PCI: Add pci_clear_master() as opposite of pci_set_master()

During an online device reset it may be useful to disable bus-mastering.
pci_disable_device() does that, and far more besides, so is not suitable
for an online reset.

Add pci_clear_master() which does just this.
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Reviewed-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>

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

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);

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);