serial: synclink_gt: dropped transmit data bugfix

Fix transmit bug that could drop send data if write() called close to
serial transmitter going idle after sending previous data.  Bug is caused
by incorrect use of device information member tx_count.

Driver originally processed one data block (write call) at a time, waiting
for transmit idle before sending more.  tx_count recorded how much data
was loaded in DMA buffers on write(), and was cleared on send completion. 
tx_count use was overloaded to record accumulated data from put_char()
callback when transmitter was idle.

A bug was introduced when transmit code was reworked to allow multiple
blocks of data in the tx DMA buffers which keeps transmitter from going
idle between blocks.  tx_count was set to size of last block loaded,
cleared when tx went idle, and monitored to know when to restart
transmitter without proper synchronization.  tx_count could be cleared
when unsent data remained in DMA buffers and transmitter required
restarting, effectively dropping unsent data.

Solution:
1. tx_count now used only to track accumulated data from put_char
2. DMA buffer state tracked by direct inspection of descriptors
   with spinlock synchronization
3. consolidate these tasks in tx_load() :
   a. check for available buffer space
   b. load buffers
   c. restart DMA and or serial transmitter as needed
   These steps were previously duplicated in multiple places,
   sometimes incompletely.
4. fix use of tx_count as active transmit indicator,
   instead using tx_active which is meant for that purpose

Signed-off-by: Paul Fulghum <paulkf@microgate.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Greg KH <greg@kroah.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/char/synclink_gt.c

@@ -468,7 +468,7 @@ static unsigned int free_tbuf_count(struct slgt_info *info);
 static unsigned int tbuf_bytes(struct slgt_info *info);
 static void reset_tbufs(struct slgt_info *info);
 static void tdma_reset(struct slgt_info *info);
-static void tx_load(struct slgt_info *info, const char *buf, unsigned int count);
+static bool tx_load(struct slgt_info *info, const char *buf, unsigned int count);
 
 static void get_signals(struct slgt_info *info);
 static void set_signals(struct slgt_info *info);
@@ -813,59 +813,32 @@ static int write(struct tty_struct *tty,
 	int ret = 0;
 	struct slgt_info *info = tty->driver_data;
 	unsigned long flags;
-	unsigned int bufs_needed;
 
 	if (sanity_check(info, tty->name, "write"))
-		goto cleanup;
+		return -EIO;
+
 	DBGINFO(("%s write count=%d\n", info->device_name, count));
 
-	if (!info->tx_buf)
-		goto cleanup;
+	if (!info->tx_buf || (count > info->max_frame_size))
+		return -EIO;
 
-	if (count > info->max_frame_size) {
-		ret = -EIO;
-		goto cleanup;
-	}
+	if (!count || tty->stopped || tty->hw_stopped)
+		return 0;
 
-	if (!count)
-		goto cleanup;
+	spin_lock_irqsave(&info->lock, flags);
 
-	if (!info->tx_active && info->tx_count) {
+	if (info->tx_count) {
 		/* send accumulated data from send_char() */
-		tx_load(info, info->tx_buf, info->tx_count);
-		goto start;
+		if (!tx_load(info, info->tx_buf, info->tx_count))
+			goto cleanup;
+		info->tx_count = 0;
 	}
-	bufs_needed = (count/DMABUFSIZE);
-	if (count % DMABUFSIZE)
-		++bufs_needed;
-	if (bufs_needed > free_tbuf_count(info))
-		goto cleanup;
 
-	ret = info->tx_count = count;
-	tx_load(info, buf, count);
-	goto start;
-
-start:
- 	if (info->tx_count && !tty->stopped && !tty->hw_stopped) {
-		spin_lock_irqsave(&info->lock,flags);
-		if (!info->tx_active)
-		 	tx_start(info);
-		else if (!(rd_reg32(info, TDCSR) & BIT0)) {
-			/* transmit still active but transmit DMA stopped */
-			unsigned int i = info->tbuf_current;
-			if (!i)
-				i = info->tbuf_count;
-			i--;
-			/* if DMA buf unsent must try later after tx idle */
-			if (desc_count(info->tbufs[i]))
-				ret = 0;
-		}
-		if (ret > 0)
-			update_tx_timer(info);
-		spin_unlock_irqrestore(&info->lock,flags);
- 	}
+	if (tx_load(info, buf, count))
+		ret = count;
 
