hps-magic.c

// SPDX-License-Identifier: GPL-2.0
/*                              
 * Userspace ioctl example for the ChromeOS snooping prevention sensor (HPS).
 *                                                                                                    
 * Copyright 2022 Google LLC.                                                                         
 */                                                                                                   
        
#include <linux/i2c.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/ioctl.h>

#define HPS_IOC_TRANSFER        _IOWR('h', 0x01, struct hps_transfer_ioctl_data)
#define HPS_MAGIC               0x9df2
#define HPS_CMD_READ_REG(n)     (0x80 | n)

struct hps_transfer_ioctl_data {
        __u32 isize;                    /* Number of bytes to send */
        unsigned char *ibuf;            /* Input buffer */
        __u32 osize;                    /* Number of bytes to receive */
        unsigned char *obuf;            /* Output buffer */
};

int main() {
        int fd = open("/dev/hps", O_RDWR);
        int ret;
        int magic;
        unsigned char cmd[1] = { HPS_CMD_READ_REG(0) };
        unsigned char out[2];
        struct hps_transfer_ioctl_data args = {
                .isize = sizeof(cmd),
                .ibuf = &cmd[0],
                .osize = sizeof(out),
                .obuf = &out[0],
        };

        if (fd < 0) {
                perror("open");
                return EXIT_FAILURE;
        }

        /* Wait for the sensor to come online. */
        usleep(1000 * 1000);

        ret = ioctl(fd, HPS_IOC_TRANSFER, &args);
        if (ret != 2) {
                perror("ioctl");
                close(fd);
                return EXIT_FAILURE;
        }
        magic = (out[0] << 8) | out[1];
        printf("HPS magic: 0x%04x (expected 0x%04x)\n", magic, HPS_MAGIC);
        close(fd);

        return magic == HPS_MAGIC;
}