From da28947e7e3602669e27d5e9ce787436ed662fa4 Mon Sep 17 00:00:00 2001
From: Clemens Ladisch <clemens@ladisch.de>
Date: Mon, 11 Apr 2011 09:57:54 +0200
Subject: firewire: ohci: avoid separate DMA mapping for small AT payloads

For AT packet payloads of up to eight bytes, we have enough unused space
in the DMA descriptors list so that we can put a copy of the payload
there and thus avoid having to create a separate streaming DMA mapping
for the payload buffer.

In a CPU-bound microbenchmark that just sends 8-byte packets, bandwidth
was measured to increase by 5.7 %, from 1009 KB/s to 1067 KB/s.  In
practice, the only performance-sensitive usage of small asynchronous
packets is the SBP-2 driver's write to the ORB_POINTER register during
SCSI command submission.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
---
 drivers/firewire/ohci.c | 25 +++++++++++++++++--------
 1 file changed, 17 insertions(+), 8 deletions(-)

diff --git a/drivers/firewire/ohci.c b/drivers/firewire/ohci.c
index dfb3cb774a79..435ed800d03f 100644
--- a/drivers/firewire/ohci.c
+++ b/drivers/firewire/ohci.c
@@ -1218,6 +1218,7 @@ static void context_stop(struct context *ctx)
 }
 
 struct driver_data {
+	u8 inline_data[8];
 	struct fw_packet *packet;
 };
 
@@ -1301,20 +1302,28 @@ static int at_context_queue_packet(struct context *ctx,
 		return -1;
 	}
 
+	BUILD_BUG_ON(sizeof(struct driver_data) > sizeof(struct descriptor));
 	driver_data = (struct driver_data *) &d[3];
 	driver_data->packet = packet;
 	packet->driver_data = driver_data;
 
 	if (packet->payload_length > 0) {
-		payload_bus =
-			dma_map_single(ohci->card.device, packet->payload,
-				       packet->payload_length, DMA_TO_DEVICE);
-		if (dma_mapping_error(ohci->card.device, payload_bus)) {
-			packet->ack = RCODE_SEND_ERROR;
-			return -1;
+		if (packet->payload_length > sizeof(driver_data->inline_data)) {
+			payload_bus = dma_map_single(ohci->card.device,
+						     packet->payload,
+						     packet->payload_length,
+						     DMA_TO_DEVICE);
+			if (dma_mapping_error(ohci->card.device, payload_bus)) {
+				packet->ack = RCODE_SEND_ERROR;
+				return -1;
+			}
+			packet->payload_bus	= payload_bus;
+			packet->payload_mapped	= true;
+		} else {
+			memcpy(driver_data->inline_data, packet->payload,
+			       packet->payload_length);
+			payload_bus = d_bus + 3 * sizeof(*d);
 		}
-		packet->payload_bus	= payload_bus;
-		packet->payload_mapped	= true;
 
 		d[2].req_count    = cpu_to_le16(packet->payload_length);
 		d[2].data_address = cpu_to_le32(payload_bus);
-- 
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