PCI: aardvark: Wait for endpoint to be ready before training link

When configuring pcie reset pin from gpio (e.g. initially set by
u-boot) to pcie function this pin goes low for a brief moment
asserting the PERST# signal. Thus connected device enters fundamental
reset process and link configuration can only begin after a minimal
100ms delay (see [1]).

Because the pin configuration comes from the "default" pinctrl it is
implicitly configured before the probe callback is called:

driver_probe_device()
  really_probe()
    ...
    pinctrl_bind_pins() /* Here pin goes from gpio to PCIE reset
                           function and PERST# is asserted */
    ...
    drv->probe()

[1] "PCI Express Base Specification", REV. 4.0
    PCI Express, February 19 2014, 6.6.1 Conventional Reset

Signed-off-by: Remi Pommarel <repk@triplefau.lt>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

drivers/pci/controller/pci-aardvark.c

@@ -337,6 +337,14 @@ static void advk_pcie_setup_hw(struct advk_pcie *pcie)
 	reg |= PIO_CTRL_ADDR_WIN_DISABLE;
 	advk_writel(pcie, reg, PIO_CTRL);
 
+	/*
+	 * PERST# signal could have been asserted by pinctrl subsystem before
+	 * probe() callback has been called, making the endpoint going into
+	 * fundamental reset. As required by PCI Express spec a delay for at
+	 * least 100ms after such a reset before link training is needed.
+	 */
+	msleep(PCI_PM_D3COLD_WAIT);
+
 	/* Start link training */
 	reg = advk_readl(pcie, PCIE_CORE_LINK_CTRL_STAT_REG);
 	reg |= PCIE_CORE_LINK_TRAINING;