NTB: Use DMA Engine to Transmit and Receive

Allocate and use a DMA engine channel to transmit and receive data over
NTB.  If none is allocated, fall back to using the CPU to transfer data.

Signed-off-by: Jon Mason <jon.mason@intel.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>

3 files changed, 295 insertions, 47 deletions

diff --git a/drivers/ntb/ntb_hw.c b/drivers/ntb/ntb_hw.c
index ab34795cf125..0345817a8355 100644
--- a/drivers/ntb/ntb_hw.c
+++ b/drivers/ntb/ntb_hw.c
@@ -350,6 +350,23 @@ int ntb_read_remote_spad(struct ntb_device *ndev, unsigned int idx, u32 *val)
 }
 
 /**
+ * ntb_get_mw_base() - get addr for the NTB memory window
+ * @ndev: pointer to ntb_device instance
+ * @mw: memory window number
+ *
+ * This function provides the base address of the memory window specified.
+ *
+ * RETURNS: address, or NULL on error.
+ */
+resource_size_t ntb_get_mw_base(struct ntb_device *ndev, unsigned int mw)
+{
+        if (mw >= ntb_max_mw(ndev))
+                return 0;
+
+        return pci_resource_start(ndev->pdev, MW_TO_BAR(mw));
+}
+
+/**
  * ntb_get_mw_vbase() - get virtual addr for the NTB memory window
  * @ndev: pointer to ntb_device instance
  * @mw: memory window number
diff --git a/drivers/ntb/ntb_hw.h b/drivers/ntb/ntb_hw.h
index d838bc13b956..4f42ed18103a 100644
--- a/drivers/ntb/ntb_hw.h
+++ b/drivers/ntb/ntb_hw.h
@@ -240,6 +240,7 @@ int ntb_write_local_spad(struct ntb_device *ndev, unsigned int idx, u32 val);
 int ntb_read_local_spad(struct ntb_device *ndev, unsigned int idx, u32 *val);
 int ntb_write_remote_spad(struct ntb_device *ndev, unsigned int idx, u32 val);
 int ntb_read_remote_spad(struct ntb_device *ndev, unsigned int idx, u32 *val);
+resource_size_t ntb_get_mw_base(struct ntb_device *ndev, unsigned int mw);
 void __iomem *ntb_get_mw_vbase(struct ntb_device *ndev, unsigned int mw);
 u64 ntb_get_mw_size(struct ntb_device *ndev, unsigned int mw);
 void ntb_ring_sdb(struct ntb_device *ndev, unsigned int idx);
diff --git a/drivers/ntb/ntb_transport.c b/drivers/ntb/ntb_transport.c
index f7380e959656..ae8657259ca0 100644
--- a/drivers/ntb/ntb_transport.c
+++ b/drivers/ntb/ntb_transport.c
@@ -47,6 +47,7 @@
  */
 #include <linux/debugfs.h>
 #include <linux/delay.h>
+#include <linux/dmaengine.h>
 #include <linux/dma-mapping.h>
 #include <linux/errno.h>
 #include <linux/export.h>
@@ -68,6 +69,10 @@ static unsigned char max_num_clients;
 module_param(max_num_clients, byte, 0644);
 MODULE_PARM_DESC(max_num_clients, "Maximum number of NTB transport clients");
 
+static unsigned int copy_bytes = 1024;
+module_param(copy_bytes, uint, 0644);
+MODULE_PARM_DESC(copy_bytes, "Threshold under which NTB will use the CPU to copy instead of DMA");
+
 struct ntb_queue_entry {
         /* ntb_queue list reference */
         struct list_head entry;
@@ -76,6 +81,13 @@ struct ntb_queue_entry {
         void *buf;
         unsigned int len;
         unsigned int flags;
+
+        struct ntb_transport_qp *qp;
+        union {
+                struct ntb_payload_header __iomem *tx_hdr;
+                struct ntb_payload_header *rx_hdr;
+        };
+        unsigned int index;
 };
 
 struct ntb_rx_info {
@@ -86,6 +98,7 @@ struct ntb_transport_qp {
         struct ntb_transport *transport;
         struct ntb_device *ndev;
         void *cb_data;
+        struct dma_chan *dma_chan;
 
