iio_writedev: Skip input channels

Calling iio_writedev without any channels specified will enable all
channels of the device. Regardless whether the channel is an input or
output channel. Change this so that input channels are skipped.

Similarly ignore explicitly specified channels if they are an input
channel.

The main advantage of this change is that iio_writedev will now exit with an
error when it is accidentally called on an IIO input device, since no
output channels will be found.

But it also future-proofs the application for a time when there is support
for devices which have both input and output channels.

Signed-off-by: Lars-Peter Clausen <[email protected]>

tests/iio_writedev.c

@@ -262,6 +262,7 @@ static struct iio_context *scan(void)
 int main(int argc, char **argv)
 {
 	unsigned int i, nb_channels;
+	unsigned int nb_active_channels = 0;
 	unsigned int buffer_size = SAMPLES_PER_READ;
 	const char *arg_uri = NULL;
 	const char *arg_ip = NULL;
@@ -391,21 +392,34 @@ int main(int argc, char **argv)
 
 	if (argc == optind + 1) {
 		/* Enable all channels */
-		for (i = 0; i < nb_channels; i++)
-			iio_channel_enable(iio_device_get_channel(dev, i));
+		for (i = 0; i < nb_channels; i++) {
+			struct iio_channel *ch = iio_device_get_channel(dev, i);
+			if (iio_channel_is_output(ch)) {
+				iio_channel_enable(ch);
+				nb_active_channels++;
+			}
+		}
 	} else {
 		for (i = 0; i < nb_channels; i++) {
 			unsigned int j;
 			struct iio_channel *ch = iio_device_get_channel(dev, i);
 			for (j = optind + 1; j < (unsigned int) argc; j++) {
 				const char *n = iio_channel_get_name(ch);
-				if (!strcmp(argv[j], iio_channel_get_id(ch)) ||
-						(n && !strcmp(n, argv[j])))
+				if ((!strcmp(argv[j], iio_channel_get_id(ch)) ||
+						(n && !strcmp(n, argv[j]))) &&
+						iio_channel_is_output(ch)) {
 					iio_channel_enable(ch);
+					nb_active_channels++;
+				}
 			}
 		}
 	}
 
+	if (!nb_active_channels) {
+		fprintf(stderr, "No output channels found\n");
+		return EXIT_FAILURE;
+	}
+
 	sample_size = iio_device_get_sample_size(dev);
 
 	buffer = iio_device_create_buffer(dev, buffer_size, cyclic_buffer);