PCI: imx6: Delay enabling reference clock for SS until it stabilizes

According to the IMX6 reference manuals, REF_SSP_EN (Reference clock enable
for SS function) must remain deasserted until the reference clock is
running at the appropriate frequency.

Delay enabling the reference clock for the SS function until it has
stabilized.  This prevents a high link failure rate (>5%) on certain IMX6
boards at various temperatures.

[bhelgaas: reword changelog slightly]
Tested-by: NFabio Estevam <festevam@gmail.com>
Signed-off-by: NTim Harvey <tharvey@gateworks.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>
Acked-by: NMarek Vasut <marex@denx.de>
Acked-by: NLucas Stach <l.stach@pengutronix.de>

drivers/pci/host/pci-imx6.c

@@ -257,11 +257,6 @@ static int imx6_pcie_deassert_core_reset(struct pcie_port *pp)
 	struct imx6_pcie *imx6_pcie = to_imx6_pcie(pp);
 	int ret;
 
-	regmap_update_bits(imx6_pcie->iomuxc_gpr, IOMUXC_GPR1,
-			IMX6Q_GPR1_PCIE_TEST_PD, 0 << 18);
-	regmap_update_bits(imx6_pcie->iomuxc_gpr, IOMUXC_GPR1,
-			IMX6Q_GPR1_PCIE_REF_CLK_EN, 1 << 16);
-
 	ret = clk_prepare_enable(imx6_pcie->pcie_phy);
 	if (ret) {
 		dev_err(pp->dev, "unable to enable pcie_phy clock\n");
@@ -283,6 +278,12 @@ static int imx6_pcie_deassert_core_reset(struct pcie_port *pp)
 	/* allow the clocks to stabilize */
 	usleep_range(200, 500);
 
+	/* power up core phy and enable ref clock */
+	regmap_update_bits(imx6_pcie->iomuxc_gpr, IOMUXC_GPR1,
+			IMX6Q_GPR1_PCIE_TEST_PD, 0 << 18);
+	regmap_update_bits(imx6_pcie->iomuxc_gpr, IOMUXC_GPR1,
+			IMX6Q_GPR1_PCIE_REF_CLK_EN, 1 << 16);
+
 	/* Some boards don't have PCIe reset GPIO. */
 	if (gpio_is_valid(imx6_pcie->reset_gpio)) {
 		gpio_set_value(imx6_pcie->reset_gpio, 0);