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

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Message-ID: <20250717004034.2998443-2-david.e.box@linux.intel.com>
Date: Wed, 16 Jul 2025 17:40:25 -0700
From: "David E. Box" <david.e.box@...ux.intel.com>
To: rafael@...nel.org,
	bhelgaas@...gle.com,
	vicamo.yang@...onical.com,
	kenny@...ix.com,
	nirmal.patel@...ux.intel.com
Cc: "David E. Box" <david.e.box@...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: [RFC 1/2] PCI/ASPM: Allow drivers to provide ASPM link state via pci_bus

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
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.

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;
 
 	/* 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 */
-- 
2.43.0