[PATCH v4 17/19] NTB: Default to CPU memcpy for performance

[Allen Hubbe]

From: Dave Jiang <dave.jiang@xxxxxxxxx>

Disable DMA usage by default, since the CPU provides much better
performance with write combining.  Provide a module parameter to enable
DMA usage when offloading the memcpy is preferred.

Signed-off-by: Dave Jiang <dave.jiang@xxxxxxxxx>
Signed-off-by: Allen Hubbe <Allen.Hubbe@xxxxxxx>
---
 drivers/ntb/ntb_transport.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/drivers/ntb/ntb_transport.c b/drivers/ntb/ntb_transport.c
index 7a765d3230d8..e07b056af3be 100644
--- a/drivers/ntb/ntb_transport.c
+++ b/drivers/ntb/ntb_transport.c
@@ -88,6 +88,10 @@ 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");
 
+static bool use_dma;
+module_param(use_dma, bool, 0644);
+MODULE_PARM_DESC(use_dma, "Use DMA engine to perform large data copy");
+
 static struct dentry *nt_debugfs_dir;
 
 struct ntb_queue_entry {
@@ -1589,10 +1593,15 @@ ntb_transport_create_queue(void *data, struct device *client_dev,
 	dma_cap_zero(dma_mask);
 	dma_cap_set(DMA_MEMCPY, dma_mask);
 
-	qp->dma_chan = dma_request_channel(dma_mask, ntb_dma_filter_fn,
-					   (void *)(unsigned long)node);
-	if (!qp->dma_chan)
-		dev_info(&pdev->dev, "Unable to allocate DMA channel, using CPU instead\n");
+	if (use_dma) {
+		qp->dma_chan = dma_request_channel(dma_mask, ntb_dma_filter_fn,
+						   (void *)(unsigned long)node);
+		if (!qp->dma_chan)
+			dev_info(&pdev->dev, "Unable to allocate DMA channel\n");
+	} else {
+		qp->dma_chan = NULL;
+	}
+	dev_dbg(&pdev->dev, "Using %s memcpy\n", qp->dma_chan ? "DMA" : "CPU");
 
 	for (i = 0; i < NTB_QP_DEF_NUM_ENTRIES; i++) {
 		entry = kzalloc_node(sizeof(*entry), GFP_ATOMIC, node);
-- 
2.4.0.rc0.43.gcf8a8c6

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/