Re: [PATCH 09/15] NTB: Use DMA Engine to Transmit and Receive
From: Dan Williams
Date: Mon Aug 19 2013 - 05:41:56 EST
On Fri, Aug 2, 2013 at 10:35 AM, Jon Mason <jon.mason@xxxxxxxxx> wrote:
> 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.
>
> Cc: Dan Williams <djbw@xxxxxx>
> Cc: Vinod Koul <vinod.koul@xxxxxxxxx>
> Cc: Dave Jiang <dave.jiang@xxxxxxxxx>
> Signed-off-by: Jon Mason <jon.mason@xxxxxxxxx>
> ---
> drivers/ntb/ntb_hw.c | 17 +++
> drivers/ntb/ntb_hw.h | 1 +
> drivers/ntb/ntb_transport.c | 285 ++++++++++++++++++++++++++++++++++++-------
> 3 files changed, 258 insertions(+), 45 deletions(-)
>
> diff --git a/drivers/ntb/ntb_hw.c b/drivers/ntb/ntb_hw.c
> index 1d8e551..014222c 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 b03de80..ab5f768 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 f7380e9..73a35e4 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_raw;
> 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;
> @@ -802,6 +832,7 @@ static void ntb_transport_init_queue(struct ntb_transport *nt,
> 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 +851,12 @@ 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;
> + qp->tx_mw_raw = ntb_get_mw_base(qp->ndev, mw_num) + qp_offset;
Just a quibble with the name, why "raw" instead of "phys"?
> 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;
> @@ -956,13 +988,22 @@ 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;
> +
> + wmb();
What is this barrier paired with?
> + hdr->flags = 0;
>
> - memcpy(entry->buf, offset, entry->len);
> + /* 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 the
> + * receive stalling forever.
> + */
Is this for the case where we are bouncing back and forth between
sync/async? Otherwise I do not see how transactions could get out of
order given you allocate a channel once per queue. Flushing the async
transactions when transitioning to sync is one option. Is this
comment warning about a bug or explaining why we do the iowrite?
> + iowrite32(entry->index, &qp->rx_info->entry);
>
> ntb_list_add(&qp->ntb_rx_free_q_lock, &entry->entry, &qp->rx_free_q);
>
> @@ -970,6 +1011,80 @@ 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;
> + dma_addr_t src, dest;
> + dma_cookie_t cookie;
> + void *buf = entry->buf;
> + unsigned long flags;
> +
> + entry->len = len;
> +
> + if (!chan)
> + goto err;
> +
> + device = chan->device;
> +
> + if (len < copy_bytes ||
> + !is_dma_copy_aligned(device, __pa(offset), __pa(buf), len))
__pa()'s necessary here? I don't think the alignment requirements
will ever cross PAGE_OFFSET.
> + 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_clear_parent(txd);
> + txd_clear_next(txd);
Neither of these txd_clear* calls should be necessary.
> + 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;
> +
> + dma_async_issue_pending(chan);
hmm... can this go in ntb_process_rx() so that the submission is
batched after all the async work is prepped?
> + 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:
> + 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 +1123,44 @@ 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;
>
> -out:
> - /* Ensure that the data is fully copied out before clearing the flag */
> - wmb();
> - hdr->flags = 0;
> - iowrite32(qp->rx_index, &qp->rx_info->entry);
> + 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);
> + wmb();
> + hdr->flags = 0;
> + iowrite32(qp->rx_index, &qp->rx_info->entry);
> +
> + goto out;
> }
>
> static void ntb_transport_rx(unsigned long data)
> @@ -1070,19 +1188,12 @@ 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);
> -
> - hdr = offset + qp->tx_max_frame - sizeof(struct ntb_payload_header);
> - iowrite32(entry->len, &hdr->len);
> - iowrite32((u32) qp->tx_pkts, &hdr->ver);
> + struct ntb_queue_entry *entry = data;
> + struct ntb_transport_qp *qp = entry->qp;
> + struct ntb_payload_header __iomem *hdr = entry->tx_hdr;
>
> - /* Ensure that the data is fully copied out before setting the flag */
> wmb();
> iowrite32(entry->flags | DESC_DONE_FLAG, &hdr->flags);
>
> @@ -1103,15 +1214,79 @@ 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 dma_async_tx_descriptor *txd;
> + struct dma_chan *chan = qp->dma_chan;
> + struct dma_device *device;
> + dma_addr_t src, dest;
> + dma_cookie_t cookie;
> + struct ntb_payload_header __iomem *hdr;
> 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;
> +
> + iowrite32(entry->len, &hdr->len);
> + iowrite32((u32) qp->tx_pkts, &hdr->ver);
> +
> + if (!chan)
> + goto err;
>
> - 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,
> + device = chan->device;
> +
> + dest = qp->tx_mw_raw + qp->tx_max_frame * qp->tx_index;
> +
> + if (len < copy_bytes ||
> + !is_dma_copy_aligned(device, __pa(buf), dest, len))
ditto on the use of __pa here.
> + 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_clear_parent(txd);
> + txd_clear_next(txd);
...again not needed.
> + 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 +1302,7 @@ 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 +1388,16 @@ 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 private DMA channel, using CPU instead\n");
You can drop "private" since this is a public allocation.
> +
> 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 +1407,7 @@ 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 +1453,14 @@ 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)
> + dmaengine_terminate_all(qp->dma_chan);
Do you need this or can you just wait for all outstanding tx / rx to quiesce?
> ntb_unregister_db_callback(qp->ndev, qp->qp_num);
> tasklet_disable(&qp->rx_work);
>
> + cancel_delaye
Soon the ability to ask the driver to unmap for you will go away. You
can get ahead of the game by unmapping manually.
d_work_sync(&qp->link_work);
> +
> while ((entry = ntb_list_rm(&qp->ntb_rx_free_q_lock, &qp->rx_free_q)))
> kfree(entry);
>
> @@ -1382,8 +1566,10 @@ 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 +1685,18 @@ 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);
Unless I missed it you need a dmaengine_get() dmaengine_put() pair in
your driver setup/teardown. dmaengine_get() notifies dmaengine of a
dma_find_channel() user.
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