         bool client_ready;
         bool qp_link;
@@ -99,6 +112,7 @@ struct ntb_transport_qp {
         struct list_head tx_free_q;
         spinlock_t ntb_tx_free_q_lock;
         void __iomem *tx_mw;
+        dma_addr_t tx_mw_phys;
         unsigned int tx_index;
         unsigned int tx_max_entry;
         unsigned int tx_max_frame;
@@ -114,6 +128,7 @@ struct ntb_transport_qp {
         unsigned int rx_index;
         unsigned int rx_max_entry;
         unsigned int rx_max_frame;
+        dma_cookie_t last_cookie;
 
         void (*event_handler) (void *data, int status);
         struct delayed_work link_work;
@@ -129,9 +144,14 @@ struct ntb_transport_qp {
         u64 rx_err_no_buf;
         u64 rx_err_oflow;
         u64 rx_err_ver;
+        u64 rx_memcpy;
+        u64 rx_async;
         u64 tx_bytes;
         u64 tx_pkts;
         u64 tx_ring_full;
+        u64 tx_err_no_buf;
+        u64 tx_memcpy;
+        u64 tx_async;
 };
 
 struct ntb_transport_mw {
@@ -381,7 +401,7 @@ static ssize_t debugfs_read(struct file *filp, char __user *ubuf, size_t count,
         char *buf;
         ssize_t ret, out_offset, out_count;
 
-        out_count = 600;
+        out_count = 1000;
 
         buf = kmalloc(out_count, GFP_KERNEL);
         if (!buf)
@@ -396,6 +416,10 @@ static ssize_t debugfs_read(struct file *filp, char __user *ubuf, size_t count,
         out_offset += snprintf(buf + out_offset, out_count - out_offset,
                                "rx_pkts - \t%llu\n", qp->rx_pkts);
         out_offset += snprintf(buf + out_offset, out_count - out_offset,
+                               "rx_memcpy - \t%llu\n", qp->rx_memcpy);
+        out_offset += snprintf(buf + out_offset, out_count - out_offset,
+                               "rx_async - \t%llu\n", qp->rx_async);
+        out_offset += snprintf(buf + out_offset, out_count - out_offset,
                                "rx_ring_empty - %llu\n", qp->rx_ring_empty);
         out_offset += snprintf(buf + out_offset, out_count - out_offset,
                                "rx_err_no_buf - %llu\n", qp->rx_err_no_buf);
@@ -415,8 +439,14 @@ static ssize_t debugfs_read(struct file *filp, char __user *ubuf, size_t count,
         out_offset += snprintf(buf + out_offset, out_count - out_offset,
                                "tx_pkts - \t%llu\n", qp->tx_pkts);
         out_offset += snprintf(buf + out_offset, out_count - out_offset,
+                               "tx_memcpy - \t%llu\n", qp->tx_memcpy);
+        out_offset += snprintf(buf + out_offset, out_count - out_offset,
+                               "tx_async - \t%llu\n", qp->tx_async);
+        out_offset += snprintf(buf + out_offset, out_count - out_offset,
                                "tx_ring_full - \t%llu\n", qp->tx_ring_full);
         out_offset += snprintf(buf + out_offset, out_count - out_offset,
+                               "tx_err_no_buf - %llu\n", qp->tx_err_no_buf);
+        out_offset += snprintf(buf + out_offset, out_count - out_offset,
                                "tx_mw - \t%p\n", qp->tx_mw);
         out_offset += snprintf(buf + out_offset, out_count - out_offset,
                                "tx_index - \t%u\n", qp->tx_index);
@@ -488,11 +518,11 @@ static void ntb_transport_setup_qp_mw(struct ntb_transport *nt,
                 num_qps_mw = nt->max_qps / mw_max;
 
         rx_size = (unsigned int) nt->mw[mw_num].size / num_qps_mw;
-        qp->remote_rx_info = nt->mw[mw_num].virt_addr +
-                             (qp_num / mw_max * rx_size);
+        qp->rx_buff = nt->mw[mw_num].virt_addr + qp_num / mw_max * rx_size;
         rx_size -= sizeof(struct ntb_rx_info);
 
-        qp->rx_buff = qp->remote_rx_info + 1;
+        qp->remote_rx_info = qp->rx_buff + rx_size;
+
         /* Due to housekeeping, there must be atleast 2 buffs */
         qp->rx_max_frame = min(transport_mtu, rx_size / 2);
         qp->rx_max_entry = rx_size / qp->rx_max_frame;
@@ -796,12 +826,13 @@ static void ntb_qp_link_work(struct work_struct *work)
                                       msecs_to_jiffies(NTB_LINK_DOWN_TIMEOUT));
 }
 
