[alsa-devel] [PATCH 1/7] PCI: Restore BARs on runtime resume despite being unbound

[Lukas Wunner]

PCI devices not bound to a driver are supposed to stay in D0 during
runtime suspend.  But they may have a parent which is bound and can be
transitioned to D3cold at runtime.  Once the parent goes to D3cold, the
unbound child may go to D3cold as well.  When the child comes out of
D3cold, its BARs are uninitialized and thus inaccessible when a driver
tries to probe.

One example are recent hybrid graphics laptops which cut power to the
discrete GPU when the root port above it goes to ACPI power state D3.
Users may provoke this by unbinding the GPU driver and allowing runtime
PM on the GPU via sysfs:  The PM core will then treat the GPU as
"suspended", which in turn allows the root port to runtime suspend,
causing the power resources listed in its _PR3 object to be powered off.
The GPU's BARs will be uninitialized when a driver later probes it.

Another example are hybrid graphics laptops where the GPU itself (rather
than the root port) is capable of runtime suspending to D3cold.  If the
GPU's integrated HDA controller is not bound and the GPU's driver
decides to runtime suspend to D3cold, the HDA controller's BARs will be
uninitialized when a driver later probes it.

Fix by restoring the BARs on runtime resume if the device is not bound.
This is sufficient to fix the above-mentioned use cases.  Other use
cases might require a full-blown pci_save_state() / pci_restore_state()
or execution of fixups.  We can add that once use cases materialize,
let's not inflate the code unnecessarily.

Cc: Bjorn Helgaas <bhelgaas at google.com>
Cc: Rafael J. Wysocki <rafael.j.wysocki at intel.com>
Signed-off-by: Lukas Wunner <lukas at wunner.de>
---
 drivers/pci/pci-driver.c | 8 ++++++--
 drivers/pci/pci.c        | 2 +-
 drivers/pci/pci.h        | 1 +
 3 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/drivers/pci/pci-driver.c b/drivers/pci/pci-driver.c
index 3bed6beda051..51b11cbd48f6 100644
--- a/drivers/pci/pci-driver.c
+++ b/drivers/pci/pci-driver.c
@@ -1277,10 +1277,14 @@ static int pci_pm_runtime_resume(struct device *dev)
 
 	/*
 	 * If pci_dev->driver is not set (unbound), the device should
-	 * always remain in D0 regardless of the runtime PM status
+	 * always remain in D0 regardless of the runtime PM status.
+	 * But if its parent can go to D3cold, this device may have
+	 * been in D3cold as well and require restoration of its BARs.
 	 */
-	if (!pci_dev->driver)
+	if (!pci_dev->driver) {
+		pci_restore_bars(pci_dev);
 		return 0;
+	}
 
 	if (!pm || !pm->runtime_resume)
 		return -ENOSYS;
diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index f6a4dd10d9b0..f694650235f2 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -563,7 +563,7 @@ int pci_wait_for_pending(struct pci_dev *dev, int pos, u16 mask)
  * Restore the BAR values for a given device, so as to make it
  * accessible by its driver.
  */
-static void pci_restore_bars(struct pci_dev *dev)
+void pci_restore_bars(struct pci_dev *dev)
 {
 	int i;
 
diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
index fcd81911b127..29dc15bbe3bf 100644
--- a/drivers/pci/pci.h
+++ b/drivers/pci/pci.h
@@ -83,6 +83,7 @@ void pci_allocate_cap_save_buffers(struct pci_dev *dev);
 void pci_free_cap_save_buffers(struct pci_dev *dev);
 bool pci_bridge_d3_possible(struct pci_dev *dev);
 void pci_bridge_d3_update(struct pci_dev *dev);
+void pci_restore_bars(struct pci_dev *dev);
 
 static inline void pci_wakeup_event(struct pci_dev *dev)
 {
-- 
2.15.1