/* fbprobe.c — inspect /dev/fb0 geometry & current pan position.
 * Device-verified ioctl numbers (do NOT swap): FBIOGET_VSCREENINFO=0x4600,
 * FBIOGET_FSCREENINFO=0x4602, FBIOBLANK=0x4611.
 */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <errno.h>
#include <stdint.h>

#define FBIOGET_VSCREENINFO 0x4600
#define FBIOGET_FSCREENINFO 0x4602
#define FBIOBLANK          0x4611
#define FBIO_PAN_DISPLAY   0x4606
#define FBIO_WAITFORVSYNC  0x4620

int main(void)
{
    int fd = open("/dev/fb0", O_RDWR);
    if (fd < 0) { perror("open /dev/fb0"); return 1; }

    unsigned char v[160], f[68];
    memset(v, 0, sizeof v); memset(f, 0, sizeof f);
    if (ioctl(fd, 0x4600, v) < 0) { perror("vscreeninfo(0x4600)"); return 1; }
    if (ioctl(fd, 0x4602, f) < 0) { perror("fscreeninfo(0x4602)"); return 1; }

    uint32_t xres  = *(uint32_t*)(v + 0);
    uint32_t yres  = *(uint32_t*)(v + 4);
    uint32_t xv    = *(uint32_t*)(v + 8);
    uint32_t yv    = *(uint32_t*)(v + 12);
    uint32_t xoff  = *(uint32_t*)(v + 16);
    uint32_t yoff  = *(uint32_t*)(v + 20);
    uint32_t bpp   = *(uint32_t*)(v + 24);
    uint32_t vmode = *(uint32_t*)(v + 80);

    uint32_t smem_len = *(uint32_t*)(f + 20);
    uint32_t visual   = *(uint32_t*)(f + 32);
    uint16_t xs       = *(uint16_t*)(f + 36);
    uint16_t ys       = *(uint16_t*)(f + 38);
    uint32_t line_len = *(uint32_t*)(f + 44);
    uint32_t type     = *(uint32_t*)(f + 24);

    printf("V: xres=%u yres=%u xres_v=%u yres_v=%u xoffset=%u yoffset=%u bpp=%u vmode=%u\n",
           xres, yres, xv, yv, xoff, yoff, bpp, vmode);
    printf("V: red(offset%u,len%u) green(off%u,len%u) blue(off%u,len%u) transp(off%u,len%u)\n",
           *(uint32_t*)(v+28)&0xFF, *(uint32_t*)(v+28)>>8,
           *(uint32_t*)(v+32)&0xFF, *(uint32_t*)(v+32)>>8,
           *(uint32_t*)(v+36)&0xFF, *(uint32_t*)(v+36)>>8,
           *(uint32_t*)(v+40)&0xFF, *(uint32_t*)(v+40)>>8);
    printf("F: id='%.16s' smem_len=%u type=%u visual=%u xpanstep=%u ypanstep=%u line_length=%u\n",
           (char*)f, smem_len, type, visual, xs, ys, line_len);

    unsigned long one = (unsigned long)line_len * yres;
    printf("frame size (line*yres) = %lu ; 2*frame = %lu ; smem_len = %u\n",
           one, one * 2, smem_len);
    printf("DOUBLE-BUFFER? %s\n",
           (smem_len >= one * 2) ? "YES (>= 2 frames in smem)" : "single (1 frame)");
    printf("YRES_VIRTUAL = %u ; yoffset = %u ; displayed buffer index = %u\n",
           yv, yoff, yv > 0 ? (yoff / (yres ? yres : 1)) : 0);
    printf("display starts at byte %u (=%u*line_len)\n", yoff * line_len, yoff);
    printf("=> visible window x=%u..%u y=%u..%u (of %ux%u virtual)\n",
           xoff, xoff + xres, yoff, yoff + yres, xv, yv);

    /* try FBIOPAN_DISPLAY to read back set-able + WAITFORVSYNC presence */
    unsigned char pan[160];
    memcpy(pan, v, sizeof pan);
    pan[20] = 0; pan[16] = 0;   /* yoffset=0 xoffset=0, only to test */
    errno = 0;
    /* don't actually pan (avoid disturbing the player); just report errno of a
     * no-op probe would need a real pan.  Instead check WAITFORVSYNC support. */
    uint32_t zero = 0;
    errno = 0;
    int wfs = ioctl(fd, 0x4620, &zero);
    printf("FBIO_WAITFORVSYNC(0x4620) = %d (errno=%d %s)\n", wfs, errno, strerror(errno));
    close(fd);
    return 0;
}