[PATCH v1 0/2] PCI: Add pci=nobwctrl to disable the PCIe bandwidth controller

[PATCH v1 0/2] PCI: Add pci=nobwctrl to disable the PCIe bandwidth controller

[Guixin Liu]

Enabling the PCIe bandwidth controller sets the Link Bandwidth Management
Interrupt Enable (LBMIE) and Link Autonomous Bandwidth Interrupt Enable
(LABIE) bits.  On some (typically old) platforms this has been observed to
cause boot hangs; commit 46a9f70e93ef ("PCI/bwctrl: Disable BW controller
on Intel P45 using a quirk") works around one such platform with a
per-device quirk.

A per-device quirk only helps once the offending device is identified and
a quirk has shipped, which is awkward for a boot hang since the machine
cannot be booted to report the culprit.  The only existing escape hatch,
pcie_ports=compat, is a blunt instrument that also disables AER, PME,
hotplug and DPC.

This series adds a pci=nobwctrl boot option to disable just the bandwidth
controller system-wide, giving users an immediate, rebuild-free workaround
while a targeted quirk is worked out.

Patch 1 adds the pci_bwctrl_available()/pci_no_bwctrl() infrastructure
(mirroring pci_aer_available()/pci_no_aer()) with no functional change.
Patch 2 wires up the pci=nobwctrl parameter and documents it.

Tested on a QEMU q35 topology with PCIe ports and switches: without the
parameter the bwctrl service devices are present as before; with
pci=nobwctrl they all disappear while PME/AER/hotplug services stay intact
and boot is clean.

Guixin Liu (2):
  PCI/bwctrl: Add pci_bwctrl_available()
  PCI: Add pci=nobwctrl parameter to disable the PCIe bandwidth
    controller

 Documentation/admin-guide/kernel-parameters.txt |  6 ++++++
 drivers/pci/pci.c                               |  2 ++
 drivers/pci/pci.h                               |  4 ++++
 drivers/pci/pcie/bwctrl.c                       | 12 ++++++++++++
 drivers/pci/pcie/portdrv.c                      |  3 ++-
 5 files changed, 26 insertions(+), 1 deletion(-)

-- 
2.43.7

[PATCH v1 1/2] PCI/bwctrl: Add pci_bwctrl_available()

[Guixin Liu]

Enabling the PCIe bandwidth controller sets the Link Bandwidth Management
Interrupt Enable (LBMIE) and Link Autonomous Bandwidth Interrupt Enable
(LABIE) bits.  On some (typically old) platforms this has been observed to
cause boot hangs, e.g. commit 46a9f70e93ef ("PCI/bwctrl: Disable BW
controller on Intel P45 using a quirk") works around one such case with a
per-device quirk.

In preparation for a system-wide switch to turn the bandwidth controller
off, add pci_bwctrl_available() and pci_no_bwctrl(), mirroring
pci_aer_available()/pci_no_aer(), and consult the former when deciding
whether to enable PCIE_PORT_SERVICE_BWCTRL.

No functional change intended: the controller stays available by default.

Signed-off-by: Guixin Liu <kanie@linux.alibaba.com>
---
 drivers/pci/pci.h          |  4 ++++
 drivers/pci/pcie/bwctrl.c  | 12 ++++++++++++
 drivers/pci/pcie/portdrv.c |  3 ++-
 3 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
index 4469e1a77f3c..7a1d4f624ed8 100644
--- a/drivers/pci/pci.h
+++ b/drivers/pci/pci.h
@@ -1121,8 +1121,12 @@ static inline void pcie_ecrc_get_policy(char *str) { }
 
 #ifdef CONFIG_PCIEPORTBUS
 void pcie_reset_lbms(struct pci_dev *port);
+void pci_no_bwctrl(void);
+bool pci_bwctrl_available(void);
 #else
 static inline void pcie_reset_lbms(struct pci_dev *port) {}
+static inline void pci_no_bwctrl(void) {}
+static inline bool pci_bwctrl_available(void) { return false; }
 #endif
 
 struct pci_dev_reset_methods {
diff --git a/drivers/pci/pcie/bwctrl.c b/drivers/pci/pcie/bwctrl.c
index c4c8d260bf96..947cb4b8697b 100644
--- a/drivers/pci/pcie/bwctrl.c
+++ b/drivers/pci/pcie/bwctrl.c
@@ -48,6 +48,18 @@ struct pcie_bwctrl_data {
 /* Prevent port removal during Link Speed changes. */
 static DECLARE_RWSEM(pcie_bwctrl_setspeed_rwsem);
 
