staging: most: aim-cdev: make syscall write accept buffers of arbitrary size

This patch allows to call the write() function for synchronous and
isochronous channels with buffers of any size. The AIM simply waits for
data to fill up the MOST buffer object according to the network interface
controller specification for streaming channels, before it submits the
buffer to the HDM.

The new behavior is backward compatible to the old applications, since
all known applications needed to fill the buffer completely anyway.

Signed-off-by: Christian Gromm <christian.gromm@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/drivers/staging/most/aim-cdev/cdev.c b/drivers/staging/most/aim-cdev/cdev.c
index 5458fb9..7f51024 100644
--- a/drivers/staging/most/aim-cdev/cdev.c
+++ b/drivers/staging/most/aim-cdev/cdev.c
@@ -57,7 +57,11 @@
 
 static inline bool ch_get_mbo(struct aim_channel *c, struct mbo **mbo)
 {
-	*mbo = most_get_mbo(c->iface, c->channel_id, &cdev_aim);
+	if (!kfifo_peek(&c->fifo, mbo)) {
+		*mbo = most_get_mbo(c->iface, c->channel_id, &cdev_aim);
+		if (*mbo)
+			kfifo_in(&c->fifo, mbo, 1);
+	}
 	return *mbo;
 }
 
@@ -184,8 +188,7 @@
 			 size_t count, loff_t *offset)
 {
 	int ret;
-	size_t actual_len;
-	size_t max_len;
+	size_t to_copy, left;
 	struct mbo *mbo = NULL;
 	struct aim_channel *c = filp->private_data;
 
@@ -205,20 +208,24 @@
 		goto unlock;
 	}
 
-	max_len = c->cfg->buffer_size;
-	actual_len = min(count, max_len);
-	mbo->buffer_length = actual_len;
-
-	if (copy_from_user(mbo->virt_address, buf, mbo->buffer_length)) {
+	to_copy = min(count, c->cfg->buffer_size - c->mbo_offs);
+	left = copy_from_user(mbo->virt_address + c->mbo_offs, buf, to_copy);
+	if (left == to_copy) {
 		ret = -EFAULT;
-		goto put_mbo;
+		goto unlock;
 	}
 
-	most_submit_mbo(mbo);
-	mutex_unlock(&c->io_mutex);
-	return actual_len;
-put_mbo:
-	most_put_mbo(mbo);
+	c->mbo_offs += to_copy - left;
+	if (c->mbo_offs >= c->cfg->buffer_size ||
+	    c->cfg->data_type == MOST_CH_CONTROL ||
+	    c->cfg->data_type == MOST_CH_ASYNC) {
+		kfifo_skip(&c->fifo);
+		mbo->buffer_length = c->mbo_offs;
+		c->mbo_offs = 0;
+		most_submit_mbo(mbo);
+	}
+
+	ret = to_copy - left;
 unlock:
 	mutex_unlock(&c->io_mutex);
 	return ret;
@@ -287,7 +294,7 @@
 		if (!kfifo_is_empty(&c->fifo))
 			mask |= POLLIN | POLLRDNORM;
 	} else {
-		if (ch_has_mbo(c))
+		if (!kfifo_is_empty(&c->fifo) || ch_has_mbo(c))
 			mask |= POLLOUT | POLLWRNORM;
 	}
 	return mask;