[PATCH v2 1/1] PCI: Do not wait for disconnected devices when resuming

["Ilpo Järvinen" <ilpo.jarvinen@linux.intel.com>]

On runtime resume, pci_dev_wait() is called:
  pci_pm_runtime_resume()
    pci_pm_bridge_power_up_actions()
      pci_bridge_wait_for_secondary_bus()
        pci_dev_wait()

While a device is runtime suspended along with its PCI hierarchy, the
device could get disconnected. In such case, the link will not come up
no matter how long pci_dev_wait() waits for it.

Besides the above mentioned case, there could be other ways to get the
device disconnected while pci_dev_wait() is waiting for the link to
come up.

Make pci_dev_wait() to exit if the device is already disconnected to
avoid unnecessary delay.

The use cases of pci_dev_wait() boil down to two:
  1. Waiting for the device after reset
  2. pci_bridge_wait_for_secondary_bus()

The callers in both cases seem to benefit from propagating the
disconnection as error even if device disconnection would be more
analoguous to the case where there is no device in the first place
which return 0 from pci_dev_wait(). In the case 2, it results in
unnecessary marking of the devices disconnected again but that is
just harmless extra work.

Also make sure compiler does not become too clever with
dev->error_state and use READ_ONCE() to force a fetch for the
up-to-date value.

Reported-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Tested-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
---

v2:

Sent independent of the other patch.

Return -ENOTTY instead of 0 because it aligns better with the
expecations of the reset use case and only causes unnecessary
disconnect marking in the pci_bridge_wait_for_secondary_bus()
case for devices that are already marked disconnected.

 drivers/pci/pci.c | 5 +++++
 drivers/pci/pci.h | 9 ++++++++-
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index ca4159472a72..14c57296a0aa 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -1250,6 +1250,11 @@ static int pci_dev_wait(struct pci_dev *dev, char *reset_type, int timeout)
 	for (;;) {
 		u32 id;
 
+		if (pci_dev_is_disconnected(dev)) {
+			pci_dbg(dev, "disconnected; not waiting\n");
+			return -ENOTTY;
+		}
+
 		pci_read_config_dword(dev, PCI_COMMAND, &id);
 		if (!PCI_POSSIBLE_ERROR(id))
 			break;
diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
index 2336a8d1edab..58a32d2d2e96 100644
--- a/drivers/pci/pci.h
+++ b/drivers/pci/pci.h
@@ -4,6 +4,8 @@
 
 #include <linux/pci.h>
 
+#include <asm/rwonce.h>
+
 /* Number of possible devfns: 0.0 to 1f.7 inclusive */
 #define MAX_NR_DEVFNS 256
 
@@ -370,7 +372,12 @@ static inline int pci_dev_set_disconnected(struct pci_dev *dev, void *unused)
 
 static inline bool pci_dev_is_disconnected(const struct pci_dev *dev)
 {
-	return dev->error_state == pci_channel_io_perm_failure;
+	/*
+	 * error_state is set in pci_dev_set_io_state() using xchg/cmpxchg()
+	 * and read w/o common lock. READ_ONCE() ensures compiler cannot cache
+	 * the value (e.g. inside the loop in pci_dev_wait()).
+	 */
+	return READ_ONCE(dev->error_state) == pci_channel_io_perm_failure;
 }
 
 /* pci_dev priv_flags */
-- 
2.39.2