[PATCH v12 2/7] PCI: Avoid saving config space state if inaccessible

[Farhan Ali <alifm@linux.ibm.com>]

The current reset process saves the device's config space state before
reset and restores it afterward. However errors may occur unexpectedly and
it may then be impossible to save config space because the device may be
inaccessible (e.g. DPC). This results in saving invalid values that get
written back to the device during state restoration.

With a reset we want to recover/restore the device into a functional state.
So avoid saving the state of the config space when the device config space
is inaccessible.

Signed-off-by: Farhan Ali <alifm@linux.ibm.com>
Reviewed-by: Niklas Schnelle <schnelle@linux.ibm.com>
Reviewed-by: Bjorn Helgaas <bhelgaas@google.com>
---
 drivers/pci/pci.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index 70162f15a72c..b36263834289 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -722,6 +722,27 @@ u16 pci_find_dvsec_capability(struct pci_dev *dev, u16 vendor, u16 dvsec)
 }
 EXPORT_SYMBOL_GPL(pci_find_dvsec_capability);
 
+static bool pci_dev_config_accessible(struct pci_dev *dev, char *msg)
+{
+	u32 val;
+
+	/*
+	 * If device's config space is inaccessible it can return ~0 for
+	 * any reads. Since VFs can also return ~0 for Device and Vendor ID
+	 * check Command and Status registers. Note that this is racy
+	 * because the device may become inaccessible partway through
+	 * next access.
+	 */
+	pci_read_config_dword(dev, PCI_COMMAND, &val);
+	if (PCI_POSSIBLE_ERROR(val)) {
+		pci_warn(dev, "Device config space inaccessible; unable to %s\n",
+				msg);
+		return false;
+	}
+
+	return true;
+}
+
 /**
  * pci_find_parent_resource - return resource region of parent bus of given
  *			      region
@@ -5029,6 +5050,9 @@ static void pci_dev_save_and_disable(struct pci_dev *dev)
 	 */
 	pci_set_power_state(dev, PCI_D0);
 
+	if (!pci_dev_config_accessible(dev, "save state"))
+		return;
+
 	pci_save_state(dev);
 	/*
 	 * Disable the device by clearing the Command register, except for
-- 
2.43.0