[[PATCHv3]] PCI: fu740: Force gen1 for initial device probe

[Ben Dooks]

The fu740 PCIe core does not probe any devices on the SiFive Unmatched
board without this fix from U-Boot (or having U-Boot explicitly start
the PCIe via either boot-script or user command).

The fix claims to set the link-speed to gen1 to get the probe
to work. As this is a copy from U-Boot, the code is assumed to be
correct and does fix the issue on the Unmatched. The code is at:
https://source.denx.de/u-boot/u-boot/-/blob/master/drivers/pci/pcie_dw_sifive.c#L271

The code has been this way since the driver was commited in:
https://source.denx.de/u-boot/u-boot/-/commit/416395c772018c6bf52aad36aca163115001793f

Signed-off-by: Ben Dooks <ben.dooks at codethink.co.uk>
---
 drivers/pci/controller/dwc/pcie-fu740.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/drivers/pci/controller/dwc/pcie-fu740.c b/drivers/pci/controller/dwc/pcie-fu740.c
index 842b7202b96e..19501ec8c487 100644
--- a/drivers/pci/controller/dwc/pcie-fu740.c
+++ b/drivers/pci/controller/dwc/pcie-fu740.c
@@ -177,11 +177,30 @@ static void fu740_pcie_init_phy(struct fu740_pcie *afp)
 	fu740_phyregwrite(1, PCIEX8MGMT_PHY_LANE3_BASE, PCIEX8MGMT_PHY_INIT_VAL, afp);
 }
 
+/* This is copied from u-boot. Force system to gen1 otherwise nothing probes
+ * as found on the SiFive Unmatched board.
+ */
+static void fu740_pcie_force_gen1(struct dw_pcie *dw, struct fu740_pcie *afp )
+{
+	unsigned val;
+
+	dw_pcie_dbi_ro_wr_en(dw);
+
+	val = dw_pcie_readl_dbi(dw, 0x70 + PCI_EXP_LNKCAP);
+	pr_info("%s: link-cap was %08x\n", __func__, val);
+	dw_pcie_writel_dbi(dw, 0x70 + PCI_EXP_LNKCAP, val | 0xf);
+
+	dw_pcie_dbi_ro_wr_dis(dw);
+}
+
 static int fu740_pcie_start_link(struct dw_pcie *pci)
 {
 	struct device *dev = pci->dev;
 	struct fu740_pcie *afp = dev_get_drvdata(dev);
 
+	/* Force PCIe gen1 otherwise Unmatched board does not probe */
+	fu740_pcie_force_gen1(pci, afp);
+
 	/* Enable LTSSM */
 	writel_relaxed(0x1, afp->mgmt_base + PCIEX8MGMT_APP_LTSSM_ENABLE);
 	return 0;
-- 
2.34.1