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

[Yogesh Gaur]

On Fri, Sep 11, 2026 at 6:44 PM Heiner Kallweit <hkallweit1@xxxxxxxxx> wrote:
>
> On 11.09.2026 15:00, Yogesh Gaur wrote:
> > On Fri, Sep 11, 2026 at 1:03 AM Heiner Kallweit <hkallweit1@xxxxxxxxx> wrote:
> >>
> >> On 10.09.2026 15:01, Yogesh Gaur wrote:
> >>> rtl_enable_ltr() programs the MAC to generate LTR messages - ALDPS_LTR_EN,
> >>>
> >>
> >> Empty lines, mail formatting issue?
> > Yes, I would correct it in v3.
> >
> >>
> >>> 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
> >>
> >> Did you verify that this is caused by LTR messages, and not by any other
> >> root cause?
> >>
> >>>
> >>> 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
> >>
> >> Did you verify that your patch fixes the reported issue? Or do you
> >> just assume this? Because in the bug comment history I don't see
> >> a statement that issue has been resolved.
> > I don't have a board with me, hence have not verified it. I did a
> > compile check only.
>
> If it's not verified that your patch fixes the reported issue, better
> remove the Closes line.
>
Sure, would remove it in v4.

Thanks.

> >>
> >>> Assisted-by: LLM
> >>>
> >>> Signed-off-by: Yogesh Gaur <yogeshgaur.83@xxxxxxxxx>
> >>>
> >>> ---
> >>>
> >>> 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@xxxxxxxxx/
> >>>
> >>>
> >>>
> >>>  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)
> >>
> >> Simpler would be:
> >> if (!IS_ENABLED(CONFIG_PCIEASPM) || !tp->pci_dev->ltr_path)
> >>
> > I tried that and it does not build, because ltr_path itself is declared
> > inside the #ifdef:
> >
> >   include/linux/pci.h:443
> >     #ifdef CONFIG_PCIEASPM
> >             struct pcie_link_state  *link_state;
> >             unsigned int    aspm_l0s_support:1;
> >             unsigned int    aspm_l1_support:1;
> >             unsigned int    ltr_path:1;
> >     #endif
> >
> > IS_ENABLED() is a C expression, so the member reference still has to
> > compile even when the left operand is 0:
> >
> >   drivers/net/ethernet/realtek/r8169_main.c: In function 'rtl_enable_ltr':
> >   error: 'struct pci_dev' has no member named 'ltr_path'
> >        if (!IS_ENABLED(CONFIG_PCIEASPM) || !tp->pci_dev->ltr_path)
> >
> Right, ofc ..
>
> >> Then you don't need the stub function.
> >>
> > You are right that the stub function should go, though. v3 drops it and
> > keeps a single rtl_enable_ltr() with the guard inline:
> >
> >         static void rtl_enable_ltr(struct rtl8169_private *tp)
> >         {
> >                 /* comment */
> >         #ifdef CONFIG_PCIEASPM
> >                 if (!tp->pci_dev->ltr_path)
> >                         return;
> >         #else
> >                 return;
> >         #endif
>
> LGTM
>
> >>>
> >>> +             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)
> >>>
> >>>  {
> >>>
> >>
>