linux-kernel - [PATCH v4 1/2] PCI: Configure root port MPS during host probing

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Message-Id: <20250510155607.390687-2-18255117159@163.com>
Date: Sat, 10 May 2025 23:56:06 +0800
From: Hans Zhang <18255117159@....com>
To: lpieralisi@...nel.org,
	kw@...ux.com,
	bhelgaas@...gle.com,
	heiko@...ech.de,
	manivannan.sadhasivam@...aro.org,
	yue.wang@...ogic.com
Cc: pali@...nel.org,
	neil.armstrong@...aro.org,
	robh@...nel.org,
	jingoohan1@...il.com,
	khilman@...libre.com,
	jbrunet@...libre.com,
	martin.blumenstingl@...glemail.com,
	linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-amlogic@...ts.infradead.org,
	linux-rockchip@...ts.infradead.org,
	Hans Zhang <18255117159@....com>,
	Niklas Cassel <cassel@...nel.org>
Subject: [PATCH v4 1/2] PCI: Configure root port MPS during host probing

Current PCIe initialization logic may leave root ports operating with
non-optimal Maximum Payload Size (MPS) settings. While downstream device
configuration is handled during bus enumeration, root port MPS values
inherited from firmware or hardware defaults might not utilize the full
capabilities supported by the controller hardware. This can result is
uboptimal data transfer efficiency across the PCIe hierarchy.

During host controller probing phase, when PCIe bus tuning is enabled,
the implementation now configures root port MPS settings to their
hardware-supported maximum values. By iterating through bridge devices
under the root bus and identifying PCIe root ports, each port's MPS is
set to 128 << pcie_mpss to match the device's maximum supported payload
size. The Max Read Request Size (MRRS) is subsequently adjusted through
existing companion logic to maintain compatibility with PCIe
specifications.

Explicit initialization at host probing stage ensures consistent PCIe
topology configuration before downstream devices perform their own MPS
negotiations. This proactive approach addresses platform-specific
requirements where controller drivers depend on properly initialized
root port settings, while maintaining backward compatibility through
PCIE_BUS_TUNE_OFF conditional checks. Hardware capabilities are fully
utilized without altering existing device negotiation behaviors.

Suggested-by: Niklas Cassel <cassel@...nel.org>
Signed-off-by: Hans Zhang <18255117159@....com>
---
 drivers/pci/probe.c | 72 ++++++++++++++++++++++++++-------------------
 1 file changed, 41 insertions(+), 31 deletions(-)

diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
index 364fa2a514f8..1f67c03d170a 100644
--- a/drivers/pci/probe.c
+++ b/drivers/pci/probe.c
@@ -2149,6 +2149,37 @@ int pci_setup_device(struct pci_dev *dev)
 	return 0;
 }
 
+static void pcie_write_mps(struct pci_dev *dev, int mps)
+{
+	int rc;
+
+	if (pcie_bus_config == PCIE_BUS_PERFORMANCE) {
+		mps = 128 << dev->pcie_mpss;
+
+		if (pci_pcie_type(dev) != PCI_EXP_TYPE_ROOT_PORT &&
+		    dev->bus->self)
+
+			/*
+			 * For "Performance", the assumption is made that
+			 * downstream communication will never be larger than
+			 * the MRRS.  So, the MPS only needs to be configured
+			 * for the upstream communication.  This being the case,
+			 * walk from the top down and set the MPS of the child
+			 * to that of the parent bus.
+			 *
+			 * Configure the device MPS with the smaller of the
+			 * device MPSS or the bridge MPS (which is assumed to be
+			 * properly configured at this point to the largest
+			 * allowable MPS based on its parent bus).
+			 */
+			mps = min(mps, pcie_get_mps(dev->bus->self));
+	}
+
+	rc = pcie_set_mps(dev, mps);
+	if (rc)
+		pci_err(dev, "Failed attempting to set the MPS\n");
+}
+
 static void pci_configure_mps(struct pci_dev *dev)
 {
 	struct pci_dev *bridge = pci_upstream_bridge(dev);
@@ -2178,6 +2209,16 @@ static void pci_configure_mps(struct pci_dev *dev)
 		return;
 	}
 
+	/*
+	 * Unless MPS strategy is PCIE_BUS_TUNE_OFF (don't touch MPS at all),
+	 * start off by setting root ports' MPS to MPSS. Depending on the MPS
+	 * strategy, and the MPSS of the devices below the root port, the MPS
+	 * of the root port might get overridden later.
+	 */
+	if (pci_pcie_type(dev) == PCI_EXP_TYPE_ROOT_PORT &&
+	    pcie_bus_config != PCIE_BUS_TUNE_OFF)
+		pcie_write_mps(dev, 128 << dev->pcie_mpss);
+
 	if (!bridge || !pci_is_pcie(bridge))
 		return;
 
@@ -2875,37 +2916,6 @@ static int pcie_find_smpss(struct pci_dev *dev, void *data)
 	return 0;
 }
 
-static void pcie_write_mps(struct pci_dev *dev, int mps)
-{
-	int rc;
-
-	if (pcie_bus_config == PCIE_BUS_PERFORMANCE) {
-		mps = 128 << dev->pcie_mpss;
-
-		if (pci_pcie_type(dev) != PCI_EXP_TYPE_ROOT_PORT &&
-		    dev->bus->self)
-
-			/*
-			 * For "Performance", the assumption is made that
-			 * downstream communication will never be larger than
-			 * the MRRS.  So, the MPS only needs to be configured
-			 * for the upstream communication.  This being the case,
-			 * walk from the top down and set the MPS of the child
-			 * to that of the parent bus.
-			 *
-			 * Configure the device MPS with the smaller of the
-			 * device MPSS or the bridge MPS (which is assumed to be
-			 * properly configured at this point to the largest
-			 * allowable MPS based on its parent bus).
-			 */
-			mps = min(mps, pcie_get_mps(dev->bus->self));
-	}
-
-	rc = pcie_set_mps(dev, mps);
-	if (rc)
-		pci_err(dev, "Failed attempting to set the MPS\n");
-}
-
 static void pcie_write_mrrs(struct pci_dev *dev)
 {
 	int rc, mrrs;
-- 
2.25.1