linux-kernel - Re: [RFC 1/2] PCI/ASPM: Allow drivers to provide ASPM link state via pci

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Message-ID: <CAJZ5v0hCTUBzM=UXf1jw6yRraVREDqpa4dAVwhHEkbxtqU9ogA@mail.gmail.com>
Date: Thu, 17 Jul 2025 12:00:06 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: "David E. Box" <david.e.box@...ux.intel.com>
Cc: rafael@...nel.org, bhelgaas@...gle.com, vicamo.yang@...onical.com, 
	kenny@...ix.com, nirmal.patel@...ux.intel.com, linux-pm@...r.kernel.org, 
	linux-pci@...r.kernel.org, ilpo.jarvinen@...ux.intel.com, 
	linux-kernel@...r.kernel.org, 
	"Rafael J . Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [RFC 1/2] PCI/ASPM: Allow drivers to provide ASPM link state via pci_bus

On Thu, Jul 17, 2025 at 2:40 AM David E. Box
<david.e.box@...ux.intel.com> wrote:
>
> Synthetic PCIe hierarchies such as those created by Intel VMD are not
> enumerated or configured by firmware, and therefore do not receive
> BIOS-provided ASPM defaults. This leaves devices behind such domains with
> ASPM effectively disabled, despite platform intent.
>
> Introduce a mechanism to allow the bus owner (e.g. a controller driver) to
> supply a default ASPM policy via a new aspm_bus_link_state field in
> pci_bus.  A new bus flag, PCI_BUS_FLAGS_ASPM_DEFAULT_OVERRIDE, indicates

This doesn't seem to match the code - the name of the new flag is
different there.

> that the core should use this value instead of the BIOS default when
> initializing link state.
>
> This avoids the need for controller-specific logic in ASPM core and allows
> for proper power savings in these otherwise unsupported hierarchies.

I'm guessing that VMD is supposed to set
PCI_BUS_FLAGS_NO_ASPM_DEFAULT, but that doesn't happen in patch [2/2]
AFAICS.

And I would just merge the two patches, IMV there's no reason to keep
them separate.

> Suggested-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> Signed-off-by: David E. Box <david.e.box@...ux.intel.com>
> ---
>  drivers/pci/pcie/aspm.c |  5 ++++-
>  include/linux/pci.h     | 12 ++++++++----
>  2 files changed, 12 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/pci/pcie/aspm.c b/drivers/pci/pcie/aspm.c
> index 29fcb0689a91..2ad1852ac9b2 100644
> --- a/drivers/pci/pcie/aspm.c
> +++ b/drivers/pci/pcie/aspm.c
> @@ -866,7 +866,10 @@ static void pcie_aspm_cap_init(struct pcie_link_state *link, int blacklist)
>         }
>
>         /* Save default state */
> -       link->aspm_default = link->aspm_enabled;
> +       if (parent->bus->bus_flags & PCI_BUS_FLAGS_NO_ASPM_DEFAULT)
> +               link->aspm_default = parent->bus->aspm_bus_link_state;
> +       else
> +               link->aspm_default = link->aspm_enabled;

Could you avoid using the new flag by assuming that if
parent->bus->aspm_bus_link_state was zero, link->aspm_enabled would
take effect?  So the check would be something like

if (parent->bus->aspm_bus_link_state)
        link->aspm_default = parent->bus->aspm_bus_link_state;
else
        link->aspm_default = link->aspm_enabled;

>
>         /* Setup initial capable state. Will be updated later */
>         link->aspm_capable = link->aspm_support;
> diff --git a/include/linux/pci.h b/include/linux/pci.h
> index 05e68f35f392..7e1c305c419c 100644
> --- a/include/linux/pci.h
> +++ b/include/linux/pci.h
> @@ -256,10 +256,11 @@ enum pci_irq_reroute_variant {
>
>  typedef unsigned short __bitwise pci_bus_flags_t;
>  enum pci_bus_flags {
> -       PCI_BUS_FLAGS_NO_MSI    = (__force pci_bus_flags_t) 1,
> -       PCI_BUS_FLAGS_NO_MMRBC  = (__force pci_bus_flags_t) 2,
> -       PCI_BUS_FLAGS_NO_AERSID = (__force pci_bus_flags_t) 4,
> -       PCI_BUS_FLAGS_NO_EXTCFG = (__force pci_bus_flags_t) 8,
> +       PCI_BUS_FLAGS_NO_MSI            = (__force pci_bus_flags_t) 1,
> +       PCI_BUS_FLAGS_NO_MMRBC          = (__force pci_bus_flags_t) 2,
> +       PCI_BUS_FLAGS_NO_AERSID         = (__force pci_bus_flags_t) 4,
> +       PCI_BUS_FLAGS_NO_EXTCFG         = (__force pci_bus_flags_t) 8,
> +       PCI_BUS_FLAGS_NO_ASPM_DEFAULT   = (__force pci_bus_flags_t) 16,
>  };
>
>  /* Values from Link Status register, PCIe r3.1, sec 7.8.8 */
> @@ -665,6 +666,9 @@ struct pci_bus {
>         void            *sysdata;       /* Hook for sys-specific extension */
>         struct proc_dir_entry *procdir; /* Directory entry in /proc/bus/pci */
>
> +#ifdef CONFIG_PCIEASPM
> +       unsigned int    aspm_bus_link_state;    /* Bus owner provided link state */
> +#endif
>         unsigned char   number;         /* Bus number */
>         unsigned char   primary;        /* Number of primary bridge */
>         unsigned char   max_bus_speed;  /* enum pci_bus_speed */
> --