core: accept zero-size time csets

This commit allows zero-size channels to exist. The feature is used
for TDC and DTC (time to digital converters and the opposite).  This
simple change in the core allows to implement TDC drivers that only
return the control (which includes a time stamp) and no data.

For DTC to work we need some modification to chardev.c, which are not
there yet.

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>
Acked-by: Federico Vaga <federico.vaga@gmail.com>

bus.c

@@ -220,7 +220,12 @@ static int _zdev_template_check_and_init(struct zio_device *zdev,
 				name, cset->index);
 			return -EINVAL;
 		}
-		if (!cset->ssize) {
+		/*
+		* Only time-type can have a zero-sized sample. This
+		* protects against users who forgot to fill fields.
+		*/
+		if (!cset->ssize &&
+		    (cset->flags & ZIO_CSET_TYPE) != ZIO_CSET_TYPE_TIME) {
 			dev_err(&zdev->head.dev,
 				"ssize can not be 0 in %s cset%i\n",
 				name, cset->index);

objects.c

@@ -527,11 +527,7 @@ static int chan_register(struct zio_channel *chan, struct zio_channel *chan_t)
 		goto out_zattr_check;
 	}
 	ctrl->nsamples = chan->cset->ti->nsamples;
-	ctrl->nbits = __get_nbits(chan);
-	if (!ctrl->nbits) {
-		err = -EINVAL; /* message already printed */
-		goto out_ctrl_bits;
-	}
+	ctrl->nbits = __get_nbits(chan); /* may be zero */
 	/* ctrl->addr.family = PF_ZIO */
 	ctrl->addr.cset = chan->cset->index;
 	ctrl->addr.chan = chan->index;