Created attachment 294525 [details]
source code of the program

There seems to be some issue with the /dev/fb0 device on my Lenovo Thinkpad X1 Carbon 7th gen.

When I write to the framebuffer device using the API in linux/fb.h, the laptop screen can only pick up the first frame. The screen freezes until I use Ctrl-C to exit the program.

Here is a minimal program that demonstrates the problem:

    /* compile with 
     *   $ cc fb.c -lm
     */

    #include <stdint.h>
    #include <assert.h>
    #include <unistd.h>
    #include <fcntl.h>
    #include <linux/fb.h>
    #include <sys/mman.h>
    #include <sys/ioctl.h>
    #include <time.h>
    #include <math.h>

    enum {
      width = 256, height = 256
    };

    static float
    t(void)
    {
      struct timespec ts;
      clock_gettime(CLOCK_MONOTONIC, &ts);
      return ts.tv_sec + ts.tv_nsec/1000000000.0;
    }

    /* draw anything that changes with time */
    static void
    draw(uint32_t *fb, int pitch)
    {
      for (int y = 0; y < height; ++y)
      for (int x = 0; x < width;  ++x) {
        uint8_t s = 255*(0.5 + 0.5 * sin(x*x + y*y + 5*t()));
        fb[y*pitch + x] = 0xff<<24 | s<<16 | s<<8 | s;
      }
    }

    int 
    main(void) {
      struct fb_fix_screeninfo finfo;
      struct fb_var_screeninfo vinfo;
      int fd;
      size_t size;
      uint32_t *fb;

      fd = open("/dev/fb0", O_RDWR);
      if (fd < 0)
        return 1;
      if (ioctl(fd, FBIOGET_FSCREENINFO, &finfo) < 0)
        return 1;
      if (ioctl(fd, FBIOGET_VSCREENINFO, &vinfo) < 0)
        return 1;

      /* we will only work with 32 bit color for simplicity */
      assert(vinfo.bits_per_pixel == 32);

      /* set up our framebuffer */
      size = vinfo.yres_virtual * finfo.line_length;
      fb = mmap(0, size, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
      if (fb == MAP_FAILED)
        return 1;
      fb = (fb + vinfo.xoffset + vinfo.yoffset*finfo.line_length);

      while (1) {
        draw(fb, finfo.line_length/4);
      }

      munmap(fb, size);
      close(fd);
    }

Make sure to run the program on TTY and you are in video group.

Strangely, the HDMI output is not affected. I can see the animation on my monitor but not on the laptop screen (1080p non-touch).

Kernel version: 5.10.4-arch2-1