/* fbpantest.c — empirically find the ioctl that pans /dev/fb0 by yoffset.
 * Confirmed already: fb0 is 480x720 RGB565, yres_virtual=1440 (double),
 * yoffset selects the displayed buffer, ypanstep=1. We try candidate ioctls
 * that set yoffset to the OTHER buffer and read yoffset back; whichever
 * changes it is the pan mechanism. We restore yoffset immediately.
 */
#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 GETV 0x4600

static int get_info(int fd, uint32_t *yoff, uint32_t *yres) {
    unsigned char v[160];
    memset(v, 0, sizeof v);
    if (ioctl(fd, GETV, v) < 0) return -1;
    *yres = *(uint32_t*)(v + 4);
    *yoff = *(uint32_t*)(v + 20);
    return 0;
}

static void write_yoffset_to(int fd, uint32_t cmd, uint32_t yoffset) {
    unsigned char v[160];
    memset(v, 0, sizeof v);
    if (ioctl(fd, GETV, v) < 0) return;
    /* modify only the offset fields, keep everything else */
    *(uint32_t*)(v + 16) = 0;
    *(uint32_t*)(v + 20) = yoffset;
    ioctl(fd, cmd, v);
}

int main(void) {
    int fd = open("/dev/fb0", O_RDWR);
    if (fd < 0) { perror("open /dev/fb0"); return 1; }
    uint32_t orig = 0, yres = 0;
    if (get_info(fd, &orig, &yres) < 0) { perror("getvar"); return 1; }
    uint32_t target = (orig < yres) ? yres : 0;
    printf("orig yoffset=%u yres=%u target(other buf)=%u\n", orig, yres, target);

    unsigned tests[] = { 0x4601, 0x4606 };  /* PUT_VSCREENINFO, PAN_DISPLAY(nr 0x06) */
    for (unsigned i = 0; i < sizeof(tests)/sizeof(tests[0]); i++) {
        uint32_t cmd = tests[i];
        write_yoffset_to(fd, cmd, target);
        uint32_t now = 0, tmp = 0;
        int ok = get_info(fd, &now, &tmp);
        printf("  cmd=0x%04X -> yoffset=%u  (target %u)  %s\n",
               cmd, now, target, (ok==0 && now==target) ? "*** PAN WORKS ***" : "");
        /* restore */
        if (ok == 0 && now == target) {
            write_yoffset_to(fd, cmd, orig);
            uint32_t back = 0;
            get_info(fd, &back, &tmp);
            printf("    restored to yoffset=%u\n", back);
            break;
        }
    }

    /* also probe FBIOBLANK candidate 0x4611 and 0x4603 just to report which blanks */
    errno = 0;
    int b1 = ioctl(fd, 0x4603, 5);
    int e1 = errno; errno = 0;
    int b2 = ioctl(fd, 0x4611, 5);
    int e2 = errno;
    printf("  blank 0x4603(5) r=%d errno=%d ; 0x4611(5) r=%d errno=%d\n", b1, e1, b2, e2);
    /* ensure unblanked back to 0 regardless */
    ioctl(fd, 0x4611, 0);   /* unblank (works per launcher) */

    close(fd);
    return 0;
}