[RFC PATCH v3 02/20] PCI/PM: Add a power-state helper that tries the bus lock

[Shameer Kolothum <skolothumtho@nvidia.com>]

PCI error recovery can hold pci_bus_sem for reading while waiting for a
lock held by a driver. If that driver also needs the bus read lock to
bring a device to D0, a queued writer can block it. Recovery, the driver
and the writer then all wait for each other.

Add pci_try_set_power_state(), which returns -EAGAIN if it cannot take
the bus lock immediately. The caller can then release its locks. Once
the bus lock is held, use pci_set_power_state_locked() for the transition.
Keep the existing cases that need no power-state change. The transition
itself can still sleep.

Callers that already hold the bus read lock must use the locked helper
instead.

Signed-off-by: Shameer Kolothum <skolothumtho@nvidia.com>
---
 include/linux/pci.h |  3 +++
 drivers/pci/pci.c   | 37 +++++++++++++++++++++++++++++++++++++
 2 files changed, 40 insertions(+)

diff --git a/include/linux/pci.h b/include/linux/pci.h
index aa3765a20a96..542fa4af0a43 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -1535,6 +1535,7 @@ int pci_load_and_free_saved_state(struct pci_dev *dev,
 int pci_platform_power_transition(struct pci_dev *dev, pci_power_t state);
 int pci_set_power_state(struct pci_dev *dev, pci_power_t state);
 int pci_set_power_state_locked(struct pci_dev *dev, pci_power_t state);
+int pci_try_set_power_state(struct pci_dev *dev, pci_power_t state);
 pci_power_t pci_choose_state(struct pci_dev *dev, pm_message_t state);
 bool pci_pme_capable(struct pci_dev *dev, pci_power_t state);
 void pci_pme_active(struct pci_dev *dev, bool enable);
@@ -2216,6 +2217,8 @@ static inline int pci_set_power_state(struct pci_dev *dev, pci_power_t state)
 { return 0; }
 static inline int pci_set_power_state_locked(struct pci_dev *dev, pci_power_t state)
 { return 0; }
+static inline int pci_try_set_power_state(struct pci_dev *dev, pci_power_t state)
+{ return 0; }
 static inline int pci_wake_from_d3(struct pci_dev *dev, bool enable)
 { return 0; }
 static inline pci_power_t pci_choose_state(struct pci_dev *dev,
diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index 7c6f2aba8144..f2ebd19204aa 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -1655,6 +1655,43 @@ int pci_set_power_state_locked(struct pci_dev *dev, pci_power_t state)
 }
 EXPORT_SYMBOL(pci_set_power_state_locked);
 
+/**
+ * pci_try_set_power_state - Set power state without waiting for the bus lock
+ * @dev: PCI device to handle
+ * @state: Target PCI power state
+ *
+ * Like pci_set_power_state(), but return -EAGAIN without changing the power
+ * state if the bus lock cannot be acquired immediately. The transition
+ * itself can sleep.
+ *
+ * Callers already holding the bus read lock must use
+ * pci_set_power_state_locked().
+ */
+int pci_try_set_power_state(struct pci_dev *dev, pci_power_t state)
+{
+	int ret;
+
+	if (state > PCI_D3cold)
+		state = PCI_D3cold;
+	else if (state < PCI_D0)
+		state = PCI_D0;
+	else if ((state == PCI_D1 || state == PCI_D2) && pci_no_d1d2(dev))
+		return 0;
+
+	if (dev->current_state == state)
+		return 0;
+	if (state >= PCI_D3hot && (dev->dev_flags & PCI_DEV_FLAGS_NO_D3))
+		return 0;
+
+	if (!down_read_trylock(&pci_bus_sem))
+		return -EAGAIN;
+	ret = pci_set_power_state_locked(dev, state);
+	up_read(&pci_bus_sem);
+
+	return ret;
+}
+EXPORT_SYMBOL(pci_try_set_power_state);
+
 #define PCI_EXP_SAVE_REGS	7
 
 static struct pci_cap_saved_state *_pci_find_saved_cap(struct pci_dev *pci_dev,
-- 
2.43.0