Add utility `cm108_set_gpio` to set CM108 GPIO pin for troubleshooting.

Also clarify why we write 5 bytes for a 4 byte HID output report in the comment
in cm108.c

src/CMakeLists.txt

@@ -420,6 +420,24 @@ if(UDEV_FOUND)
 endif()
 
 
+# Utility for setting the CM108 GPIO pins.
+if(UDEV_FOUND)
+  list(APPEND cm108_set_gpio_SOURCES
+    cm108_set_gpio.c
+    ${cm108_SOURCES}
+    )
+
+  add_executable(cm108_set_gpio
+    ${cm108_set_gpio_SOURCES}
+    )
+
+  target_link_libraries(cm108_set_gpio
+    ${MISC_LIBRARIES}
+    ${UDEV_LIBRARIES}
+    )
+endif()
+
+
 # Touch Tone to Speech sample application.
 # ttcalc
 list(APPEND ttcalc_SOURCES

src/cm108.c

@@ -836,10 +836,11 @@ static int cm108_write (char *name, int iomask, int iodata)
 	io[3] = iomask;
 	io[4] = 0;
 
-	// Writing 4 bytes fails with errno 32, EPIPE, "broken pipe."
-	// Hamlib writes 5 bytes which I don't understand.
-	// Writing 5 bytes works.
-	// I have no idea why.  From the CMedia datasheet it looks like we need 4.
+	// Per https://www.kernel.org/doc/Documentation/hid/hidraw.txt, the
+	// first byte is the report number, which is always 0 since the device
+	// only has 1 report per the datasheet. The remaining 4 bytes follow the
+	// structure specified in the datasheet, although we only care about the
+	// GPIO bytes here.
 
 	n = write (fd, io, sizeof(io));
 	if (n != sizeof(io)) {

src/cm108_set_gpio.c

@@ -0,0 +1,54 @@
+//
+//    This file is part of Dire Wolf, an amateur radio packet TNC.
+//
+//    Copyright (C) 2014, 2016, 2017  John Langner, WB2OSZ
+//
+//    This program is free software: you can redistribute it and/or modify
+//    it under the terms of the GNU General Public License as published by
+//    the Free Software Foundation, either version 2 of the License, or
+//    (at your option) any later version.
+//
+//    This program is distributed in the hope that it will be useful,
+//    but WITHOUT ANY WARRANTY; without even the implied warranty of
+//    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+//    GNU General Public License for more details.
+//
+//    You should have received a copy of the GNU General Public License
+//    along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+
+
+/*------------------------------------------------------------------
+ *
+ * Module:      cm108_set_gpio.c
+ *
+ * Purpose:   	Utility to set GPIO pins on a CM108.
+ *
+ * Description:	Test utility to set the GPIO pins on a CM108 USB sound device.
+ *
+ *---------------------------------------------------------------*/
+
+#include <stdlib.h>
+
+#include "cm108.h"
+#include "textcolor.h"
+
+void usage() {
+	text_color_set(DW_COLOR_INFO);
+	dw_printf("\n");
+	dw_printf("cm108_set_gpio - Utility to set a CM108 GPIO pin.\n");
+	dw_printf("\n");
+	dw_printf("Usage:	cm108_set_gpio /dev/hidrawN PIN_NUMBER <0|1>\n");
+	dw_printf("\n");
+}
+
+int main (int argc, char **argv) {
+	if (argc != 4) {
+		usage();
+		return 1;
+	}
+	char* hidraw_filename = argv[1];
+	int pin_num = atoi(argv[2]);
+	int state = atoi(argv[3]);
+	return cm108_set_gpio_pin(hidraw_filename, pin_num, state);
+}