[PATCH v2] PCI: Move pci_dev->is_busmaster into priv_flags

[Maurice Hieronymus <mhi@xxxxxxxxxxx>]

`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@xxxxxxxxxx/ 
[1]
Suggested-by: Danilo Krummrich <dakr@xxxxxxxxxx>
Suggested-by: Lukas Wunner <lukas@xxxxxxxxx>
Cc: rust-for-linux@xxxxxxxxxxxxxxx
Signed-off-by: Maurice Hieronymus <mhi@xxxxxxxxxxx>
---
`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@xxxxxxxxxxx
---
 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@xxxxxxxxxxx>