PCI: PM: Add special case handling for PCIe device wakeup

From: Rafael J. Wysocki <[email protected]>

Some PCIe devices only support PME (Power Management Event) from D3cold. One example is the ASMedia xHCI controller:

11:00.0 USB controller: ASMedia Technology Inc. ASM1042A USB 3.0 Host Controller (prog-if 30 [XHCI])
...
Capabilities: [78] Power Management version 3
Flags: PMEClk- DSI- D1- D2- AuxCurrent=55mA PME(D0-,D1-,D2-,D3hot-,D3cold+)
Status: D0 NoSoftRst+ PME-Enable- DSel=0 DScale=0 PME-

In those cases, if the device is expected to generate wakeup events from its final power state, pci_target_state() returns D0, which prevents the PCIe hierarchy above the device from entering any low-power states too, but the device cannot signal PME from D0 either. However, if the device were allowed to go into D3hot, its parent PCIe port and its ancestors would also be able to go into D3 and if any of them goes into D3cold, the device would end up in D3cold too (as per the PCI PM spec v1.2, Table 6-1), in which case it would be able to signal PME.

This means that the system could be put into a lower-power configuration while meeting the requirement to enable the device to generate PME from the final state (which is not the case if the device stays in D0 along with the entire hierarchy above it).

In order to avoid missing that opportunity, extend pci_pme_capable() to return 'true' in the special case when the target state is D3hot and the device can only signal PME from D3cold and update pci_target_state() to return the current target state if pci_pme_capable() returns 'true' for it.

This change can be regarded as a pci_target_state() fix, because that function should ignore its 'wakeup' argument if signaling PME from any power states shallower than the current candidate one (including D0) is not supported.

---

Hi Mika,

IMO it is better to address the case in which the device cannot signal PME from D0 (as well as from any power states shallower than D3cold), which appears to be the one at hand, to start with and then, if need be, take care of the case in which signaling PME from both D0 and D3cold is supported separately.

If the device cannot signal PME from D0, then there is no point returning D0 from pci_target_state() just because 'wakeup' is set, so this is a bug, and then enabling PME in case the device ends up in D3cold can be regarded as "best effort" (we cannot guarantee that this will always happen, but then it is the only way to enable it to signal wakeup anyway).

The case when signaling PME from D0 and D3cold (if we need to worry about it at all) is more controversial, because in that case leaving the device in D0 really allows it to signal wakeup, while putting it into D3hot in hope that it will end up in D3cold may not work.

Cheers!

-> v2:
* Instead of checking the direct parent of the device in question, which doesn't work, check if the device can signal PME from any power states shallower than D3cold, including D0. If not, return 'true' from pci_pme_capable() for D3hot as a way to indicate that the device should be able to signal wakeup from the final state conditional on getting one of its ancestors into D3cold.

---
 drivers/pci/pci.c |   34 +++++++++++++++++++++-------------
 1 file changed, 21 insertions(+), 13 deletions(-)

Index: linux-pm/drivers/pci/pci.c
===================================================================
@@ -2301,7 +2300,21 @@ bool pci_pme_capable(struct pci_dev *dev
 	if (!dev->pm_cap)
 		return false;
 
-	return !!(dev->pme_support & (1 << state));
+	if (dev->pme_support & (1 << state))
+		return true;
+
+	/*
+	 * Special case: The target state is D3hot and the device (which is a
+	 * PCIe one) only supports signaling PME from D3cold.  In that case, if
+	 * the device is allowed to go into D3hot, its ancestor PCIe port may go
+	 * into D3cold which will cause the device to end up in D3cold too
+	 * (along the lines of the PCI PM spec v1.2, Table 6-1), so it will be
+	 * able to signal PME from the final state.  It will not be able to
+	 * signal PME if left in D0, however.
+	 */
+	return state == PCI_D3hot && pci_is_pcie(dev) &&
+		(dev->pme_support & (1 << PCI_D3cold)) &&
+		!(dev->pme_support ^ (1 << PCI_D3cold));
 }
 EXPORT_SYMBOL(pci_pme_capable);
 

@@ -2595,17 +2608,12 @@ static pci_power_t pci_target_state(stru
 	if (dev->current_state == PCI_D3cold)
 		target_state = PCI_D3cold;
 
-	if (wakeup) {
-		/*
-		 * Find the deepest state from which the device can generate
-		 * PME#.
-		 */
-		if (dev->pme_support) {
-			while (target_state
-			      && !(dev->pme_support & (1 << target_state)))
-				target_state--;
-		}
-	}
+	if (!wakeup || !dev->pme_support || pci_pme_capable(dev, target_state))
+		return target_state;
+
+	/* Find the deepest state from which the device can generate PME#. */
+	while (target_state && !(dev->pme_support & (1 << target_state)))
+		target_state--;
 
 	return target_state;
 }

Hi Rafael,

Tried this now and it fixes the issue! Also checked with another device that actually supports PME from other states than D3cold and it also works (as expected).

Feel free to add my,

Thanks!

Hi Mika,

Thank you!

However, after giving some more consideration to this, I've realized that it may still not be the cleanest way to address the issue at hand.

Namely, there are two things we want to happen: (1) pci_target_state() should return the original candidate target state if 'wakeup' is true, but the device cannot signal PME from any states shallower than the original one (including D0) and (2) __pci_enable_wake() should call pci_pme_active() for devices that support signaling PME from D3cold regardless of whether or not they support signaling PME from the power state passed to it (because a device may end up in D3cold as a result of a transition of the hierarchy above it and PME should be signaled then). They are only related to each other because the PME signaling capability is taken into account in both places, but in fact (1) has nothing to do with D3cold (and making it effectively depend on whether or not the device can signal PME from D3cold is somewhat artificial) and (2) has nothing to do with what the target power state of the device is. And so each of these two things should be covered by a separate patch.

Let me post an alternative patch series in accordance with the above.