Remember the MAC address that firmware writes into RAL0/RAH0 after reset.
On later probes, wait for that cached passthrough address instead of using
any valid register change as the completion condition.

This keeps the first probe conservative and lets later dock replug cycles
use the known passthrough address once the platform has already shown that
firmware can rewrite the address registers.

Signed-off-by: Chia-Lin Kao (AceLan) <acelan.kao@canonical.com>
---
 drivers/net/ethernet/intel/igc/igc_main.c | 41 +++++++++++++++++++++--
 1 file changed, 39 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/intel/igc/igc_main.c b/drivers/net/ethernet/intel/igc/igc_main.c
index fa9752ed8bc5..cf87a82f1bf8 100644
--- a/drivers/net/ethernet/intel/igc/igc_main.c
+++ b/drivers/net/ethernet/intel/igc/igc_main.c
@@ -12,6 +12,7 @@
 #include <linux/bpf_trace.h>
 #include <net/xdp_sock_drv.h>
 #include <linux/etherdevice.h>
+#include <linux/mutex.h>
 #include <linux/pci.h>
 #include <linux/mdio.h>
 
@@ -70,6 +71,32 @@ static const struct pci_device_id igc_pci_tbl[] = {
 
 MODULE_DEVICE_TABLE(pci, igc_pci_tbl);
 
+static bool igc_mac_passthrough_cached;
+static u8 igc_mac_passthrough_addr[ETH_ALEN] __aligned(2);
+static DEFINE_MUTEX(igc_mac_passthrough_lock);
+
+static bool igc_get_mac_passthrough_addr(u8 *addr)
+{
+	mutex_lock(&igc_mac_passthrough_lock);
+	if (!igc_mac_passthrough_cached) {
+		mutex_unlock(&igc_mac_passthrough_lock);
+		return false;
+	}
+
+	ether_addr_copy(addr, igc_mac_passthrough_addr);
+	mutex_unlock(&igc_mac_passthrough_lock);
+
+	return true;
+}
+
+static void igc_cache_mac_passthrough_addr(u8 *addr)
+{
+	mutex_lock(&igc_mac_passthrough_lock);
+	ether_addr_copy(igc_mac_passthrough_addr, addr);
+	igc_mac_passthrough_cached = true;
+	mutex_unlock(&igc_mac_passthrough_lock);
+}
+
 static void igc_read_rar0(struct igc_hw *hw, u8 *addr, u32 *ral, u32 *rah)
 {
 	*ral = rd32(IGC_RAL(0));
@@ -97,22 +124,32 @@ static bool igc_is_lmvp_device(struct pci_dev *pdev)
 static void igc_wait_for_lmvp_mac_passthrough(struct pci_dev *pdev,
 					      struct igc_hw *hw)
 {
+	u8 cached_addr[ETH_ALEN] __aligned(2);
 	u8 addr[ETH_ALEN] __aligned(2);
 	u32 orig_ral, orig_rah;
 	u32 ral, rah;
+	bool cached;
 	int i;
 
 	if (!igc_is_lmvp_device(pdev))
 		return;
 
+	cached = igc_get_mac_passthrough_addr(cached_addr);
 	igc_read_rar0(hw, addr, &orig_ral, &orig_rah);
+	if (cached && ether_addr_equal(addr, cached_addr))
+		return;
 
 	for (i = 0; i < 100; i++) {
 		msleep(100);
 		igc_read_rar0(hw, addr, &ral, &rah);
-		if ((ral != orig_ral || rah != orig_rah) &&
-		    is_valid_ether_addr(addr))
+		if (cached) {
+			if (ether_addr_equal(addr, cached_addr))
+				return;
+		} else if ((ral != orig_ral || rah != orig_rah) &&
+			   is_valid_ether_addr(addr)) {
+			igc_cache_mac_passthrough_addr(addr);
 			return;
+		}
 	}
 }
 
-- 
2.53.0