Linux code for reading a GM1356 USB-based decibel sound monitor. See https://oldvcr.blogspot.com/2021/12/monitoring-vintage-server-room-and.html

gm1356.c

/*

(C)2020-1 Cameron Kaiser, ckaiser@floodgap.com
All rights reserved.
Distributed under the Floodgap Free Software License.

Credit to https://github.com/dobra-noc/gm1356/blob/master/PROTOCOL.md for the
original protocol description.

Linux:
gcc -o gm1356 gm1356.c -lusb

This device doesn't register as a HID on Mac OS X (polling time of 0?!).
This device freaks out NetBSD and messes with the console.
Even Linux's libhid doesn't like it, so we have to talk to it the hard way.

*/

#define DEBUG 0
#define DSLM_VENDID 0x64bd
#define DSLM_DEVID 0x74e3

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <getopt.h>

#define SEND_PACKET_LENGTH 8
unsigned char PACKET[SEND_PACKET_LENGTH] = {
	0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
};
int ret;

/* #if defined(__linux__) */
#include <usb.h>

/* this is written like this in the hopes that there will be ports to
   other operating systems, but this device is a piece of crap and has
   stymied all of my current attempts. */

struct usb_bus *busses;
struct usb_bus *bus;
struct usb_device *dev, *dslm = NULL;
struct usb_dev_handle *hdslm;

void release_dslm_handle() {
	(void)usb_release_interface(hdslm, 1);
	(void)usb_close(hdslm);
}

int claim_dslm_handle() {
	if (DEBUG)
		usb_set_debug(255);

	usb_init();
	usb_find_busses();
	ret = usb_find_devices();
	if (ret < 0) {
		perror("usb_find_devices");
		return 1;
	}
	busses = usb_get_busses();
	for(bus = busses; bus; bus = bus->next) {
		for(dev = bus->devices; dev; dev = dev->next) {
			if ((dev->descriptor.idVendor == DSLM_VENDID) &&
			    (dev->descriptor.idProduct == DSLM_DEVID)) {
				dslm = dev;
			}
		}
	}
	if (!dslm) {
		fprintf(stderr, "unable to find DSLM\n");
		return 1;
	}
	hdslm = usb_open(dslm);
	usb_detach_kernel_driver_np(hdslm, 0);
	ret = usb_claim_interface(hdslm, 0);
	if (ret < 0) {
		release_dslm_handle();
		perror("usb_claim_interface");
		return 1;
	}

	return 0;
}

/*
int usb_interrupt_write(usb_dev_handle *dev, int ep, const char *bytes,
	int size, int timeout);
*/

int send_dslm_packet() {
	ret = usb_interrupt_write(
		hdslm,
		0x02, /* endpoint 2 OUT */
		(const char *)PACKET,
		SEND_PACKET_LENGTH,
		10000);
	if (DEBUG) {
		fprintf(stderr, "usb_interrupt_write: %d\n", ret);
		if (ret != SEND_PACKET_LENGTH)
			return 1;
	} else if (ret != SEND_PACKET_LENGTH) {
		fprintf(stderr, "usb_interrupt_write: %d\n", ret);
		return 1;
	}

	return 0;
}

/*
int usb_interrupt_read(usb_dev_handle *dev, int ep, char *bytes, int size,
        int timeout);
*/

int get_dslm_report() {
	ret = usb_interrupt_read(
		hdslm,
		0x81, /* endpoint 1 IN */
		(char *)PACKET,
		SEND_PACKET_LENGTH,
		10000);
	if (DEBUG) {
		fprintf(stderr, "usb_interrupt_read: %d\n", ret);
	}
	return ret;
}

/* #endif */

void usage(char **argv) {
	fprintf(stderr, "usage: %s [-s] [-c] [-m] [-r 0-4]\n", argv[0]);
}

int main(int argc, char** argv) {
	int ch;
	size_t tries = 0;
	float db, maxdb = 130.0f;
	long r;

	/* send settings */
	PACKET[0] = 0x56;
	PACKET[1] = 0x40; /* filter A, not max, fast, full 30-130dB range */
	while ((ch = getopt(argc, argv, "?hr:csm")) != -1) {
		switch(ch) {
			case 's': {
				/* use 1 second averaged sample, not instant */
				PACKET[1] &= 0xbf; /* turn off fast bit */
				break;
			}
			case 'c': {
				/* use low-pass filter "filter C" */
				PACKET[1] |= 0x10; /* turn on C bit */
				break;
			}
			case 'm': {
				/* return max reading rather than current */
				PACKET[1] |= 0x20; /* turn on max bit */
				break;
			}
			case 'r': {
				/* set decibel range for greater precision */
				/*
					0: 30-130dB range
					1: 30-60dB
					2: 50-100dB
					3: 60-110dB
					4: 80-130dB
				*/
				ret = atoi(optarg);
				if (ret < 0 || ret > 4) {
					fprintf(stderr, "%s: -r 0-4 only\n",
						argv[0]);
					return 1;
				}
				maxdb =
					(maxdb == 1) ? 60 :
					(maxdb == 2) ? 100 :
					(maxdb == 3) ? 110 :
					130;
				PACKET[1] |= ret;
				break;
			}
			case '?' :
			case 'h' :
			default: {
				usage(argv);
				return 0;
			}
		}
	}
	
	if (claim_dslm_handle())
		return 1;

	if (send_dslm_packet())
		return 1;

	/* request dB */
	PACKET[0] = 0xb3;
	/* needs a random ID number */
	r = random();
	PACKET[1] = r & 0xff;
	PACKET[2] = ( r >> 8 ) & 0xff;
	PACKET[3] = ( r >>16 ) & 0xff;

	if (DEBUG) {
		for(ret=0;ret<8;ret++) {
			fprintf(stderr, "%02x ", PACKET[ret]);
		}
	}

	for(;;) {
		/* sometimes have to try this a few times */
		if (send_dslm_packet())
			return 1;

		if (get_dslm_report() == SEND_PACKET_LENGTH) {
			if (DEBUG) {
				for(ret=0;ret<8;ret++) {
					fprintf(stderr, "%02x ", PACKET[ret]);
				}
			}
			/* check for preposterous values: cannot be > range */
			db = ((256*PACKET[0]) + PACKET[1]) / 10.0f;
			if (db <= maxdb)
				break;
			if (DEBUG)
				fprintf(stderr, "preposterous value %2.1f\n",
					db);
		}
		if (++tries > 8) {
			fprintf(stderr, "read failed\n");
			release_dslm_handle();
			return 1;
		}
	}
	fprintf(stdout, "%2.1f\n", db);

	release_dslm_handle();
	return 0;
}