-static void ntb_transport_init_queue(struct ntb_transport *nt,
+static int ntb_transport_init_queue(struct ntb_transport *nt,
                                      unsigned int qp_num)
 {
         struct ntb_transport_qp *qp;
         unsigned int num_qps_mw, tx_size;
         u8 mw_num, mw_max;
+        u64 qp_offset;
 
         mw_max = ntb_max_mw(nt->ndev);
         mw_num = QP_TO_MW(nt->ndev, qp_num);
@@ -820,11 +851,18 @@ static void ntb_transport_init_queue(struct ntb_transport *nt,
                 num_qps_mw = nt->max_qps / mw_max;
 
         tx_size = (unsigned int) ntb_get_mw_size(qp->ndev, mw_num) / num_qps_mw;
-        qp->rx_info = ntb_get_mw_vbase(nt->ndev, mw_num) +
-                      (qp_num / mw_max * tx_size);
+        qp_offset = qp_num / mw_max * tx_size;
+        qp->tx_mw = ntb_get_mw_vbase(nt->ndev, mw_num) + qp_offset;
+        if (!qp->tx_mw)
+                return -EINVAL;
+
+        qp->tx_mw_phys = ntb_get_mw_base(qp->ndev, mw_num) + qp_offset;
+        if (!qp->tx_mw_phys)
+                return -EINVAL;
+
         tx_size -= sizeof(struct ntb_rx_info);
+        qp->rx_info = qp->tx_mw + tx_size;
 
-        qp->tx_mw = qp->rx_info + 1;
         /* Due to housekeeping, there must be atleast 2 buffs */
         qp->tx_max_frame = min(transport_mtu, tx_size / 2);
         qp->tx_max_entry = tx_size / qp->tx_max_frame;
@@ -851,6 +889,8 @@ static void ntb_transport_init_queue(struct ntb_transport *nt,
         INIT_LIST_HEAD(&qp->rx_pend_q);
         INIT_LIST_HEAD(&qp->rx_free_q);
         INIT_LIST_HEAD(&qp->tx_free_q);
+
+        return 0;
 }
 
 int ntb_transport_init(struct pci_dev *pdev)
@@ -889,8 +929,11 @@ int ntb_transport_init(struct pci_dev *pdev)
 
         nt->qp_bitmap = ((u64) 1 << nt->max_qps) - 1;
 
-        for (i = 0; i < nt->max_qps; i++)
-                ntb_transport_init_queue(nt, i);
+        for (i = 0; i < nt->max_qps; i++) {
+                rc = ntb_transport_init_queue(nt, i);
+                if (rc)
+                        goto err3;
+        }
 
         INIT_DELAYED_WORK(&nt->link_work, ntb_transport_link_work);
         INIT_WORK(&nt->link_cleanup, ntb_transport_link_cleanup);
@@ -956,13 +999,19 @@ void ntb_transport_free(void *transport)
         kfree(nt);
 }
 
-static void ntb_rx_copy_task(struct ntb_transport_qp *qp,
-                             struct ntb_queue_entry *entry, void *offset)
+static void ntb_rx_copy_callback(void *data)
 {
+        struct ntb_queue_entry *entry = data;
+        struct ntb_transport_qp *qp = entry->qp;
         void *cb_data = entry->cb_data;
         unsigned int len = entry->len;
+        struct ntb_payload_header *hdr = entry->rx_hdr;
+
+        /* Ensure that the data is fully copied out before clearing the flag */
+        wmb();
+        hdr->flags = 0;
 
-        memcpy(entry->buf, offset, entry->len);
+        iowrite32(entry->index, &qp->rx_info->entry);
 
         ntb_list_add(&qp->ntb_rx_free_q_lock, &entry->entry, &qp->rx_free_q);
 
@@ -970,6 +1019,86 @@ static void ntb_rx_copy_task(struct ntb_transport_qp *qp,
                 qp->rx_handler(qp, qp->cb_data, cb_data, len);
 }
 
