firewire: ohci: avoid separate DMA mapping for small AT payloads (da28947e) · Commits · 戴 / test

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

+17 −8

Original line number	Diff line number	Diff line
		@@ -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 (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);
		}

		d[2].req_count = cpu_to_le16(packet->payload_length);
		d[2].data_address = cpu_to_le32(payload_bus);

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