Per PCIe Base Specification r6.0, sec 8.4.4 ("Lane Margining at
Receiver"), PCIe devices operating at 16.0 GT/s (Gen 4) or higher data
rates support the Lane Margining at Receiver Extended Capability
(ID 0x27), and it is mandatory for receivers operating at 64.0 GT/s
(Gen 6) or higher data rates. Lane Margining allows software to
evaluate high-speed link margins by measuring timing and voltage steps
for each individual physical lane and receiver.
Add driver and debugfs support for PCIe Lane Margining at Receiver:
- Add Lane Margining at Receiver Extended Capability register
definitions (PCI_EXT_CAP_ID_LMR, PCI_LMR_PORT_CAP, PCI_LMR_PORT_STS,
PCI_LMR_LANE_CTRL, PCI_LMR_LANE_STS) to <uapi/linux/pci_regs.h>.
- Add Kconfig option CONFIG_PCIE_LMR (under drivers/pci/pcie/Kconfig)
dependent on DEBUG_FS.
- Implement drivers/pci/pcie/margin.c to probe the capability on Gen4+
links and expose per-device debugfs entries under:
/sys/kernel/debug/pci/pcie_lmr_<pci_dev_name>/
providing control over margining enablement, receiver selection, and
execution of timing/voltage margin step commands. Distinguish
between missing mandatory LMR capability on Gen6+ vs optional on
Gen4/Gen5.
- Hook pci_lmr_init() into pci_init_capabilities() during device probe
in drivers/pci/probe.c and pci_lmr_exit() into drivers/pci/remove.c.
- Add kselftest script under tools/testing/selftests/pcie_lmt/pcie_lmt.sh
to test debugfs capability reads, enablement, and stepping.
Signed-off-by: Priyank Rathod <[email protected]>
---
Per PCIe Base Specification r6.0, section 8.4.4 ("Lane Margining at Receiver"),
PCIe devices operating at 16.0 GT/s (Gen 4) or higher data rates support the
Lane Margining at Receiver Extended Capability (ID 0x27), and it is mandatory
for receivers operating at 64.0 GT/s (Gen 6) or higher data rates.
Lane Margining allows system software to evaluate high-speed link signal
integrity and margins by measuring timing and voltage steps for each physical
lane and receiver independently.
This series introduces kernel driver support, debugfs controls, and a
kselftest automation script for PCIe Lane Margining at Receiver (LMR/LMT).
==============================================================================
1. How to Enable & Configure
==============================================================================
Enable the Kconfig option under PCI support:
CONFIG_PCIE_LMR=y (or =m)
(Depends on CONFIG_PCI and CONFIG_DEBUG_FS)
Upon boot or device hotplug on Gen4+ links (>= 16.0 GT/s), the driver probes
Extended Capability ID 0x27 and exposes per-device debugfs interfaces:
/sys/kernel/debug/pci/pcie_lmr_<domain>:<bus>:<dev>.<func>/
==============================================================================
2. How to Use the Debugfs Interface (Manual Margining)
==============================================================================
Inspect device-wide margining capabilities and port status:
# Inspect root device LMR capabilities & status
cat /sys/kernel/debug/pci/pcie_lmr_0000:01:00.0/capabilities
cat /sys/kernel/debug/pci/pcie_lmr_0000:01:00.0/port_status
Enable active Lane Margining on the device:
# Enable Lane Margining state machine
echo 1 > /sys/kernel/debug/pci/pcie_lmr_0000:01:00.0/enable
Inspect and step individual lanes (e.g. lane0):
# Select target receiver (0 = local receiver, 1..6 = retimers/link partners)
echo 0 > /sys/kernel/debug/pci/pcie_lmr_0000:01:00.0/lane0/receiver
# Check available timing and voltage steps for this receiver
cat /sys/kernel/debug/pci/pcie_lmr_0000:01:00.0/lane0/caps
cat /sys/kernel/debug/pci/pcie_lmr_0000:01:00.0/lane0/num_timing_steps
cat /sys/kernel/debug/pci/pcie_lmr_0000:01:00.0/lane0/num_voltage_steps
# Step timing margin or voltage margin offset
echo 2 > /sys/kernel/debug/pci/pcie_lmr_0000:01:00.0/lane0/margin_timing
echo 1 > /sys/kernel/debug/pci/pcie_lmr_0000:01:00.0/lane0/margin_voltage
# Reset margin offset back to nominal (0)
echo 0 > /sys/kernel/debug/pci/pcie_lmr_0000:01:00.0/lane0/margin_timing
echo 0 > /sys/kernel/debug/pci/pcie_lmr_0000:01:00.0/lane0/margin_voltage
Disable Lane Margining when finished:
echo 0 > /sys/kernel/debug/pci/pcie_lmr_0000:01:00.0/enable
==============================================================================
3. How to Run Automated Kselftests Using the Test Script
==============================================================================
An automated kselftest script is included to test capability reads, receiver
selection, and margining commands across all enumerated LMR devices:
