fb0-fails-with-invalid-argument

However, you can try the DRM way:
https://stackoverflow.com/questions/36960747/calling-mmap-on-dumbbuffer-with-linux-direct-rendering-manager-in-rust-fails-wh

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