firewire: core: add CSR MAINT_UTILITY support

Implement the MAIN_UTILITY register, which is utterly optional
but useful as a safe target for diagnostic read/write/broadcast
transactions.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>

drivers/firewire/core-transaction.c

@@ -1140,6 +1140,15 @@ static void handle_registers(struct fw_card *card, struct fw_request *request,
 			rcode = RCODE_TYPE_ERROR;
 		break;
 
+	case CSR_MAINT_UTILITY:
+		if (tcode == TCODE_READ_QUADLET_REQUEST)
+			*data = card->maint_utility_register;
+		else if (tcode == TCODE_WRITE_QUADLET_REQUEST)
+			card->maint_utility_register = *data;
+		else
+			rcode = RCODE_TYPE_ERROR;
+		break;
+
 	case CSR_BROADCAST_CHANNEL:
 		if (tcode == TCODE_READ_QUADLET_REQUEST)
 			*data = cpu_to_be32(card->broadcast_channel);

include/linux/firewire.h

@@ -38,6 +38,7 @@
 #define CSR_CHANNELS_AVAILABLE		0x224
 #define CSR_CHANNELS_AVAILABLE_HI	0x224
 #define CSR_CHANNELS_AVAILABLE_LO	0x228
+#define CSR_MAINT_UTILITY		0x230
 #define CSR_BROADCAST_CHANNEL		0x234
 #define CSR_CONFIG_ROM			0x400
 #define CSR_CONFIG_ROM_END		0x800
@@ -122,6 +123,8 @@ struct fw_card {
 	bool broadcast_channel_allocated;
 	u32 broadcast_channel;
 	__be32 topology_map[(CSR_TOPOLOGY_MAP_END - CSR_TOPOLOGY_MAP) / 4];
+
+	__be32 maint_utility_register;
 };
 
 struct fw_attribute_group {