Re: [PATCH] PCI/ASPM: Enable L0s/L1 for removable devices when BIOS didn't configure ASPM

[Mika Westerberg <mika.westerberg@linux.intel.com>]

Hi,

On Tue, May 05, 2026 at 08:09:22PM +0200, Rafael J. Wysocki wrote:
> +Mika Westerberg
> 
> On Tue, May 5, 2026 at 12:53 AM Mario Limonciello
> <mario.limonciello@amd.com> wrote:
> >
> > When comparing lspci output between Windows and Linux for hotplugged
> > Thunderbolt 5 eGPU devices, Windows enables ASPM L1 but Linux doesn't:
> >
> >   Windows: LnkCtl: ASPM L1 Enabled
> >   Linux:   LnkCtl: ASPM Disabled
> >
> > This difference in ASPM configuration can cause behavioral differences
> > between the two operating systems for the same hardware.
> >
> > The root cause is that Linux's default ASPM policy (POLICY_DEFAULT) relies
> > on firmware/BIOS configuration. For hotplugged devices like Thunderbolt/USB4
> > eGPUs, the BIOS may not have configured ASPM since the device wasn't present
> > at boot time. As a result, link->aspm_enabled is 0, link->aspm_default is
> > set to 0, and Linux never enables ASPM for these devices.
> >
> > Devicetree platforms already have special handling to enable L0s/L1 by
> > default regardless of firmware configuration. Extend this same logic to
> > removable devices when firmware hasn't configured any ASPM states.
> >
> > This makes Linux behavior more consistent with Windows for hotplugged
> > Thunderbolt/USB4 devices.
> >
> > Link: https://bugzilla.kernel.org/show_bug.cgi?id=221319
> > Assisted-by: Claude Opus 4.6 <noreply@anthropic.com>
> > Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
> > ---
> >  drivers/pci/pcie/aspm.c | 11 +++++++++--
> >  1 file changed, 9 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/pci/pcie/aspm.c b/drivers/pci/pcie/aspm.c
> > index 925373b98dff0..77497d90be0b7 100644
> > --- a/drivers/pci/pcie/aspm.c
> > +++ b/drivers/pci/pcie/aspm.c
> > @@ -804,8 +804,15 @@ static void pcie_aspm_override_default_link_state(struct pcie_link_state *link)
> >         struct pci_dev *pdev = link->downstream;
> >         u32 override;
> >
> > -       /* For devicetree platforms, enable L0s and L1 by default */
> > -       if (of_have_populated_dt()) {
> > +       /*
> > +        * For devicetree platforms, enable L0s and L1 by default.
> > +        *
> > +        * For removable devices (e.g., Thunderbolt/USB4), enable L0s and L1
> > +        * by default if BIOS didn't configure any ASPM states. This handles
> > +        * hotplugged devices where firmware may not have configured ASPM.
> > +        */

Only L1 is supported over TB/USB4 tunnel (no L0s, no L1 substates). The
PCIe endpoint and the downstream port it connects to of course can support
the full range as that's a real PCIe link.

> > +       if (of_have_populated_dt() ||
> > +           (dev_is_removable(&pdev->dev) && !link->aspm_enabled)) {
> >                 if (link->aspm_support & PCIE_LINK_STATE_L0S)
> >                         link->aspm_default |= PCIE_LINK_STATE_L0S;
> >                 if (link->aspm_support & PCIE_LINK_STATE_L1)
> > --
> > 2.43.0
> >