[PATCH 1/2] PCI: Replace pci_dev->is_busmaster with accessors

[Maurice Hieronymus <mhi@mailbox.org>]

`is_busmaster` is one bit of a ~60-bit C bitfield in `struct pci_dev`.
Bits sharing a bitfield word must not be modified concurrently, but its
writers take no common lock: `pci_set_master()` can run without the
device lock (e.g. from runtime PM resume paths), `pci_disable_device()`
clears the bit, and other bits in the same word are written from
entirely different contexts, e.g. `broken_parity_status` from sysfs.
Concurrent read-modify-write cycles of the shared word can then lose
updates.

Move `is_busmaster` into a new `flags` bitmap modified with atomic
bitops and accessed through generated accessor functions, following the
example of commit a7cc262a1135 ("driver core: Replace dev->offline +
->offline_disabled with accessors"). More bitfield flags can follow the
same pattern later.

This also unblocks the Rust device enabling API rework [1], where a
guard object calls `pci_disable_device()` from contexts that may run
concurrently with `pci_set_master()`.

Link: https://lore.kernel.org/rust-for-linux/DJOEYVBS17MJ.1YD3TNGQBWHNK@kernel.org/ [1]
Suggested-by: Danilo Krummrich <dakr@kernel.org>
Cc: rust-for-linux@vger.kernel.org
Signed-off-by: Maurice Hieronymus <mhi@mailbox.org>
---
 drivers/net/ethernet/sfc/falcon/farch.c     |  2 +-
 drivers/net/ethernet/sfc/siena/farch.c      |  2 +-
 drivers/pci/pci-driver.c                    |  2 +-
 drivers/pci/pci.c                           |  6 ++---
 drivers/scsi/lpfc/lpfc_init.c               |  4 ++--
 drivers/xen/xen-pciback/conf_space_header.c |  4 ++--
 drivers/xen/xen-pciback/pciback_ops.c       |  4 ++--
 include/linux/pci.h                         | 37 ++++++++++++++++++++++++++++-
 8 files changed, 48 insertions(+), 13 deletions(-)

diff --git a/drivers/net/ethernet/sfc/falcon/farch.c b/drivers/net/ethernet/sfc/falcon/farch.c
index 23d507a3820d..42594bd7e818 100644
--- a/drivers/net/ethernet/sfc/falcon/farch.c
+++ b/drivers/net/ethernet/sfc/falcon/farch.c
@@ -724,7 +724,7 @@ int ef4_farch_fini_dmaq(struct ef4_nic *efx)
 	/* Do not attempt to write to the NIC during EEH recovery */
 	if (efx->state != STATE_RECOVERY) {
 		/* Only perform flush if DMA is enabled */
-		if (efx->pci_dev->is_busmaster) {
+		if (pci_dev_busmaster(efx->pci_dev)) {
 			efx->type->prepare_flush(efx);
 			rc = ef4_farch_do_flush(efx);
 			efx->type->finish_flush(efx);
diff --git a/drivers/net/ethernet/sfc/siena/farch.c b/drivers/net/ethernet/sfc/siena/farch.c
index 7613d7988894..f673af4c77b6 100644
--- a/drivers/net/ethernet/sfc/siena/farch.c
+++ b/drivers/net/ethernet/sfc/siena/farch.c
@@ -723,7 +723,7 @@ int efx_farch_fini_dmaq(struct efx_nic *efx)
 	/* Do not attempt to write to the NIC during EEH recovery */
 	if (efx->state != STATE_RECOVERY) {
 		/* Only perform flush if DMA is enabled */
-		if (efx->pci_dev->is_busmaster) {
+		if (pci_dev_busmaster(efx->pci_dev)) {
 			efx->type->prepare_flush(efx);
 			rc = efx_farch_do_flush(efx);
 			efx->type->finish_flush(efx);
diff --git a/drivers/pci/pci-driver.c b/drivers/pci/pci-driver.c
index f36778e62ac1..412afa12a285 100644
--- a/drivers/pci/pci-driver.c
+++ b/drivers/pci/pci-driver.c
@@ -649,7 +649,7 @@ static int pci_pm_reenable_device(struct pci_dev *pci_dev)
 	 * if the device was busmaster before the suspend, make it busmaster
 	 * again
 	 */
-	if (pci_dev->is_busmaster)
+	if (pci_dev_busmaster(pci_dev))
 		pci_set_master(pci_dev);
 
 	return retval;
diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index 77b17b13ee61..c4fd6fe6098d 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -2045,7 +2045,7 @@ static void pci_enable_bridge(struct pci_dev *dev)
 		pci_enable_bridge(bridge);
 
 	if (pci_is_enabled(dev)) {
-		if (!dev->is_busmaster)
+		if (!pci_dev_busmaster(dev))
 			pci_set_master(dev);
 		return;
 	}
@@ -2205,7 +2205,7 @@ void pci_disable_device(struct pci_dev *dev)
 
 	do_pci_disable_device(dev);
 
-	dev->is_busmaster = 0;
+	pci_dev_assign_busmaster(dev, false);
 }
 EXPORT_SYMBOL(pci_disable_device);
 
@@ -4120,7 +4120,7 @@ static void __pci_set_master(struct pci_dev *dev, bool enable)
 			enable ? "enabling" : "disabling");
 		pci_write_config_word(dev, PCI_COMMAND, cmd);
 	}
-	dev->is_busmaster = enable;
+	pci_dev_assign_busmaster(dev, enable);
 }
 
