[PATCH net v2] r8169: don't enable chip LTR when the platform has not enabled LTR

[Yogesh Gaur <yogeshgaur.83@gmail.com>]

rtl_enable_ltr() programs the MAC to generate LTR messages - ALDPS_LTR_EN,
LTR_SNOOP_EN, LTR_OBFF_LOCK_EN, plus LINK_SPEED_CHANGE_EN on
RTL8125/RTL8126/RTL8127 - and rtl_hw_aspm_clkreq_enable() calls it on
every ASPM enable, then goes on to let the chip trigger L1.2.

The only gate is tp->aspm_manageable, which records that the OS is allowed
to control ASPM. It says nothing about LTR. LTR is a separate PCIe
capability that only works if every device on the path to the root port
supports it. The PCI core works that out in pci_configure_ltr() and
records the result in pci_dev->ltr_path; per PCIe r6.0 sec 7.5.3.16 a
function must not issue LTR messages while LTR Mechanism Enable is clear.

So on a platform whose hierarchy has no LTR path, the driver tells the
chip to start sending LTR messages nothing will honour, and ties ALDPS -
the PHY's link-down power saving - to them. A report against RTL8125B
(rev 05, firmware rtl8125b-2_0.0.2) in a mini PC shows the effect: 291
link down/up transitions in one eight-hour boot, with repeated downshifts
to 100Mbps, against four transitions at boot and then a stable link on
the kernel before the LTR change.

Gate the chip's LTR programming on pci_dev->ltr_path. That field lives
inside CONFIG_PCIEASPM, and without ASPM support there is nothing here to
enable LTR for, so compile the function out in that configuration.

Note this does change CONFIG_PCIEASPM=n builds, which used to program
chip LTR unconditionally: pci_disable_link_state() is a stub that returns
success there, so tp->aspm_manageable ends up set. The PCI core does not
configure LTR in that configuration either.

Fixes: 9ab94a32af70 ("r8169: enable LTR support")
Closes: https://bugzilla.redhat.com/show_bug.cgi?id=2529752
Assisted-by: LLM
Signed-off-by: Yogesh Gaur <yogeshgaur.83@gmail.com>
---
v2:
 - Gate on pci_dev->ltr_path instead of reading PCI_EXP_DEVCTL2 directly,
   and compile rtl_enable_ltr() out for CONFIG_PCIEASPM=n, where that
   field does not exist and there is nothing to enable LTR for.
   Suggested by Heiner Kallweit.
 - Commit message notes the resulting change for CONFIG_PCIEASPM=n builds.
 - No change to the RTL8125 register programming itself.

v1: https://lore.kernel.org/all/20260909110554.1977-1-yogeshgaur.83@gmail.com/

 drivers/net/ethernet/realtek/r8169_main.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/drivers/net/ethernet/realtek/r8169_main.c b/drivers/net/ethernet/realtek/r8169_main.c
index ec4fc21fa21f..05f7a44aff01 100644
--- a/drivers/net/ethernet/realtek/r8169_main.c
+++ b/drivers/net/ethernet/realtek/r8169_main.c
@@ -3035,8 +3035,16 @@ static void rtl_disable_exit_l1(struct rtl8169_private *tp)
 	}
 }
 
+#ifdef CONFIG_PCIEASPM
 static void rtl_enable_ltr(struct rtl8169_private *tp)
 {
+	/* The chip must not issue LTR messages unless LTR is enabled on the
+	 * whole path up to the root port. pci_configure_ltr() works that out
+	 * and records the result in pci_dev->ltr_path.
+	 */
+	if (!tp->pci_dev->ltr_path)
+		return;
+
 	switch (tp->mac_version) {
 	case RTL_GIGA_MAC_VER_80:
 		r8168_mac_ocp_write(tp, 0xcdd0, 0x9003);
@@ -3120,6 +3128,11 @@ static void rtl_enable_ltr(struct rtl8169_private *tp)
 	/* chip can trigger LTR */
 	r8168_mac_ocp_modify(tp, LTR_OBFF_LOCK, 0x0003, LTR_OBFF_LOCK_EN);
 }
+#else
+static void rtl_enable_ltr(struct rtl8169_private *tp)
+{
+}
+#endif
 
 static void rtl_hw_aspm_clkreq_enable(struct rtl8169_private *tp, bool enable)
 {
-- 
2.34.1