From: Bjorn Helgaas <bhelg...@google.com>

Add details about how we decide whether we can put a PCI bridge in D3.
9d26d3a8f1b0 ("PCI: Put PCIe ports into D3 during suspend") added this
support to reduce power consumption on Intel Sunrise Point and Broxton
platforms.

In some cases we don't use D3 for bridges even when it should work, simply
because it's impractical to test the configuration, or we tripped over some
possible hardware issue on older platforms.  Links to discussion of the
PCIe port runtime power management patches, which includes mention of these
issues, are below.

No functional change.

Link: v1: 
https://lkml.kernel.org/r/1456750566-116248-1-git-send-email-mika.westerb...@linux.intel.com
Link: v2: 
https://lkml.kernel.org/r/1460111790-92836-1-git-send-email-mika.westerb...@linux.intel.com
Link: v3: 
https://lkml.kernel.org/r/1460628268-16204-1-git-send-email-mika.westerb...@linux.intel.com
Link: v4: 
https://lkml.kernel.org/r/1461578004-129094-1-git-send-email-mika.westerb...@linux.intel.com
Link: v5: 
https://lkml.kernel.org/r/1461919919-120102-1-git-send-email-mika.westerb...@linux.intel.com
Link: v6: 
https://lkml.kernel.org/r/1464855435-32960-1-git-send-email-mika.westerb...@linux.intel.com
Link: https://lkml.kernel.org/r/2858019.9tucwsd...@aspire.rjw.lan
Signed-off-by: Bjorn Helgaas <bhelg...@google.com>
---
 drivers/pci/pci.c |   19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index f6a4dd10d9b0..75db77cf3f8f 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -2260,6 +2260,13 @@ bool pci_bridge_d3_possible(struct pci_dev *bridge)
 {
        unsigned int year;
 
+       /*
+        * In principle we should be able to put conventional PCI bridges
+        * into D3.  We only support it for PCIe because (a) we want to
+        * save power on new (2015 and newer) SoCs that can enter deep
+        * low-power states only if PCIe Root Ports are in D3 and (b) we
+        * don't want to risk regressions on older hardware.
+        */
        if (!pci_is_pcie(bridge))
                return false;
 
@@ -2276,6 +2283,14 @@ bool pci_bridge_d3_possible(struct pci_dev *bridge)
                 * hotplug ports handled by firmware in System Management Mode
                 * may not be put into D3 by the OS (Thunderbolt on non-Macs).
                 * For simplicity, disallow in general for now.
+                *
+                * Per PCIe r4.0, sec 6.7.3.4, if the form factor requires
+                * wake support, a hot-plug capable Downstream Port must
+                * support generation of a wakeup event on hot-plug events
+                * that occur when the system is in a sleep state or the
+                * Port is in device state D1, D2, or D3hot.  Therefore, it
+                * might be possible to use D3 even for hot-plug Ports, but
+                * for now we do not.
                 */
                if (bridge->is_hotplug_bridge)
                        return false;
@@ -2285,7 +2300,9 @@ bool pci_bridge_d3_possible(struct pci_dev *bridge)
 
                /*
                 * It should be safe to put PCIe ports from 2015 or newer
-                * to D3.
+                * to D3.  We have vague reports of possible hardware
+                * issues when putting older PCIe ports into D3.  See
+                * changelog.
                 */
                if (dmi_get_date(DMI_BIOS_DATE, &year, NULL, NULL) &&
                    year >= 2015) {

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