 /**
diff --git a/drivers/scsi/lpfc/lpfc_init.c b/drivers/scsi/lpfc/lpfc_init.c
index 82af59c913e9..08dc06e7dfc2 100644
--- a/drivers/scsi/lpfc/lpfc_init.c
+++ b/drivers/scsi/lpfc/lpfc_init.c
@@ -14398,7 +14398,7 @@ lpfc_io_slot_reset_s3(struct pci_dev *pdev)
 
 	pci_restore_state(pdev);
 
-	if (pdev->is_busmaster)
+	if (pci_dev_busmaster(pdev))
 		pci_set_master(pdev);
 
 	spin_lock_irq(&phba->hbalock);
@@ -15251,7 +15251,7 @@ lpfc_io_slot_reset_s4(struct pci_dev *pdev)
 	 */
 	pci_save_state(pdev);
 
-	if (pdev->is_busmaster)
+	if (pci_dev_busmaster(pdev))
 		pci_set_master(pdev);
 
 	spin_lock_irq(&phba->hbalock);
diff --git a/drivers/xen/xen-pciback/conf_space_header.c b/drivers/xen/xen-pciback/conf_space_header.c
index 8b50cbcbdfe1..59a89f915916 100644
--- a/drivers/xen/xen-pciback/conf_space_header.c
+++ b/drivers/xen/xen-pciback/conf_space_header.c
@@ -81,10 +81,10 @@ static int command_write(struct pci_dev *dev, int offset, u16 value, void *data)
 			dev_data->enable_intx = 0;
 	}
 
-	if (!dev->is_busmaster && is_master_cmd(value)) {
+	if (!pci_dev_busmaster(dev) && is_master_cmd(value)) {
 		dev_dbg(&dev->dev, "set bus master\n");
 		pci_set_master(dev);
-	} else if (dev->is_busmaster && !is_master_cmd(value)) {
+	} else if (pci_dev_busmaster(dev) && !is_master_cmd(value)) {
 		dev_dbg(&dev->dev, "clear bus master\n");
 		pci_clear_master(dev);
 	}
diff --git a/drivers/xen/xen-pciback/pciback_ops.c b/drivers/xen/xen-pciback/pciback_ops.c
index bfc186bf05bc..01f4705421c9 100644
--- a/drivers/xen/xen-pciback/pciback_ops.c
+++ b/drivers/xen/xen-pciback/pciback_ops.c
@@ -125,14 +125,14 @@ void xen_pcibk_reset_device(struct pci_dev *dev)
 		if (pci_is_enabled(dev))
 			pci_disable_device(dev);
 
-		dev->is_busmaster = 0;
+		pci_dev_assign_busmaster(dev, false);
 	} else {
 		pci_read_config_word(dev, PCI_COMMAND, &cmd);
 		if (cmd & (PCI_COMMAND_INVALIDATE)) {
 			cmd &= ~(PCI_COMMAND_INVALIDATE);
 			pci_write_config_word(dev, PCI_COMMAND, cmd);
 
-			dev->is_busmaster = 0;
+			pci_dev_assign_busmaster(dev, false);
 		}
 	}
 }
diff --git a/include/linux/pci.h b/include/linux/pci.h
index ebb5b9d76360..9964646bdd46 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -336,6 +336,25 @@ struct pci_sriov;
 struct pci_p2pdma;
 struct rcec_ea;
 
+/**
+ * enum struct_pci_dev_flags - Flags in struct pci_dev
+ *
+ * Each flag has a set of accessor functions created via
+ * __create_pci_dev_flag_accessors() and must only be accessed through
+ * them.
+ *
+ * @PCI_DEV_FLAG_BUSMASTER: Bus mastering is enabled on the device. Pure
+ *		bookkeeping state, maintained by pci_set_master(),
+ *		pci_clear_master() and pci_disable_device(); modifying it
+ *		does not itself change the hardware state.
+ * @PCI_DEV_FLAG_COUNT: Number of defined struct_pci_dev_flags.
+ */
+enum struct_pci_dev_flags {
+	PCI_DEV_FLAG_BUSMASTER = 0,
+
+	PCI_DEV_FLAG_COUNT
+};
+
 /* struct pci_dev - describes a PCI device
  *
  * @supported_speeds:	PCIe Supported Link Speeds Vector (+ reserved 0 at
@@ -461,7 +480,6 @@ struct pci_dev {
 	unsigned int	pref_64_window:1;	/* Pref mem window is 64-bit */
 	unsigned int	multifunction:1;	/* Multi-function device */
 
-	unsigned int	is_busmaster:1;		/* Is busmaster */
 	unsigned int	no_msi:1;		/* May not use MSI */
 	unsigned int	block_cfg_access:1;	/* Config space access blocked */
 	unsigned int	broken_parity_status:1;	/* Generates false positive parity */
@@ -592,8 +610,25 @@ struct pci_dev {
 	u8		tph_mode;	/* TPH mode */
 	u8		tph_req_type;	/* TPH requester type */
 #endif
+
+	/* PCI_DEV_FLAG_XXX flags. Use atomic bitfield operations to modify. */
+	DECLARE_BITMAP(flags, PCI_DEV_FLAG_COUNT);
 };
 
+#define __create_pci_dev_flag_accessors(accessor_name, flag_name) \
+static inline bool pci_dev_##accessor_name(const struct pci_dev *pdev) \
+{ \
+	return test_bit(flag_name, pdev->flags); \
+} \
+static inline void pci_dev_assign_##accessor_name(struct pci_dev *pdev, bool value) \
+{ \
+	assign_bit(flag_name, pdev->flags, value); \
+}
+
+__create_pci_dev_flag_accessors(busmaster, PCI_DEV_FLAG_BUSMASTER);
+
+#undef __create_pci_dev_flag_accessors
+
 static inline struct pci_dev *pci_physfn(struct pci_dev *dev)
 {
 #ifdef CONFIG_PCI_IOV

-- 
2.51.2