linux-kernel - [PATCH] PCI: mvebu: Set Target Link Speed for 2.5GT downstream devices

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Date:   Mon, 07 Nov 2022 08:13:27 +0000
From:   Nathan Rossi <nathan@...hanrossi.com>
To:     linux-pci@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Cc:     Nathan Rossi <nathan@...hanrossi.com>,
        Nathan Rossi <nathan.rossi@...i.com>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Pali Rohár <pali@...nel.org>
Subject: [PATCH] PCI: mvebu: Set Target Link Speed for 2.5GT downstream devices

From: Nathan Rossi <nathan.rossi@...i.com>

There is a known issue with the mvebu PCIe controller when triggering
retraining of the link (via Link Control) where the link is dropped
completely causing significant delay in the renegotiation of the link.
This occurs only when the downstream device is 2.5GT and the upstream
port is configured to support both 2.5GT and 5GT.

It is possible to prevent this link dropping by setting the associated
link speed in Target Link Speed of the Link Control 2 register. This
only needs to be done when the downstream is specifically 2.5GT.

This change applies the required Target Link Speed value during
mvebu_pcie_setup_hw conditionally depending on the current link speed
from the Link Status register, only applying the change when the link
is configured to 2.5GT already.

Signed-off-by: Nathan Rossi <nathan.rossi@...i.com>
---
 drivers/pci/controller/pci-mvebu.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/drivers/pci/controller/pci-mvebu.c b/drivers/pci/controller/pci-mvebu.c
index 1ced73726a..6a869a33ba 100644
--- a/drivers/pci/controller/pci-mvebu.c
+++ b/drivers/pci/controller/pci-mvebu.c
@@ -248,7 +248,7 @@ static void mvebu_pcie_setup_wins(struct mvebu_pcie_port *port)
 
 static void mvebu_pcie_setup_hw(struct mvebu_pcie_port *port)
 {
-	u32 ctrl, lnkcap, cmd, dev_rev, unmask, sspl;
+	u32 ctrl, lnkcap, cmd, dev_rev, unmask, sspl, lnksta, lnkctl2;
 
 	/* Setup PCIe controller to Root Complex mode. */
 	ctrl = mvebu_readl(port, PCIE_CTRL_OFF);
@@ -339,6 +339,22 @@ static void mvebu_pcie_setup_hw(struct mvebu_pcie_port *port)
 	unmask |= PCIE_INT_INTX(0) | PCIE_INT_INTX(1) |
 		  PCIE_INT_INTX(2) | PCIE_INT_INTX(3);
 	mvebu_writel(port, unmask, PCIE_INT_UNMASK_OFF);
+
+	/*
+	 * Set Target Link Speed within the Link Control 2 register when the
+	 * linked downstream device is connected at 2.5GT. This is configured
+	 * in order to avoid issues with the controller when the upstream port
+	 * is configured to support 2.5GT and 5GT and the downstream device is
+	 * linked at 2.5GT, retraining the link in this case causes the link to
+	 * drop taking significant time to retrain.
+	 */
+	lnksta = mvebu_readl(port, PCIE_CAP_PCIEXP + PCI_EXP_LNKCTL) >> 16;
+	if ((lnksta & PCI_EXP_LNKSTA_CLS) == PCI_EXP_LNKSTA_CLS_2_5GB) {
+		lnkctl2 = mvebu_readl(port, PCIE_CAP_PCIEXP + PCI_EXP_LNKCTL2);
+		lnkctl2 &= ~PCI_EXP_LNKCTL2_TLS;
+		lnkctl2 |= PCI_EXP_LNKCTL2_TLS_2_5GT;
+		mvebu_writel(port, lnkctl2, PCIE_CAP_PCIEXP + PCI_EXP_LNKCTL2);
+	}
 }
 
 static struct mvebu_pcie_port *mvebu_pcie_find_port(struct mvebu_pcie *pcie,
---
2.37.2