 cleanup:
+	spin_unlock_irqrestore(&info->lock, flags);
 	DBGINFO(("%s write rc=%d\n", info->device_name, ret));
 	return ret;
 }
@@ -882,7 +855,7 @@ static int put_char(struct tty_struct *tty, unsigned char ch)
 	if (!info->tx_buf)
 		return 0;
 	spin_lock_irqsave(&info->lock,flags);
-	if (!info->tx_active && (info->tx_count < info->max_frame_size)) {
+	if (info->tx_count < info->max_frame_size) {
 		info->tx_buf[info->tx_count++] = ch;
 		ret = 1;
 	}
@@ -981,10 +954,8 @@ static void flush_chars(struct tty_struct *tty)
 	DBGINFO(("%s flush_chars start transmit\n", info->device_name));
 
 	spin_lock_irqsave(&info->lock,flags);
-	if (!info->tx_active && info->tx_count) {
-		tx_load(info, info->tx_buf,info->tx_count);
-	 	tx_start(info);
-	}
+	if (info->tx_count && tx_load(info, info->tx_buf, info->tx_count))
+		info->tx_count = 0;
 	spin_unlock_irqrestore(&info->lock,flags);
 }
 
@@ -997,10 +968,9 @@ static void flush_buffer(struct tty_struct *tty)
 		return;
 	DBGINFO(("%s flush_buffer\n", info->device_name));
 
-	spin_lock_irqsave(&info->lock,flags);
-	if (!info->tx_active)
-		info->tx_count = 0;
-	spin_unlock_irqrestore(&info->lock,flags);
+	spin_lock_irqsave(&info->lock, flags);
+	info->tx_count = 0;
+	spin_unlock_irqrestore(&info->lock, flags);
 
 	tty_wakeup(tty);
 }
@@ -1033,12 +1003,10 @@ static void tx_release(struct tty_struct *tty)
 	if (sanity_check(info, tty->name, "tx_release"))
 		return;
 	DBGINFO(("%s tx_release\n", info->device_name));
-	spin_lock_irqsave(&info->lock,flags);
-	if (!info->tx_active && info->tx_count) {
-		tx_load(info, info->tx_buf, info->tx_count);
-	 	tx_start(info);
-	}
-	spin_unlock_irqrestore(&info->lock,flags);
+	spin_lock_irqsave(&info->lock, flags);
+	if (info->tx_count && tx_load(info, info->tx_buf, info->tx_count))
+		info->tx_count = 0;
+	spin_unlock_irqrestore(&info->lock, flags);
 }
 
 /*
@@ -1506,27 +1474,25 @@ static netdev_tx_t hdlcdev_xmit(struct sk_buff *skb,
 
 	DBGINFO(("%s hdlc_xmit\n", dev->name));
 
+	if (!skb->len)
+		return NETDEV_TX_OK;
+
 	/* stop sending until this frame completes */
 	netif_stop_queue(dev);
 
-	/* copy data to device buffers */
-	info->tx_count = skb->len;
-	tx_load(info, skb->data, skb->len);
-
 	/* update network statistics */
 	dev->stats.tx_packets++;
 	dev->stats.tx_bytes += skb->len;
 
-	/* done with socket buffer, so free it */
-	dev_kfree_skb(skb);
-
 	/* save start time for transmit timeout detection */
 	dev->trans_start = jiffies;
 
-	spin_lock_irqsave(&info->lock,flags);
-	tx_start(info);
-	update_tx_timer(info);
-	spin_unlock_irqrestore(&info->lock,flags);
+	spin_lock_irqsave(&info->lock, flags);
+	tx_load(info, skb->data, skb->len);
+	spin_unlock_irqrestore(&info->lock, flags);
+
+	/* done with socket buffer, so free it */
+	dev_kfree_skb(skb);
 
 	return NETDEV_TX_OK;
 }
@@ -2180,7 +2146,7 @@ static void isr_serial(struct slgt_info *info)
 
 	if (info->params.mode == MGSL_MODE_ASYNC) {
 		if (status & IRQ_TXIDLE) {
-			if (info->tx_count)
+			if (info->tx_active)
 				isr_txeom(info, status);
 		}
 		if (info->rx_pio && (status & IRQ_RXDATA))
@@ -2276,13 +2242,42 @@ static void isr_tdma(struct slgt_info *info)
 	}
 }
 