+static bool pcie_bwctrl_disable;
+
+void pci_no_bwctrl(void)
+{
+	pcie_bwctrl_disable = true;
+}
+
+bool pci_bwctrl_available(void)
+{
+	return !pcie_bwctrl_disable;
+}
+
 static bool pcie_valid_speed(enum pci_bus_speed speed)
 {
 	return (speed >= PCIE_SPEED_2_5GT) && (speed <= PCIE_SPEED_64_0GT);
diff --git a/drivers/pci/pcie/portdrv.c b/drivers/pci/pcie/portdrv.c
index 2d6aa488fe7b..e212c34f7f4a 100644
--- a/drivers/pci/pcie/portdrv.c
+++ b/drivers/pci/pcie/portdrv.c
@@ -274,7 +274,8 @@ static int get_port_device_capability(struct pci_dev *dev)
 
 		pcie_capability_read_dword(dev, PCI_EXP_LNKCAP, &linkcap);
 		if (linkcap & PCI_EXP_LNKCAP_LBNC &&
-		    hweight8(dev->supported_speeds) > 1)
+		    hweight8(dev->supported_speeds) > 1 &&
+		    pci_bwctrl_available())
 			services |= PCIE_PORT_SERVICE_BWCTRL;
 	}
 
-- 
2.43.7

[PATCH v1 2/2] PCI: Add pci=nobwctrl parameter to disable the PCIe bandwidth controller

[Guixin Liu]

The PCIe bandwidth controller enables the Link Bandwidth Management
Interrupt Enable (LBMIE) and Link Autonomous Bandwidth Interrupt Enable
(LABIE) bits.  On some (typically old) platforms this has been observed to
cause boot hangs; commit 46a9f70e93ef ("PCI/bwctrl: Disable BW controller
on Intel P45 using a quirk") addresses one such platform with a per-device
quirk.

Per-device quirks only help once the offending device has been identified
and a quirk has shipped, which is awkward for a boot hang since the machine
cannot be booted to report it.  The only existing escape hatch,
pcie_ports=compat, is a blunt instrument that also disables AER, PME,
hotplug and DPC.

Add a pci=nobwctrl boot option to disable just the bandwidth controller
system-wide, giving users an immediate, rebuild-free workaround while a
targeted quirk is worked out.

Signed-off-by: Guixin Liu <kanie@linux.alibaba.com>
---
 Documentation/admin-guide/kernel-parameters.txt | 6 ++++++
 drivers/pci/pci.c                               | 2 ++
 2 files changed, 8 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index b5493a7f8f22..7621f0b61dc8 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5053,6 +5053,12 @@ Kernel parameters
 		noaer		[PCIE] If the PCIEAER kernel config parameter is
 				enabled, this kernel boot option can be used to
 				disable the use of PCIE advanced error reporting.
+		nobwctrl	[PCIE] Disable the PCIe bandwidth controller,
+				which manages Link Bandwidth Management
+				Interrupts. Enabling these interrupts has been
+				observed to cause boot hangs on some (typically
+				old) platforms; use this option to work around
+				such issues.
 		nodomains	[PCI] Disable support for multiple PCI
 				root domains (aka PCI segments, in ACPI-speak).
 		nommconf	[X86] Disable use of MMCONFIG for PCI
diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index 77b17b13ee61..36c0b3fe4017 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -6756,6 +6756,8 @@ static int __init pci_setup(char *str)
 				pcie_ats_disabled = true;
 			} else if (!strcmp(str, "noaer")) {
 				pci_no_aer();
+			} else if (!strcmp(str, "nobwctrl")) {
+				pci_no_bwctrl();
 			} else if (!strcmp(str, "earlydump")) {
 				pci_early_dump = true;
 			} else if (!strncmp(str, "realloc=", 8)) {
-- 
2.43.7

Re: [PATCH v1 2/2] PCI: Add pci=nobwctrl parameter to disable the PCIe bandwidth controller

[sashiko-bot]

> The PCIe bandwidth controller enables the Link Bandwidth Management
> Interrupt Enable (LBMIE) and Link Autonomous Bandwidth Interrupt Enable
> (LABIE) bits.  On some (typically old) platforms this has been observed to
> cause boot hangs; commit 46a9f70e93ef ("PCI/bwctrl: Disable BW controller
> on Intel P45 using a quirk") addresses one such platform with a per-device
> quirk.
> 
> Per-device quirks only help once the offending device has been identified
> and a quirk has shipped, which is awkward for a boot hang since the machine
> cannot be booted to report it.  The only existing escape hatch,
> pcie_ports=compat, is a blunt instrument that also disables AER, PME,
> hotplug and DPC.
> 
> Add a pci=nobwctrl boot option to disable just the bandwidth controller
> system-wide, giving users an immediate, rebuild-free workaround while a
> targeted quirk is worked out.
> 
> Signed-off-by: Guixin Liu <kanie@linux.alibaba.com>

Sashiko has reviewed this patch and found no issues. It looks great!

