PCI: pciehp: Support interrupts sent from D3hot

If a hotplug port is able to send an interrupt, one would naively assume
that it is accessible at that moment.  After all, if it wouldn't be
accessible, i.e. if its parent is in D3hot and the link to the hotplug
port is thus down, how should an interrupt come through?

It turns out that assumption is wrong at least for Thunderbolt:  Even
though its parents are in D3hot, a Thunderbolt hotplug port is able to
signal interrupts.  Because the port's config space is inaccessible and
resuming the parents may sleep, the hard IRQ handler has to defer
runtime resuming the parents and reading the Slot Status register to the
IRQ thread.

If the hotplug port uses a level-triggered INTx interrupt, it needs to
be masked until the IRQ thread has cleared the signaled events.  For
simplicity, this commit also masks edge-triggered MSI/MSI-X interrupts.
Note that if the interrupt is shared (which can only happen for INTx),
other devices are starved from receiving interrupts until the IRQ thread
is scheduled, has runtime resumed the hotplug port's parents and has
read and cleared the Slot Status register.

That delay is dominated by the 10 ms D3hot->D0 transition time of each
parent port.  The worst case is a Thunderbolt downstream port at the
end of a daisy chain:  There may be up to six Thunderbolt controllers
in-between it and the root port, each comprising an upstream and
downstream port, plus its own upstream port.  That's 13 x 10 = 130 ms.
Possible mitigations are polling the interrupt while it's disabled or
reducing the d3_delay of Thunderbolt ports if possible.

Open code masking of the interrupt instead of requesting it with the
IRQF_ONESHOT flag to minimize the period during which it is masked.
(IRQF_ONESHOT unmasks the IRQ only after the IRQ thread has finished.)

PCIe r4.0 sec 6.7.3.4 states that "If wake generation is required by the
associated form factor specification, a hotplug capable Downstream Port
must support generation of a wakeup event (using the PME mechanism) on
hotplug events that occur when the system is in a sleep state or the
Port is in device state D1, D2, or D3Hot."

This would seem to imply that PME needs to be enabled on the hotplug
port when it is runtime suspended.  pci_enable_wake() currently doesn't
enable PME on bridges, it may be necessary to add an exemption for
hotplug bridges there.  On "Light Ridge" Thunderbolt controllers, the
PME_Status bit is not set when an interrupt occurs while the hotplug
port is in D3hot, even if PME is enabled.  (I've tested this on a Mac
and we hardcode the OSC_PCI_EXPRESS_PME_CONTROL bit to 0 on Macs in
negotiate_os_control(), modifying it to 1 didn't change the behavior.)

(Side note:  Section 6.7.3.4 also states that "PME and Hot-Plug Event
interrupts (when both are implemented) always share the same MSI or
MSI-X vector".  That would only seem to apply to Root Ports, however
the section never mentions Root Ports, only Downstream Ports.  This is
explained in the definition of "Downstream Port" in the "Terms and
Acronyms" section of the PCIe Base Spec:  "The Ports on a Switch that
are not the Upstream Port are Downstream Ports.  All Ports on a Root
Complex are Downstream Ports.")

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: Mika Westerberg <mika.westerberg@linux.intel.com>
Cc: Ashok Raj <ashok.raj@intel.com>
Cc: Keith Busch <keith.busch@intel.com>
Cc: Yinghai Lu <yinghai@kernel.org>

drivers/pci/hotplug/pciehp.h

@@ -156,8 +156,13 @@ struct controller {
  *
  * %DISABLE_SLOT: Disable the slot in response to a user request via sysfs or
  *	an Attention Button press after the 5 second delay
+ * %RERUN_ISR: Used by the IRQ handler to inform the IRQ thread that the
+ *	hotplug port was inaccessible when the interrupt occurred, requiring
+ *	that the IRQ handler is rerun by the IRQ thread after it has made the
+ *	hotplug port accessible by runtime resuming its parents to D0
  */
 #define DISABLE_SLOT		(1 << 16)
+#define RERUN_ISR		(1 << 17)
 
