`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 the existing `priv_flags` bitmap, which is
modified with atomic bitops. The bit definition stays private to
drivers/pci; xen-pciback, lpfc and sfc access the flag through the new
exported accessors `pci_dev_is_busmaster()` and
`pci_dev_assign_busmaster()`.

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>
Suggested-by: Lukas Wunner <lukas@wunner.de>
Cc: rust-for-linux@vger.kernel.org
Signed-off-by: Maurice Hieronymus <mhi@mailbox.org>
---
`struct pci_dev` keeps ~60 flags in one C bitfield. Bits sharing a
word must not be modified concurrently, but several writers take no
common lock: `pci_set_master()` writes `is_busmaster` and can run
without the device lock (e.g. runtime PM resume paths),
`pci_disable_device()` clears it, and `broken_parity_status_store()`
writes the same word from sysfs at any time without any lock.

Move `is_busmaster` into the existing `priv_flags` bitmap, which is
modified with atomic bitops. The bit definition stays private to
drivers/pci; the drivers that use the flag (xen-pciback, lpfc, sfc)
access it through new exported accessor functions.

This is a prerequisite for the Rust device enabling API rework [1]:
the guard object planned there calls `pci_disable_device()` from
contexts that may run concurrently with `pci_set_master()`.
---
Changes in v2:
- Move the bit into the existing priv_flags bitmap instead of adding a
  new public flags bitmap (Lukas). The bit definition stays private to
  drivers/pci; outside users go through new exported accessors.
- Drop the broken_parity_status conversion (Lukas).
- Link to v1: https://lore.kernel.org/r/20260711-pci-dev-flags-v1-0-2fcf2811138c@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                           | 33 ++++++++++++++++++++++++++---
 drivers/pci/pci.h                           |  1 +
 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                         |  3 ++-
 9 files changed, 42 insertions(+), 13 deletions(-)

diff --git a/drivers/net/ethernet/sfc/falcon/farch.c b/drivers/net/ethernet/sfc/falcon/farch.c
index 23d507a3820d..c73db6081190 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_is_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..815487b1b227 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_is_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..f3b2255258df 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_is_busmaster(pci_dev))
 		pci_set_master(pci_dev);
 
 	return retval;
diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index 77b17b13ee61..80c962811737 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_is_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);
 
@@ -4106,6 +4106,33 @@ void pci_unmap_iospace(struct resource *res)
 }
 EXPORT_SYMBOL(pci_unmap_iospace);
 
+/**
+ * pci_dev_is_busmaster - Query the bus mastering bookkeeping flag
+ * @pdev: the PCI device to query
+ *
+ * Returns: the current value of the bookkeeping flag; the PCI_COMMAND
+ * register is not consulted.
+ */
+bool pci_dev_is_busmaster(const struct pci_dev *pdev)
+{
+	return test_bit(PCI_DEV_BUSMASTER, &pdev->priv_flags);
+}
+EXPORT_SYMBOL(pci_dev_is_busmaster);
+
+/**
+ * pci_dev_assign_busmaster - Set the bus mastering bookkeeping flag
+ * @pdev: the PCI device
+ * @busmaster: new flag value
+ *
+ * Only updates the bookkeeping flag; the PCI_COMMAND register is left
+ * untouched.
+ */
+void pci_dev_assign_busmaster(struct pci_dev *pdev, bool busmaster)
+{
+	assign_bit(PCI_DEV_BUSMASTER, &pdev->priv_flags, busmaster);
+}
+EXPORT_SYMBOL(pci_dev_assign_busmaster);
+
 static void __pci_set_master(struct pci_dev *dev, bool enable)
 {
 	u16 old_cmd, cmd;
@@ -4120,7 +4147,7 @@ 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/pci/pci.h b/drivers/pci/pci.h
index 4469e1a77f3c..faa4a676e62c 100644
--- a/drivers/pci/pci.h
+++ b/drivers/pci/pci.h
@@ -773,6 +773,7 @@ static inline int pci_dev_set_disconnected(struct pci_dev *dev, void *unused)
 #define PCI_LINK_CHANGING 5
 #define PCI_LINK_LBMS_SEEN	6
 #define PCI_DEV_ALLOW_BINDING 7
+#define PCI_DEV_BUSMASTER 8
 
 static inline void pci_dev_assign_added(struct pci_dev *dev)
 {
diff --git a/drivers/scsi/lpfc/lpfc_init.c b/drivers/scsi/lpfc/lpfc_init.c
index 82af59c913e9..657453e6d7d2 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_is_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_is_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..ecc9adba369c 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_is_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_is_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 64b308b6e61c..0542221ad996 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -461,7 +461,6 @@ 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 */
@@ -1446,6 +1445,8 @@ void pci_disable_device(struct pci_dev *dev);
 extern unsigned int pcibios_max_latency;
 void pci_set_master(struct pci_dev *dev);
 void pci_clear_master(struct pci_dev *dev);
+bool pci_dev_is_busmaster(const struct pci_dev *pdev);
+void pci_dev_assign_busmaster(struct pci_dev *pdev, bool busmaster);
 
 int pci_set_pcie_reset_state(struct pci_dev *dev, enum pcie_reset_state state);
 int pci_set_cacheline_size(struct pci_dev *dev);

---
base-commit: a13c140cc289c0b7b3770bce5b3ad42ab35074aa
change-id: 20260711-pci-dev-flags-fbbcf4ff9031

Best regards,
-- 
Maurice Hieronymus <mhi@mailbox.org>

On Tue, Jul 14, 2026 at 09:37:07PM +0200, Maurice Hieronymus wrote:
> +++ b/include/linux/pci.h
> @@ -1446,6 +1445,8 @@ void pci_disable_device(struct pci_dev *dev);
>  extern unsigned int pcibios_max_latency;
>  void pci_set_master(struct pci_dev *dev);
>  void pci_clear_master(struct pci_dev *dev);
> +bool pci_dev_is_busmaster(const struct pci_dev *pdev);
> +void pci_dev_assign_busmaster(struct pci_dev *pdev, bool busmaster);

pci_dev_assign_busmaster() should not have public visibility.
Drivers should really use pci_set_master() / pci_clear_master()
and nothing else.

It seems Xen is the only one in the tree which needs this:

> +++ 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);
>  		}
>  	}
>  }

Please change these direct assignments to pci_clear_master(),
preferably in a separate patch to ease bisecting if anything
breaks.

Then you don't need pci_dev_assign_busmaster() and can just inline
setting/clearing the bit in __pci_set_master() and pci_disable_device().

Thanks,

Lukas

On Wed Jul 15, 2026 at 6:54 AM CEST, Lukas Wunner wrote:

> pci_dev_assign_busmaster() should not have public visibility.
> Drivers should really use pci_set_master() / pci_clear_master()
> and nothing else.
>
> It seems Xen is the only one in the tree which needs this:
>
>> +++ 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);
>>  		}
>>  	}
>>  }
>
> Please change these direct assignments to pci_clear_master(),
> preferably in a separate patch to ease bisecting if anything
> breaks.
This conversion does not preserve behavior. The direct assignments
clear only the software flag, while pci_clear_master() also clears
PCI_COMMAND_MASTER in config space.

I am not an expert on xen devices. That's why I want to clarify first
that this does not break anything. Especially since commit
7681f31ec9cd ("xen/pciback: Don't disable PCI_COMMAND on PCI device
reset.") deliberately removed the PCI_COMMAND write right above the
first assignment, and pci_clear_master() would reintroduce a
PCI_COMMAND write there.

Best,

Maurice