NTB: Address out of DMA descriptor issue with NTB

The transport right now does not handle the case where we run out of DMA
descriptors. We just fail when we do not succeed. Adding code to retry for
a bit attempting to use the DMA engine instead of instantly fail to CPU
copy.
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Allen Hubbe <Allen.Hubbe@emc.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>

drivers/ntb/ntb_transport.c

@@ -171,12 +171,14 @@ struct ntb_transport_qp {
 	u64 rx_err_ver;
 	u64 rx_memcpy;
 	u64 rx_async;
+	u64 dma_rx_prep_err;
 	u64 tx_bytes;
 	u64 tx_pkts;
 	u64 tx_ring_full;
 	u64 tx_err_no_buf;
 	u64 tx_memcpy;
 	u64 tx_async;
+	u64 dma_tx_prep_err;
 };
 
 struct ntb_transport_mw {
@@ -249,6 +251,8 @@ enum {
 #define QP_TO_MW(nt, qp)	((qp) % nt->mw_count)
 #define NTB_QP_DEF_NUM_ENTRIES	100
 #define NTB_LINK_DOWN_TIMEOUT	10
+#define DMA_RETRIES		20
+#define DMA_OUT_RESOURCE_TO	50
 
 static void ntb_transport_rxc_db(unsigned long data);
 static const struct ntb_ctx_ops ntb_transport_ops;
@@ -501,6 +505,12 @@ static ssize_t debugfs_read(struct file *filp, char __user *ubuf, size_t count,
 	out_offset += snprintf(buf + out_offset, out_count - out_offset,
 			       "free tx - \t%u\n",
 			       ntb_transport_tx_free_entry(qp));
+	out_offset += snprintf(buf + out_offset, out_count - out_offset,
+			       "DMA tx prep err - \t%llu\n",
+			       qp->dma_tx_prep_err);
+	out_offset += snprintf(buf + out_offset, out_count - out_offset,
+			       "DMA rx prep err - \t%llu\n",
+			       qp->dma_rx_prep_err);
 
 	out_offset += snprintf(buf + out_offset, out_count - out_offset,
 			       "\n");
@@ -726,6 +736,8 @@ static void ntb_qp_link_down_reset(struct ntb_transport_qp *qp)
 	qp->tx_err_no_buf = 0;
 	qp->tx_memcpy = 0;
 	qp->tx_async = 0;
+	qp->dma_tx_prep_err = 0;
+	qp->dma_rx_prep_err = 0;
 }
 
 static void ntb_qp_link_cleanup(struct ntb_transport_qp *qp)
@@ -1228,6 +1240,7 @@ static void ntb_async_rx(struct ntb_queue_entry *entry, void *offset)
 	struct dmaengine_unmap_data *unmap;
 	dma_cookie_t cookie;
 	void *buf = entry->buf;
+	int retries = 0;
 
 	len = entry->len;
 
@@ -1263,11 +1276,21 @@ static void ntb_async_rx(struct ntb_queue_entry *entry, void *offset)
 
 	unmap->from_cnt = 1;
 
+	for (retries = 0; retries < DMA_RETRIES; retries++) {
 		txd = device->device_prep_dma_memcpy(chan, unmap->addr[1],
 						     unmap->addr[0], len,
 						     DMA_PREP_INTERRUPT);
-	if (!txd)
+		if (txd)
+			break;
+
+		set_current_state(TASK_INTERRUPTIBLE);
+		schedule_timeout(DMA_OUT_RESOURCE_TO);
+	}
+
+	if (!txd) {
+		qp->dma_rx_prep_err++;
 		goto err_get_unmap;
+	}
 
 	txd->callback = ntb_rx_copy_callback;
 	txd->callback_param = entry;
@@ -1460,6 +1483,7 @@ static void ntb_async_tx(struct ntb_transport_qp *qp,
 	void __iomem *offset;
 	size_t len = entry->len;
 	void *buf = entry->buf;
+	int retries = 0;
 
 	offset = qp->tx_mw + qp->tx_max_frame * qp->tx_index;
 	hdr = offset + qp->tx_max_frame - sizeof(struct ntb_payload_header);
@@ -1494,10 +1518,20 @@ static void ntb_async_tx(struct ntb_transport_qp *qp,
 
 	unmap->to_cnt = 1;
 
-	txd = device->device_prep_dma_memcpy(chan, dest, unmap->addr[0], len,
-					     DMA_PREP_INTERRUPT);
-	if (!txd)
+	for (retries = 0; retries < DMA_RETRIES; retries++) {
+		txd = device->device_prep_dma_memcpy(chan, dest, unmap->addr[0],
+						     len, DMA_PREP_INTERRUPT);
+		if (txd)
+			break;
+
+		set_current_state(TASK_INTERRUPTIBLE);
+		schedule_timeout(DMA_OUT_RESOURCE_TO);
+	}
+
+	if (!txd) {
+		qp->dma_tx_prep_err++;
 		goto err_get_unmap;
+	}
 
 	txd->callback = ntb_tx_copy_callback;
 	txd->callback_param = entry;