 #define ATTN_BUTTN(ctrl)	((ctrl)->slot_cap & PCI_EXP_SLTCAP_ABP)
 #define POWER_CTRL(ctrl)	((ctrl)->slot_cap & PCI_EXP_SLTCAP_PCP)

drivers/pci/hotplug/pciehp_hpc.c

@@ -19,6 +19,7 @@
 #include <linux/jiffies.h>
 #include <linux/kthread.h>
 #include <linux/pci.h>
+#include <linux/pm_runtime.h>
 #include <linux/interrupt.h>
 #include <linux/time.h>
 #include <linux/slab.h>
@@ -521,6 +522,7 @@ static irqreturn_t pciehp_isr(int irq, void *dev_id)
 {
 	struct controller *ctrl = (struct controller *)dev_id;
 	struct pci_dev *pdev = ctrl_dev(ctrl);
+	struct device *parent = pdev->dev.parent;
 	u16 status, events;
 
 	/*
@@ -529,9 +531,26 @@ static irqreturn_t pciehp_isr(int irq, void *dev_id)
 	if (pdev->current_state == PCI_D3cold)
 		return IRQ_NONE;
 
+	/*
+	 * Keep the port accessible by holding a runtime PM ref on its parent.
+	 * Defer resume of the parent to the IRQ thread if it's suspended.
+	 * Mask the interrupt until then.
+	 */
+	if (parent) {
+		pm_runtime_get_noresume(parent);
+		if (!pm_runtime_active(parent)) {
+			pm_runtime_put(parent);
+			disable_irq_nosync(irq);
+			atomic_or(RERUN_ISR, &ctrl->pending_events);
+			return IRQ_WAKE_THREAD;
+		}
+	}
+
 	pcie_capability_read_word(pdev, PCI_EXP_SLTSTA, &status);
 	if (status == (u16) ~0) {
 		ctrl_info(ctrl, "%s: no response from device\n", __func__);
+		if (parent)
+			pm_runtime_put(parent);
 		return IRQ_NONE;
 	}
 
@@ -550,11 +569,16 @@ static irqreturn_t pciehp_isr(int irq, void *dev_id)
 	if (ctrl->power_fault_detected)
 		events &= ~PCI_EXP_SLTSTA_PFD;
 
-	if (!events)
+	if (!events) {
+		if (parent)
+			pm_runtime_put(parent);
 		return IRQ_NONE;
+	}
 
 	pcie_capability_write_word(pdev, PCI_EXP_SLTSTA, events);
 	ctrl_dbg(ctrl, "pending interrupts %#06x from Slot Status\n", events);
+	if (parent)
+		pm_runtime_put(parent);
 
 	/*
 	 * Command Completed notifications are not deferred to the
@@ -584,13 +608,29 @@ static irqreturn_t pciehp_isr(int irq, void *dev_id)
 static irqreturn_t pciehp_ist(int irq, void *dev_id)
 {
 	struct controller *ctrl = (struct controller *)dev_id;
+	struct pci_dev *pdev = ctrl_dev(ctrl);
 	struct slot *slot = ctrl->slot;
+	irqreturn_t ret;
 	u32 events;
 
+	pci_config_pm_runtime_get(pdev);
+
+	/* rerun pciehp_isr() if the port was inaccessible on interrupt */
+	if (atomic_fetch_and(~RERUN_ISR, &ctrl->pending_events) & RERUN_ISR) {
+		ret = pciehp_isr(irq, dev_id);
+		enable_irq(irq);
+		if (ret != IRQ_WAKE_THREAD) {
+			pci_config_pm_runtime_put(pdev);
+			return ret;
+		}
+	}
+
 	synchronize_hardirq(irq);
 	events = atomic_xchg(&ctrl->pending_events, 0);
-	if (!events)
+	if (!events) {
+		pci_config_pm_runtime_put(pdev);
 		return IRQ_NONE;
+	}
 
 	/* Check Attention Button Pressed */
 	if (events & PCI_EXP_SLTSTA_ABP) {
@@ -618,6 +658,7 @@ static irqreturn_t pciehp_ist(int irq, void *dev_id)
 		pciehp_green_led_off(slot);
 	}
 
+	pci_config_pm_runtime_put(pdev);
 	wake_up(&ctrl->requester);
 	return IRQ_HANDLED;
 }