PCI: aerdrv: introduce default_downstream_reset_link

I noticed that when I inject a fatal error to an endpoint via
aer-inject, aer_root_reset() is called as reset_link for a
downstream port at upstream of the endpoint:

  pcieport 0000:00:06.0: AER: Uncorrected (Fatal) error received: id=5401
   :
  pcieport 0000:52:02.0: Root Port link has been reset

It externally appears to be working, but internally issues some
accesses to PCI_ERR_ROOT_COMMAND/STATUS registers that is for
root port so not available on downstream port.

This patch introduces default_downstream_reset_link that is
a version of aer_root_reset() with no accesses to root port's
register. It is used for downstream ports that has no reset_link
function its specific.

This patch also updates related description in pcieaer-howto.txt.
Some minor fixes are included.

Signed-off-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Reviewed-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>

drivers/pci/pcie/aer/aerdrv.c

@@ -341,7 +341,6 @@ static int __devinit aer_probe(struct pcie_device *dev)
  **/
 static pci_ers_result_t aer_root_reset(struct pci_dev *dev)
 {
-	u16 p2p_ctrl;
 	u32 reg32;
 	int pos;
 
@@ -352,27 +351,7 @@ static pci_ers_result_t aer_root_reset(struct pci_dev *dev)
 	reg32 &= ~ROOT_PORT_INTR_ON_MESG_MASK;
 	pci_write_config_dword(dev, pos + PCI_ERR_ROOT_COMMAND, reg32);
 
-	/* Assert Secondary Bus Reset */
-	pci_read_config_word(dev, PCI_BRIDGE_CONTROL, &p2p_ctrl);
-	p2p_ctrl |= PCI_BRIDGE_CTL_BUS_RESET;
-	pci_write_config_word(dev, PCI_BRIDGE_CONTROL, p2p_ctrl);
-
-	/*
-	 * we should send hot reset message for 2ms to allow it time to
-	 * propogate to all downstream ports
-	 */
-	msleep(2);
-
-	/* De-assert Secondary Bus Reset */
-	p2p_ctrl &= ~PCI_BRIDGE_CTL_BUS_RESET;
-	pci_write_config_word(dev, PCI_BRIDGE_CONTROL, p2p_ctrl);
-
-	/*
-	 * System software must wait for at least 100ms from the end
-	 * of a reset of one or more device before it is permitted
-	 * to issue Configuration Requests to those devices.
-	 */
-	msleep(200);
+	aer_do_secondary_bus_reset(dev);
 	dev_printk(KERN_DEBUG, &dev->dev, "Root Port link has been reset\n");
 
 	/* Clear Root Error Status */