Date: Mon, 11 Apr 2011 09:57:54 +0200 From: Clemens Ladisch 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 Signed-off-by: Stefan Richter --- drivers/firewire/ohci.c | 25 +++++++++++++++++-------- 1 file changed, 17 insertions(+), 8 deletions(-) Index: b/drivers/firewire/ohci.c =================================================================== --- a/drivers/firewire/ohci.c +++ b/drivers/firewire/ohci.c @@ -1218,6 +1218,7 @@ static void context_stop(struct context } struct driver_data { + u8 inline_data[8]; struct fw_packet *packet; }; @@ -1301,20 +1302,28 @@ static int at_context_queue_packet(struc 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);