firewire: ohci: 0 may be a valid DMA address

I was told that there are obscure architectures with non-coherent DMA
which may DMA-map to bus address 0.  We shall not use 0 as a magic
number of uninitialized bus address variables.

The packet->payload_length > 0 test cannot be used either (except in
at_context_queue_packet) because local requests are not DMA-mapped
regardless of payload_length.  Hence add a state flag to struct
fw_packet.
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>

drivers/firewire/core-transaction.c

@@ -226,7 +226,7 @@ static void fw_fill_request(struct fw_packet *packet, int tcode, int tlabel,
 	packet->speed = speed;
 	packet->generation = generation;
 	packet->ack = 0;
-	packet->payload_bus = 0;
+	packet->payload_mapped = false;
 }
 
 /**
@@ -601,7 +601,7 @@ void fw_fill_response(struct fw_packet *response, u32 *request_header,
 		WARN(1, KERN_ERR "wrong tcode %d", tcode);
 	}
 
-	response->payload_bus = 0;
+	response->payload_mapped = false;
 }
 EXPORT_SYMBOL(fw_fill_response);
 

drivers/firewire/ohci.c

@@ -996,6 +996,7 @@ static int at_context_queue_packet(struct context *ctx,
 			return -1;
 		}
 		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);
@@ -1023,7 +1024,7 @@ static int at_context_queue_packet(struct context *ctx,
 	 */
 	if (ohci->generation != packet->generation ||
 	    reg_read(ohci, OHCI1394_IntEventSet) & OHCI1394_busReset) {
-		if (packet->payload_length > 0)
+		if (packet->payload_mapped)
 			dma_unmap_single(ohci->card.device, payload_bus,
 					 packet->payload_length, DMA_TO_DEVICE);
 		packet->ack = RCODE_GENERATION;
@@ -1059,7 +1060,7 @@ static int handle_at_packet(struct context *context,
 		/* This packet was cancelled, just continue. */
 		return 1;
 
-	if (packet->payload_bus)
+	if (packet->payload_mapped)
 		dma_unmap_single(ohci->card.device, packet->payload_bus,
 				 packet->payload_length, DMA_TO_DEVICE);
 
@@ -1723,7 +1724,7 @@ static int ohci_cancel_packet(struct fw_card *card, struct fw_packet *packet)
 	if (packet->ack != 0)
 		goto out;
 
-	if (packet->payload_bus)
+	if (packet->payload_mapped)
 		dma_unmap_single(ohci->card.device, packet->payload_bus,
 				 packet->payload_length, DMA_TO_DEVICE);
 

include/linux/firewire.h

@@ -267,6 +267,7 @@ struct fw_packet {
 	void *payload;
 	size_t payload_length;
 	dma_addr_t payload_bus;
+	bool payload_mapped;
 	u32 timestamp;
 
 	/*