+static void ntb_memcpy_rx(struct ntb_queue_entry *entry, void *offset)
+{
+        void *buf = entry->buf;
+        size_t len = entry->len;
+
+        memcpy(buf, offset, len);
+
+        ntb_rx_copy_callback(entry);
+}
+
+static void ntb_async_rx(struct ntb_queue_entry *entry, void *offset,
+                         size_t len)
+{
+        struct dma_async_tx_descriptor *txd;
+        struct ntb_transport_qp *qp = entry->qp;
+        struct dma_chan *chan = qp->dma_chan;
+        struct dma_device *device;
+        size_t pay_off, buff_off;
+        dma_addr_t src, dest;
+        dma_cookie_t cookie;
+        void *buf = entry->buf;
+        unsigned long flags;
+
+        entry->len = len;
+
+        if (!chan)
+                goto err;
+
+        if (len < copy_bytes) 
+                goto err1;
+
+        device = chan->device;
+        pay_off = (size_t) offset & ~PAGE_MASK;
+        buff_off = (size_t) buf & ~PAGE_MASK;
+
+        if (!is_dma_copy_aligned(device, pay_off, buff_off, len))
+                goto err1;
+
+        dest = dma_map_single(device->dev, buf, len, DMA_FROM_DEVICE);
+        if (dma_mapping_error(device->dev, dest))
+                goto err1;
+
+        src = dma_map_single(device->dev, offset, len, DMA_TO_DEVICE);
+        if (dma_mapping_error(device->dev, src))
+                goto err2;
+
+        flags = DMA_COMPL_DEST_UNMAP_SINGLE | DMA_COMPL_SRC_UNMAP_SINGLE |
+                DMA_PREP_INTERRUPT;
+        txd = device->device_prep_dma_memcpy(chan, dest, src, len, flags);
+        if (!txd)
+                goto err3;
+
+        txd->callback = ntb_rx_copy_callback;
+        txd->callback_param = entry;
+
+        cookie = dmaengine_submit(txd);
+        if (dma_submit_error(cookie))
+                goto err3;
+
+        qp->last_cookie = cookie;
+
+        qp->rx_async++;
+
+        return;
+
+err3:
+        dma_unmap_single(device->dev, src, len, DMA_TO_DEVICE);
+err2:
+        dma_unmap_single(device->dev, dest, len, DMA_FROM_DEVICE);
+err1:
+        /* If the callbacks come out of order, the writing of the index to the
+         * last completed will be out of order.  This may result in the
+         * receive stalling forever.
+         */
+        dma_sync_wait(chan, qp->last_cookie);
+err:
+        ntb_memcpy_rx(entry, offset);
+        qp->rx_memcpy++;
+}
+
 static int ntb_process_rxc(struct ntb_transport_qp *qp)
 {
         struct ntb_payload_header *hdr;
@@ -1008,41 +1137,45 @@ static int ntb_process_rxc(struct ntb_transport_qp *qp)
         if (hdr->flags & LINK_DOWN_FLAG) {
                 ntb_qp_link_down(qp);
 
-                ntb_list_add(&qp->ntb_rx_pend_q_lock, &entry->entry,
-                             &qp->rx_pend_q);
-                goto out;
+                goto err;
         }
 
         dev_dbg(&ntb_query_pdev(qp->ndev)->dev,
                 "rx offset %u, ver %u - %d payload received, buf size %d\n",
                 qp->rx_index, hdr->ver, hdr->len, entry->len);
 
-        if (hdr->len <= entry->len) {
-                entry->len = hdr->len;
-                ntb_rx_copy_task(qp, entry, offset);
-        } else {
-                ntb_list_add(&qp->ntb_rx_pend_q_lock, &entry->entry,
-                             &qp->rx_pend_q);
+        qp->rx_bytes += hdr->len;
+        qp->rx_pkts++;
 
+        if (hdr->len > entry->len) {
                 qp->rx_err_oflow++;
                 dev_dbg(&ntb_query_pdev(qp->ndev)->dev,
                         "RX overflow! Wanted %d got %d\n",
                         hdr->len, entry->len);
+
+                goto err;
         }
 
-        qp->rx_bytes += hdr->len;
-        qp->rx_pkts++;
+        entry->index = qp->rx_index;
+        entry->rx_hdr = hdr;
+
+        ntb_async_rx(entry, offset, hdr->len);
 
 out:
+        qp->rx_index++;
+        qp->rx_index %= qp->rx_max_entry;
+
+        return 0;
+
+err:
+        ntb_list_add(&qp->ntb_rx_pend_q_lock, &entry->entry,
+                     &qp->rx_pend_q);
         /* Ensure that the data is fully copied out before clearing the flag */
         wmb();
         hdr->flags = 0;
         iowrite32(qp->rx_index, &qp->rx_info->entry);
 