+/*
+ * return true if there are unsent tx DMA buffers, otherwise false
+ *
+ * if there are unsent buffers then info->tbuf_start
+ * is set to index of first unsent buffer
+ */
+static bool unsent_tbufs(struct slgt_info *info)
+{
+	unsigned int i = info->tbuf_current;
+	bool rc = false;
+
+	/*
+	 * search backwards from last loaded buffer (precedes tbuf_current)
+	 * for first unsent buffer (desc_count > 0)
+	 */
+
+	do {
+		if (i)
+			i--;
+		else
+			i = info->tbuf_count - 1;
+		if (!desc_count(info->tbufs[i]))
+			break;
+		info->tbuf_start = i;
+		rc = true;
+	} while (i != info->tbuf_current);
+
+	return rc;
+}
+
 static void isr_txeom(struct slgt_info *info, unsigned short status)
 {
 	DBGISR(("%s txeom status=%04x\n", info->device_name, status));
 
 	slgt_irq_off(info, IRQ_TXDATA + IRQ_TXIDLE + IRQ_TXUNDER);
 	tdma_reset(info);
-	reset_tbufs(info);
 	if (status & IRQ_TXUNDER) {
 		unsigned short val = rd_reg16(info, TCR);
 		wr_reg16(info, TCR, (unsigned short)(val | BIT2)); /* set reset bit */
@@ -2297,8 +2292,12 @@ static void isr_txeom(struct slgt_info *info, unsigned short status)
 				info->icount.txok++;
 		}
 
+		if (unsent_tbufs(info)) {
+			tx_start(info);
+			update_tx_timer(info);
+			return;
+		}
 		info->tx_active = false;
-		info->tx_count = 0;
 
 		del_timer(&info->tx_timer);
 
@@ -3949,7 +3948,7 @@ static void tx_start(struct slgt_info *info)
 		info->tx_enabled = true;
 	}
 
-	if (info->tx_count) {
+	if (desc_count(info->tbufs[info->tbuf_start])) {
 		info->drop_rts_on_tx_done = false;
 
 		if (info->params.mode != MGSL_MODE_ASYNC) {
@@ -4772,25 +4771,36 @@ static unsigned int tbuf_bytes(struct slgt_info *info)
 }
 
 /*
- * load transmit DMA buffer(s) with data
+ * load data into transmit DMA buffer ring and start transmitter if needed
+ * return true if data accepted, otherwise false (buffers full)
  */
-static void tx_load(struct slgt_info *info, const char *buf, unsigned int size)
+static bool tx_load(struct slgt_info *info, const char *buf, unsigned int size)
 {
 	unsigned short count;
 	unsigned int i;
 	struct slgt_desc *d;
 
-	if (size == 0)
-		return;
+	/* check required buffer space */
+	if (DIV_ROUND_UP(size, DMABUFSIZE) > free_tbuf_count(info))
+		return false;
 
 	DBGDATA(info, buf, size, "tx");
 
+	/*
+	 * copy data to one or more DMA buffers in circular ring
+	 * tbuf_start   = first buffer for this data
+	 * tbuf_current = next free buffer
+	 *
+	 * Copy all data before making data visible to DMA controller by
+	 * setting descriptor count of the first buffer.
+	 * This prevents an active DMA controller from reading the first DMA
+	 * buffers of a frame and stopping before the final buffers are filled.
+	 */
+
 	info->tbuf_start = i = info->tbuf_current;
 
 	while (size) {
 		d = &info->tbufs[i];
-		if (++i == info->tbuf_count)
-			i = 0;
 
 		count = (unsigned short)((size > DMABUFSIZE) ? DMABUFSIZE : size);
 		memcpy(d->buf, buf, count);
@@ -4808,11 +4818,27 @@ static void tx_load(struct slgt_info *info, const char *buf, unsigned int size)
 		else
 			set_desc_eof(*d, 0);
 
-		set_desc_count(*d, count);
+		/* set descriptor count for all but first buffer */
+		if (i != info->tbuf_start)
+			set_desc_count(*d, count);
 		d->buf_count = count;
+
+		if (++i == info->tbuf_count)
+			i = 0;
 	}
 
 	info->tbuf_current = i;
+
+	/* set first buffer count to make new data visible to DMA controller */
+	d = &info->tbufs[info->tbuf_start];
+	set_desc_count(*d, d->buf_count);
+
+	/* start transmitter if needed and update transmit timeout */
+	if (!info->tx_active)
+		tx_start(info);
+	update_tx_timer(info);
+
+	return true;
 }
 
 static int register_test(struct slgt_info *info)
@@ -4934,9 +4960,7 @@ static int loopback_test(struct slgt_info *info)
 	spin_lock_irqsave(&info->lock,flags);
 	async_mode(info);
 	rx_start(info);
-	info->tx_count = count;
 	tx_load(info, buf, count);
-	tx_start(info);
 	spin_unlock_irqrestore(&info->lock, flags);
 
 	/* wait for receive complete */