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260729034505.469107-1-kanie@linux.alibaba.com?part=2

Re: [PATCH v1 1/2] PCI/bwctrl: Add pci_bwctrl_available()

[sashiko-bot]

> Enabling the PCIe bandwidth controller sets the Link Bandwidth Management
> Interrupt Enable (LBMIE) and Link Autonomous Bandwidth Interrupt Enable
> (LABIE) bits.  On some (typically old) platforms this has been observed to
> cause boot hangs, e.g. commit 46a9f70e93ef ("PCI/bwctrl: Disable BW
> controller on Intel P45 using a quirk") works around one such case with a
> per-device quirk.
> 
> In preparation for a system-wide switch to turn the bandwidth controller
> off, add pci_bwctrl_available() and pci_no_bwctrl(), mirroring
> pci_aer_available()/pci_no_aer(), and consult the former when deciding
> whether to enable PCIE_PORT_SERVICE_BWCTRL.
> 
> No functional change intended: the controller stays available by default.
> 
> Signed-off-by: Guixin Liu <kanie@linux.alibaba.com>

Sashiko has reviewed this patch and found no issues. It looks great!

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260729034505.469107-1-kanie@linux.alibaba.com?part=1

Re: [PATCH v1 0/2] PCI: Add pci=nobwctrl to disable the PCIe bandwidth controller

[Lukas Wunner]

On Wed, Jul 29, 2026 at 11:45:03AM +0800, Guixin Liu wrote:
> Enabling the PCIe bandwidth controller sets the Link Bandwidth Management
> Interrupt Enable (LBMIE) and Link Autonomous Bandwidth Interrupt Enable
> (LABIE) bits.  On some (typically old) platforms this has been observed to
> cause boot hangs; commit 46a9f70e93ef ("PCI/bwctrl: Disable BW controller
> on Intel P45 using a quirk") works around one such platform with a
> per-device quirk.
> 
> A per-device quirk only helps once the offending device is identified and
> a quirk has shipped, which is awkward for a boot hang since the machine
> cannot be booted to report the culprit.  The only existing escape hatch,
> pcie_ports=compat, is a blunt instrument that also disables AER, PME,
> hotplug and DPC.

Ideally we want to identify all devices that have problems if the
bandwidth controller is enabled and add quirks for every one of them.

In a sense, a command line option disincentivizes reporting and
fixing affected devices because users can just work around the problem.

Additionally, there is a general dislike in the community for
adding new command line options.

Are you seeing issues on different devices than the one that's
already been quirked?  If so, a patch to add quirks for them
would be welcome.

Thanks,

Lukas

Re: [PATCH v1 0/2] PCI: Add pci=nobwctrl to disable the PCIe bandwidth controller

[Guixin Liu]

在 2026/7/29 15:01, Lukas Wunner 写道:
> On Wed, Jul 29, 2026 at 11:45:03AM +0800, Guixin Liu wrote:
>> Enabling the PCIe bandwidth controller sets the Link Bandwidth Management
>> Interrupt Enable (LBMIE) and Link Autonomous Bandwidth Interrupt Enable
>> (LABIE) bits.  On some (typically old) platforms this has been observed to
>> cause boot hangs; commit 46a9f70e93ef ("PCI/bwctrl: Disable BW controller
>> on Intel P45 using a quirk") works around one such platform with a
>> per-device quirk.
>>
>> A per-device quirk only helps once the offending device is identified and
>> a quirk has shipped, which is awkward for a boot hang since the machine
>> cannot be booted to report the culprit.  The only existing escape hatch,
>> pcie_ports=compat, is a blunt instrument that also disables AER, PME,
>> hotplug and DPC.
> Ideally we want to identify all devices that have problems if the
> bandwidth controller is enabled and add quirks for every one of them.
>
> In a sense, a command line option disincentivizes reporting and
> fixing affected devices because users can just work around the problem.
>
> Additionally, there is a general dislike in the community for
> adding new command line options.
>
> Are you seeing issues on different devices than the one that's
> already been quirked?  If so, a patch to add quirks for them
> would be welcome.
>
> Thanks,
>
> Lukas
Thanks Lukas, that's fair - I fully agree that per-device quirks are
the right long-term fix, and I'll gladly send quirks for any device I
can reproduce this on.

I'd argue this option isn't really an alternative to quirks, though.
It's what lets you *get to* a quirk in the first place, and it closes
a gap that is specific to bwctrl:

- To write a bwctrl quirk you first have to prove bwctrl is the
   culprit. pcie_ports=compat can get a hung machine booted, but it
   tears down *all* port services at once, so it can't tell you whether
   the offender was bwctrl, AER, PME or hotplug. pci=nobwctrl isolates
   exactly one variable, which is precisely the information a quirk
   needs. In that sense it feeds the quirk pipeline rather than
   discouraging reports.

- compat is also too blunt as a workaround on modern systems: losing
   native AER/DPC/hotplug just to silence bwctrl is a large regression
   in error handling. A surgical switch keeps everything else native.

- bwctrl is currently the only PCIe port service with neither a
   native/_OSC gate nor a disable of its own. hotplug, AER and PME are
   all gated on pcie_ports_native || host->native_*, and pci=noaer
   already exists as precedent. This mostly brings bwctrl up to parity
   rather than inventing a new class of knob.

If the command-line parameter itself is the sticking point, I'm happy
to implement this in whatever form you prefer - e.g. gating bwctrl
registration the same way the other services are gated, or a per-port
sysfs runtime control. My goal is to close the gap; the mechanism is
negotiable.

Best Regards,
Guixin Liu