r8169: disable RTL8126 ZRX-DC timeout

JIRA: https://issues.redhat.com/browse/RHEL-88646

commit b48688e
Author: ChunHao Lin <hau@realtek.com>
Date:   Tue Mar 18 16:37:21 2025 +0800

    r8169: disable RTL8126 ZRX-DC timeout

    Disable it due to it dose not meet ZRX-DC specification. If it is enabled,
    device will exit L1 substate every 100ms. Disable it for saving more power
    in L1 substate.

    Signed-off-by: ChunHao Lin <hau@realtek.com>
    Reviewed-by: Heiner Kallweit <hkallweit1@gmail.com>
    Link: https://patch.msgid.link/20250318083721.4127-3-hau@realtek.com
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Signed-off-by: CKI Backport Bot <cki-ci-bot+cki-gitlab-backport-bot@redhat.com>

Author: CKI Backport Bot

drivers/net/ethernet/realtek/r8169_main.c

@@ -2850,6 +2850,32 @@ static u32 rtl_csi_read(struct rtl8169_private *tp, int addr)
 		RTL_R32(tp, CSIDR) : ~0;
 }
 
+static void rtl_disable_zrxdc_timeout(struct rtl8169_private *tp)
+{
+	struct pci_dev *pdev = tp->pci_dev;
+	u32 csi;
+	int rc;
+	u8 val;
+
+#define RTL_GEN3_RELATED_OFF	0x0890
+#define RTL_GEN3_ZRXDC_NONCOMPL	0x1
+	if (pdev->cfg_size > RTL_GEN3_RELATED_OFF) {
+		rc = pci_read_config_byte(pdev, RTL_GEN3_RELATED_OFF, &val);
+		if (rc == PCIBIOS_SUCCESSFUL) {
+			val &= ~RTL_GEN3_ZRXDC_NONCOMPL;
+			rc = pci_write_config_byte(pdev, RTL_GEN3_RELATED_OFF,
+						   val);
+			if (rc == PCIBIOS_SUCCESSFUL)
+				return;
+		}
+	}
+
+	netdev_notice_once(tp->dev,
+		"No native access to PCI extended config space, falling back to CSI\n");
+	csi = rtl_csi_read(tp, RTL_GEN3_RELATED_OFF);
+	rtl_csi_write(tp, RTL_GEN3_RELATED_OFF, csi & ~RTL_GEN3_ZRXDC_NONCOMPL);
+}
+
 static void rtl_set_aspm_entry_latency(struct rtl8169_private *tp, u8 val)
 {
 	struct pci_dev *pdev = tp->pci_dev;
@@ -3816,6 +3842,7 @@ static void rtl_hw_start_8125b(struct rtl8169_private *tp)
 
 static void rtl_hw_start_8126a(struct rtl8169_private *tp)
 {
+	rtl_disable_zrxdc_timeout(tp);
 	rtl_set_def_aspm_entry_latency(tp);
 	rtl_hw_start_8125_common(tp);
 }