PCI: dra7xx: Add support to force RC to work in GEN1 mode

PCIe in AM57x/DRA7x devices is by default configured to work in GEN2 mode.
However there may be situations when working in GEN1 mode is desired.  One
example is limitation i925 (PCIe GEN2 mode not supported at junction
temperatures < 0C).

Add support to force Root Complex to work in GEN1 mode if so desired, but
don't force GEN1 mode on any board just yet.
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

drivers/pci/dwc/pci-dra7xx.c

@@ -17,6 +17,7 @@
 #include <linux/kernel.h>
 #include <linux/init.h>
 #include <linux/of_gpio.h>
+#include <linux/of_pci.h>
 #include <linux/pci.h>
 #include <linux/phy/phy.h>
 #include <linux/platform_device.h>
@@ -63,11 +64,14 @@
 #define	LINK_UP						BIT(16)
 #define	DRA7XX_CPU_TO_BUS_ADDR				0x0FFFFFFF
 
+#define EXP_CAP_ID_OFFSET				0x70
+
 struct dra7xx_pcie {
 	struct pcie_port	pp;
 	void __iomem		*base;		/* DT ti_conf */
 	int			phy_count;	/* DT phy-names count */
 	struct phy		**phy;
+	int			link_gen;
 };
 
 #define to_dra7xx_pcie(x)	container_of((x), struct dra7xx_pcie, pp)
@@ -96,12 +100,33 @@ static int dra7xx_pcie_establish_link(struct dra7xx_pcie *dra7xx)
 	struct pcie_port *pp = &dra7xx->pp;
 	struct device *dev = pp->dev;
 	u32 reg;
+	u32 exp_cap_off = EXP_CAP_ID_OFFSET;
 
 	if (dw_pcie_link_up(pp)) {
 		dev_err(dev, "link is already up\n");
 		return 0;
 	}
 
+	if (dra7xx->link_gen == 1) {
+		dw_pcie_cfg_read(pp->dbi_base + exp_cap_off + PCI_EXP_LNKCAP,
+				 4, &reg);
+		if ((reg & PCI_EXP_LNKCAP_SLS) != PCI_EXP_LNKCAP_SLS_2_5GB) {
+			reg &= ~((u32)PCI_EXP_LNKCAP_SLS);
+			reg |= PCI_EXP_LNKCAP_SLS_2_5GB;
+			dw_pcie_cfg_write(pp->dbi_base + exp_cap_off +
+					  PCI_EXP_LNKCAP, 4, reg);
+		}
+
+		dw_pcie_cfg_read(pp->dbi_base + exp_cap_off + PCI_EXP_LNKCTL2,
+				 2, &reg);
+		if ((reg & PCI_EXP_LNKCAP_SLS) != PCI_EXP_LNKCAP_SLS_2_5GB) {
+			reg &= ~((u32)PCI_EXP_LNKCAP_SLS);
+			reg |= PCI_EXP_LNKCAP_SLS_2_5GB;
+			dw_pcie_cfg_write(pp->dbi_base + exp_cap_off +
+					  PCI_EXP_LNKCTL2, 2, reg);
+		}
+	}
+
 	reg = dra7xx_pcie_readl(dra7xx, PCIECTRL_DRA7XX_CONF_DEVICE_CMD);
 	reg |= LTSSM_EN;
 	dra7xx_pcie_writel(dra7xx, PCIECTRL_DRA7XX_CONF_DEVICE_CMD, reg);
@@ -397,6 +422,10 @@ static int __init dra7xx_pcie_probe(struct platform_device *pdev)
 	reg &= ~LTSSM_EN;
 	dra7xx_pcie_writel(dra7xx, PCIECTRL_DRA7XX_CONF_DEVICE_CMD, reg);
 
+	dra7xx->link_gen = of_pci_get_max_link_speed(np);
+	if (dra7xx->link_gen < 0 || dra7xx->link_gen > 2)
+		dra7xx->link_gen = 2;
+
 	ret = dra7xx_add_pcie_port(dra7xx, pdev);
 	if (ret < 0)
 		goto err_gpio;