thunderbolt: Allow specifying custom credits for DMA tunnels

The default ones should be find but this allows the user to tweak the
credits to get more performance out of the P2P connection.
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>

drivers/thunderbolt/tunnel.c

@@ -41,9 +41,14 @@
  * Number of credits we try to allocate for each DMA path if not limited
  * by the host router baMaxHI.
  */
-#define TB_DMA_CREDITS			14U
+#define TB_DMA_CREDITS			14
 /* Minimum number of credits for DMA path */
-#define TB_MIN_DMA_CREDITS		1U
+#define TB_MIN_DMA_CREDITS		1
+
+static unsigned int dma_credits = TB_DMA_CREDITS;
+module_param(dma_credits, uint, 0444);
+MODULE_PARM_DESC(dma_credits, "specify custom credits for DMA tunnels (default: "
+                __MODULE_STRING(TB_DMA_CREDITS) ")");
 
 static bool bw_alloc_mode = true;
 module_param(bw_alloc_mode, bool, 0444);
@@ -95,7 +100,7 @@ static unsigned int tb_available_credits(const struct tb_port *port,
 	pcie = tb_acpi_may_tunnel_pcie() ? sw->max_pcie_credits : 0;
 
 	if (tb_acpi_is_xdomain_allowed()) {
-		spare = min_not_zero(sw->max_dma_credits, TB_DMA_CREDITS);
+		spare = min_not_zero(sw->max_dma_credits, dma_credits);
 		/* Add some credits for potential second DMA tunnel */
 		spare += TB_MIN_DMA_CREDITS;
 	} else {
@@ -1472,7 +1477,7 @@ struct tb_tunnel *tb_tunnel_alloc_dma(struct tb *tb, struct tb_port *nhi,
 	tunnel->dst_port = dst;
 	tunnel->deinit = tb_dma_deinit;
 
-	credits = min_not_zero(TB_DMA_CREDITS, nhi->sw->max_dma_credits);
+	credits = min_not_zero(dma_credits, nhi->sw->max_dma_credits);
 
 	if (receive_ring > 0) {
 		path = tb_path_alloc(tb, dst, receive_path, nhi, receive_ring, 0,