PCI: aardvark: Don't touch PCIe registers if no card connected

When there is no PCIe card connected and advk_pcie_rd_conf() or
advk_pcie_wr_conf() is called for PCI bus which doesn't belong to emulated
root bridge, the aardvark driver throws the following error message:

  advk-pcie d0070000.pcie: config read/write timed out

Obviously accessing PCIe registers of disconnected card is not possible.

Extend check in advk_pcie_valid_device() function for validating
availability of PCIe bus. If PCIe link is down, then the device is marked
as Not Found and the driver does not try to access these registers.

This is just an optimization to prevent accessing PCIe registers when card
is disconnected. Trying to access PCIe registers of disconnected card does
not cause any crash, kernel just needs to wait for a timeout. So if card
disappear immediately after checking for PCIe link (before accessing PCIe
registers), it does not cause any problems.

Link: https://lore.kernel.org/r/20200702083036.12230-1-pali@kernel.org
Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>

drivers/pci/controller/pci-aardvark.c

@@ -644,6 +644,13 @@ static bool advk_pcie_valid_device(struct advk_pcie *pcie, struct pci_bus *bus,
 	if ((bus->number == pcie->root_bus_nr) && PCI_SLOT(devfn) != 0)
 		return false;
 
+	/*
+	 * If the link goes down after we check for link-up, nothing bad
+	 * happens but the config access times out.
+	 */
+	if (bus->number != pcie->root_bus_nr && !advk_pcie_link_up(pcie))
+		return false;
+
 	return true;
 }