PCI: tegra: Set target speed as Gen1 before starting LTSSM [Linux 5.3]

This Linux kernel change "PCI: tegra: Set target speed as Gen1 before starting LTSSM" is included in the Linux 5.3 release. This change is authored by Manikanta Maddireddy <mmaddireddy [at] nvidia.com> on Tue Jun 18 23:31:55 2019 +0530. The commit for this change in Linux stable tree is c23ae2a (patch).

PCI: tegra: Set target speed as Gen1 before starting LTSSM

PCIe link up fails with few legacy endpoints if root port advertises both
Gen-1 and Gen-2 speeds in Tegra. This is because link number negotiation
fails if both Gen1 & Gen2 are advertised. Tegra doesn't retry link up by
advertising only Gen1. Hence, the strategy followed here is to initially
advertise only Gen-1 and after link is up, retrain link to Gen-2 speed.

Tegra doesn't support HW autonomous speed change. Link comes up in Gen1
even if Gen2 is advertised, so there is no downside of this change.

This behavior is observed with following two PCIe devices on Tegra:

- Fusion HDTV 5 Express card
- IOGear SIL - PCIE - SATA card

Signed-off-by: Manikanta Maddireddy <mmaddireddy@nvidia.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Thierry Reding <treding@nvidia.com>

There are 11 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 drivers/pci/controller/pci-tegra.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/drivers/pci/controller/pci-tegra.c b/drivers/pci/controller/pci-tegra.c
index cf2398a..ffe3ad5 100644
--- a/drivers/pci/controller/pci-tegra.c
+++ b/drivers/pci/controller/pci-tegra.c
@@ -676,6 +676,17 @@ static void tegra_pcie_apply_sw_fixup(struct tegra_pcie_port *port)
        value |= soc->update_fc_threshold;
        writel(value, port->base + RP_VEND_XP);
    }
+
+   /*
+    * PCIe link doesn't come up with few legacy PCIe endpoints if
+    * root port advertises both Gen-1 and Gen-2 speeds in Tegra.
+    * Hence, the strategy followed here is to initially advertise
+    * only Gen-1 and after link is up, retrain link to Gen-2 speed
+    */
+   value = readl(port->base + RP_LINK_CONTROL_STATUS_2);
+   value &= ~PCI_EXP_LNKSTA_CLS;
+   value |= PCI_EXP_LNKSTA_CLS_2_5GB;
+   writel(value, port->base + RP_LINK_CONTROL_STATUS_2);
 }

 static void tegra_pcie_port_enable(struct tegra_pcie_port *port)

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