-        qp->rx_index++;
-        qp->rx_index %= qp->rx_max_entry;
-
-        return 0;
+        goto out;
 }
 
 static void ntb_transport_rx(unsigned long data)
@@ -1058,6 +1191,9 @@ static void ntb_transport_rx(unsigned long data)
                 if (rc)
                         break;
         }
+
+        if (qp->dma_chan)
+                dma_async_issue_pending(qp->dma_chan);
 }
 
 static void ntb_transport_rxc_db(void *data, int db_num)
@@ -1070,19 +1206,13 @@ static void ntb_transport_rxc_db(void *data, int db_num)
         tasklet_schedule(&qp->rx_work);
 }
 
-static void ntb_tx_copy_task(struct ntb_transport_qp *qp,
-                             struct ntb_queue_entry *entry,
-                             void __iomem *offset)
+static void ntb_tx_copy_callback(void *data)
 {
-        struct ntb_payload_header __iomem *hdr;
-
-        memcpy_toio(offset, entry->buf, entry->len);
+        struct ntb_queue_entry *entry = data;
+        struct ntb_transport_qp *qp = entry->qp;
+        struct ntb_payload_header __iomem *hdr = entry->tx_hdr;
 
-        hdr = offset + qp->tx_max_frame - sizeof(struct ntb_payload_header);
-        iowrite32(entry->len, &hdr->len);
-        iowrite32((u32) qp->tx_pkts, &hdr->ver);
-
-        /* Ensure that the data is fully copied out before setting the flag */
+        /* Ensure that the data is fully copied out before setting the flags */
         wmb();
         iowrite32(entry->flags | DESC_DONE_FLAG, &hdr->flags);
 
@@ -1103,15 +1233,81 @@ static void ntb_tx_copy_task(struct ntb_transport_qp *qp,
         ntb_list_add(&qp->ntb_tx_free_q_lock, &entry->entry, &qp->tx_free_q);
 }
 
-static int ntb_process_tx(struct ntb_transport_qp *qp,
-                          struct ntb_queue_entry *entry)
+static void ntb_memcpy_tx(struct ntb_queue_entry *entry, void __iomem *offset)
 {
+        memcpy_toio(offset, entry->buf, entry->len);
+
+        ntb_tx_copy_callback(entry);
+}
+
+static void ntb_async_tx(struct ntb_transport_qp *qp,
+                         struct ntb_queue_entry *entry)
+{
+        struct ntb_payload_header __iomem *hdr;
+        struct dma_async_tx_descriptor *txd;
+        struct dma_chan *chan = qp->dma_chan;
+        struct dma_device *device;
+        size_t dest_off, buff_off;
+        dma_addr_t src, dest;
+        dma_cookie_t cookie;
         void __iomem *offset;
+        size_t len = entry->len;
+        void *buf = entry->buf;
+        unsigned long flags;
 
         offset = qp->tx_mw + qp->tx_max_frame * qp->tx_index;
+        hdr = offset + qp->tx_max_frame - sizeof(struct ntb_payload_header);
+        entry->tx_hdr = hdr;
 
-        dev_dbg(&ntb_query_pdev(qp->ndev)->dev, "%lld - offset %p, tx %u, entry len %d flags %x buff %p\n",
-                qp->tx_pkts, offset, qp->tx_index, entry->len, entry->flags,
+        iowrite32(entry->len, &hdr->len);
+        iowrite32((u32) qp->tx_pkts, &hdr->ver);
+
+        if (!chan)
+                goto err;
+
+        if (len < copy_bytes)
+                goto err;
+
+        device = chan->device;
+        dest = qp->tx_mw_phys + qp->tx_max_frame * qp->tx_index;
+        buff_off = (size_t) buf & ~PAGE_MASK;
+        dest_off = (size_t) dest & ~PAGE_MASK;
+
+        if (!is_dma_copy_aligned(device, buff_off, dest_off, len))
+                goto err;
+
+        src = dma_map_single(device->dev, buf, len, DMA_TO_DEVICE);
+        if (dma_mapping_error(device->dev, src))
+                goto err;
+
+        flags = DMA_COMPL_SRC_UNMAP_SINGLE | DMA_PREP_INTERRUPT;
+        txd = device->device_prep_dma_memcpy(chan, dest, src, len, flags);
+        if (!txd)
+                goto err1;
+
+        txd->callback = ntb_tx_copy_callback;
+        txd->callback_param = entry;
+
+        cookie = dmaengine_submit(txd);
+        if (dma_submit_error(cookie))
+                goto err1;
+
+        dma_async_issue_pending(chan);
+        qp->tx_async++;
+
+        return;
+err1:
+        dma_unmap_single(device->dev, src, len, DMA_TO_DEVICE);
+err:
+        ntb_memcpy_tx(entry, offset);
+        qp->tx_memcpy++;
+}
+
+static int ntb_process_tx(struct ntb_transport_qp *qp,
+                          struct ntb_queue_entry *entry)
+{
+        dev_dbg(&ntb_query_pdev(qp->ndev)->dev, "%lld - tx %u, entry len %d flags %x buff %p\n",
+                qp->tx_pkts, qp->tx_index, entry->len, entry->flags,
                 entry->buf);
         if (qp->tx_index == qp->remote_rx_info->entry) {
                 qp->tx_ring_full++;
@@ -1127,7 +1323,7 @@ static int ntb_process_tx(struct ntb_transport_qp *qp,
                 return 0;
         }
 
