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 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.
>
> > 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)

> 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
> >
> > +             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)
> >
> >  {
> >
>