3 files changed, 31 insertions, 12 deletions

diff --git a/Documentation/PCI/pci.txt b/Documentation/PCI/pci.txt
index fd4907a2968c..7f6de6ea5b47 100644
--- a/Documentation/PCI/pci.txt
+++ b/Documentation/PCI/pci.txt
@@ -294,7 +294,8 @@ NOTE: pci_enable_device() can fail! Check the return value.
 
 pci_set_master() will enable DMA by setting the bus master bit
 in the PCI_COMMAND register. It also fixes the latency timer value if
-it's set to something bogus by the BIOS.
+it's set to something bogus by the BIOS.  pci_clear_master() will
+disable DMA by clearing the bus master bit.
 
 If the PCI device can use the PCI Memory-Write-Invalidate transaction,
 call pci_set_mwi().  This enables the PCI_COMMAND bit for Mem-Wr-Inval
diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index c824dc8d617c..f3fd55df67db 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -1667,6 +1667,22 @@ int pci_request_regions_exclusive(struct pci_dev *pdev, const char *res_name)
                                         ((1 << 6) - 1), res_name);
 }
 
+static void __pci_set_master(struct pci_dev *dev, bool enable)
+{
+        u16 old_cmd, cmd;
+
+        pci_read_config_word(dev, PCI_COMMAND, &old_cmd);
+        if (enable)
+                cmd = old_cmd | PCI_COMMAND_MASTER;
+        else
+                cmd = old_cmd & ~PCI_COMMAND_MASTER;
+        if (cmd != old_cmd) {
+                dev_dbg(&dev->dev, "%s bus mastering\n",
+                        enable ? "enabling" : "disabling");
+                pci_write_config_word(dev, PCI_COMMAND, cmd);
+        }
+        dev->is_busmaster = enable;
+}
 
 /**
  * pci_set_master - enables bus-mastering for device dev
@@ -1675,21 +1691,21 @@ int pci_request_regions_exclusive(struct pci_dev *pdev, const char *res_name)
  * Enables bus-mastering on the device and calls pcibios_set_master()
  * to do the needed arch specific settings.
  */
-void
-pci_set_master(struct pci_dev *dev)
+void pci_set_master(struct pci_dev *dev)
 {
-        u16 cmd;
-
-        pci_read_config_word(dev, PCI_COMMAND, &cmd);
-        if (! (cmd & PCI_COMMAND_MASTER)) {
-                dev_dbg(&dev->dev, "enabling bus mastering\n");
-                cmd |= PCI_COMMAND_MASTER;
-                pci_write_config_word(dev, PCI_COMMAND, cmd);
-        }
-        dev->is_busmaster = 1;
+        __pci_set_master(dev, true);
         pcibios_set_master(dev);
 }
 
+/**
+ * pci_clear_master - disables bus-mastering for device dev
+ * @dev: the PCI device to disable
+ */
+void pci_clear_master(struct pci_dev *dev)
+{
+        __pci_set_master(dev, false);
+}
+
 #ifdef PCI_DISABLE_MWI
 int pci_set_mwi(struct pci_dev *dev)
 {
@@ -2346,6 +2362,7 @@ EXPORT_SYMBOL(pci_release_selected_regions);
 EXPORT_SYMBOL(pci_request_selected_regions);
 EXPORT_SYMBOL(pci_request_selected_regions_exclusive);
 EXPORT_SYMBOL(pci_set_master);
+EXPORT_SYMBOL(pci_clear_master);
 EXPORT_SYMBOL(pci_set_mwi);
 EXPORT_SYMBOL(pci_try_set_mwi);
 EXPORT_SYMBOL(pci_clear_mwi);
diff --git a/include/linux/pci.h b/include/linux/pci.h
index 7cbecef19bb6..0f6d2bb1df9c 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -642,6 +642,7 @@ static inline int pci_is_managed(struct pci_dev *pdev)
 
 void pci_disable_device(struct pci_dev *dev);
 void pci_set_master(struct pci_dev *dev);
+void pci_clear_master(struct pci_dev *dev);
 int pci_set_pcie_reset_state(struct pci_dev *dev, enum pcie_reset_state state);
 #define HAVE_PCI_SET_MWI
 int __must_check pci_set_mwi(struct pci_dev *dev);