-        ntb_tx_copy_task(qp, entry, offset);
+        ntb_async_tx(qp, entry);
 
         qp->tx_index++;
         qp->tx_index %= qp->tx_max_entry;
@@ -1213,11 +1409,18 @@ ntb_transport_create_queue(void *data, struct pci_dev *pdev,
         qp->tx_handler = handlers->tx_handler;
         qp->event_handler = handlers->event_handler;
 
+        qp->dma_chan = dma_find_channel(DMA_MEMCPY);
+        if (!qp->dma_chan)
+                dev_info(&pdev->dev, "Unable to allocate DMA channel, using CPU instead\n");
+        else
+                dmaengine_get();
+
         for (i = 0; i < NTB_QP_DEF_NUM_ENTRIES; i++) {
                 entry = kzalloc(sizeof(struct ntb_queue_entry), GFP_ATOMIC);
                 if (!entry)
                         goto err1;
 
+                entry->qp = qp;
                 ntb_list_add(&qp->ntb_rx_free_q_lock, &entry->entry,
                              &qp->rx_free_q);
         }
@@ -1227,6 +1430,7 @@ ntb_transport_create_queue(void *data, struct pci_dev *pdev,
                 if (!entry)
                         goto err2;
 
+                entry->qp = qp;
                 ntb_list_add(&qp->ntb_tx_free_q_lock, &entry->entry,
                              &qp->tx_free_q);
         }
@@ -1272,11 +1476,26 @@ void ntb_transport_free_queue(struct ntb_transport_qp *qp)
 
         pdev = ntb_query_pdev(qp->ndev);
 
-        cancel_delayed_work_sync(&qp->link_work);
+        if (qp->dma_chan) {
+                struct dma_chan *chan = qp->dma_chan;
+                /* Putting the dma_chan to NULL will force any new traffic to be
+                 * processed by the CPU instead of the DAM engine
+                 */
+                qp->dma_chan = NULL;
+
+                /* Try to be nice and wait for any queued DMA engine
+                 * transactions to process before smashing it with a rock
+                 */
+                dma_sync_wait(chan, qp->last_cookie);
+                dmaengine_terminate_all(chan);
+                dmaengine_put();
+        }
 
         ntb_unregister_db_callback(qp->ndev, qp->qp_num);
         tasklet_disable(&qp->rx_work);
 
+        cancel_delayed_work_sync(&qp->link_work);
+
         while ((entry = ntb_list_rm(&qp->ntb_rx_free_q_lock, &qp->rx_free_q)))
                 kfree(entry);
 
@@ -1382,8 +1601,10 @@ int ntb_transport_tx_enqueue(struct ntb_transport_qp *qp, void *cb, void *data,
                 return -EINVAL;
 
         entry = ntb_list_rm(&qp->ntb_tx_free_q_lock, &qp->tx_free_q);
-        if (!entry)
+        if (!entry) {
+                qp->tx_err_no_buf++;
                 return -ENOMEM;
+        }
 
         entry->cb_data = cb;
         entry->buf = data;
@@ -1499,9 +1720,18 @@ EXPORT_SYMBOL_GPL(ntb_transport_qp_num);
  */
 unsigned int ntb_transport_max_size(struct ntb_transport_qp *qp)
 {
+        unsigned int max;
+
         if (!qp)
                 return 0;
 
-        return qp->tx_max_frame - sizeof(struct ntb_payload_header);
+        if (!qp->dma_chan)
+                return qp->tx_max_frame - sizeof(struct ntb_payload_header);
+
+        /* If DMA engine usage is possible, try to find the max size for that */
+        max = qp->tx_max_frame - sizeof(struct ntb_payload_header);
+        max -= max % (1 << qp->dma_chan->device->copy_align);
+
+        return max;
 }
 EXPORT_SYMBOL_GPL(ntb_transport_max_size);