# Run directly as root
sudo ./tools/testing/selftests/pcie_lmt/pcie_lmt.sh
Or run via the kselftest Makefile harness:
make -C tools/testing/selftests TARGETS=pcie_lmt run_tests
Sample script output on an LMR-capable device:
pcie_lmt: testing PCIe LMR debugfs entries
pcie_lmt: probing device pcie_lmr_0000:01:00.0
pcie_lmr_0000:01:00.0: capabilities read OK
pcie_lmr_0000:01:00.0: port_status read OK
pcie_lmr_0000:01:00.0: margining enabled OK
pcie_lmr_0000:01:00.0: testing lane0
pcie_lmr_0000:01:00.0: testing lane1
pcie_lmr_0000:01:00.0: margining disabled OK
pcie_lmt [PASS]
---
drivers/pci/pci.h | 8 +
drivers/pci/pcie/Kconfig | 12 +
drivers/pci/pcie/Makefile | 1 +
drivers/pci/pcie/margin.c | 673 +++++++++++++++++++++++++++
drivers/pci/probe.c | 1 +
drivers/pci/remove.c | 1 +
include/linux/pci.h | 6 +
include/uapi/linux/pci_regs.h | 19 +
tools/testing/selftests/Makefile | 1 +
tools/testing/selftests/pcie_lmt/Makefile | 3 +
tools/testing/selftests/pcie_lmt/pcie_lmt.sh | 105 +++++
11 files changed, 830 insertions(+)
diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
index 4469e1a77f3c..c3449c6a5b79 100644
--- a/drivers/pci/pci.h
+++ b/drivers/pci/pci.h
@@ -1023,6 +1023,14 @@ static inline void pci_no_tph(void) { }
static inline void pci_tph_init(struct pci_dev *dev) { }
#endif
+#ifdef CONFIG_PCIE_LMR
+void pci_lmr_init(struct pci_dev *dev);
+void pci_lmr_exit(struct pci_dev *dev);
+#else
+static inline void pci_lmr_init(struct pci_dev *dev) { }
+static inline void pci_lmr_exit(struct pci_dev *dev) { }
+#endif
+
#ifdef CONFIG_PCIE_PTM
void pci_ptm_init(struct pci_dev *dev);
void pci_save_ptm_state(struct pci_dev *dev);
diff --git a/drivers/pci/pcie/Kconfig b/drivers/pci/pcie/Kconfig
index 207c2deae35f..3b021ca2fe84 100644
--- a/drivers/pci/pcie/Kconfig
+++ b/drivers/pci/pcie/Kconfig
@@ -137,6 +137,18 @@ config PCIE_PTM
This is only useful if you have devices that support PTM, but it
is safe to enable even if you don't.
+config PCIE_LMR
+ bool "PCI Express Lane Margining at Receiver Support"
+ depends on DEBUG_FS
+ help
+ This enables the PCI Express Lane Margining at Receiver support.
+ Lane Margining allows software to determine the voltage and
+ timing margin of each lane on a PCIe link (16.0 GT/s and above).
+ The margining data is exposed via debugfs.
+
+ This is only useful if you have devices that support lane
+ margining, but it is safe to enable even if you don't.
+
config PCIE_EDR
bool "PCI Express Error Disconnect Recover support"
depends on PCIE_DPC && ACPI
diff --git a/drivers/pci/pcie/Makefile b/drivers/pci/pcie/Makefile
index b0b43a18c304..aac45ae0402e 100644
--- a/drivers/pci/pcie/Makefile
+++ b/drivers/pci/pcie/Makefile
@@ -13,4 +13,5 @@ obj-$(CONFIG_PCIEAER_INJECT) += aer_inject.o
obj-$(CONFIG_PCIE_PME) += pme.o
obj-$(CONFIG_PCIE_DPC) += dpc.o
obj-$(CONFIG_PCIE_PTM) += ptm.o
+obj-$(CONFIG_PCIE_LMR) += margin.o
obj-$(CONFIG_PCIE_EDR) += edr.o
diff --git a/drivers/pci/pcie/margin.c b/drivers/pci/pcie/margin.c
new file mode 100644
index 000000000000..a1ccd473a407
--- /dev/null
+++ b/drivers/pci/pcie/margin.c
@@ -0,0 +1,673 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * PCI Express Lane Margining at Receiver
+ *
+ * Copyright (C) 2026 Google LLC
+ * Author: Priyank Rathod <[email protected]>
+ *
+ * Lane Margining at Receiver (PCIe r6.0, sec 8.4.4) allows the software
+ * to determine the voltage and timing margins of each lane on the PCIe
+ * link. The extended capability (ID 0x27) is available for receivers
+ * operating at 16.0 GT/s (Gen4) or higher data rates, and is mandatory
+ * for receivers operating at 64.0 GT/s (Gen6) or higher data rates.
+ *
+ * This implementation exposes per-device debugfs entries to read the margining
+ * capabilities and issues margining commands through custom file_ops.
+ */
+
+#include <linux/pci.h>
+#include <linux/slab.h>
+#include <linux/mutex.h>
+#include <linux/delay.h>
+#include <linux/jiffies.h>
+#include <linux/seq_file.h>
+#include <linux/bitfield.h>
+#include <linux/debugfs.h>
+
+#include "../pci.h"
+
+/* Margin type encodings per the PCIe spec */
+#define LMR_TYPE_DEMARGIN 0x0
+#define LMR_TYPE_REPORT_CAPS 0x1
+#define LMR_TYPE_REPORT_VOLTAGE_STEPS 0x2
+#define LMR_TYPE_REPORT_TIMING_STEPS 0x3
+#define LMR_TYPE_TIMING 0x4
+#define LMR_TYPE_VOLTAGE 0x5
+#define LMR_TYPE_NO_CMD 0x7
+
+/* LMR command timing parameters */
+#define LMR_CMD_TIMEOUT_MS 100
+#define LMR_CMD_SLEEP_MIN_US 100
+#define LMR_CMD_SLEEP_MAX_US 200
+#define LMR_ENABLE_TIMEOUT_MS 100
+#define LMR_ENABLE_SLEEP_MIN_US 1000
+#define LMR_ENABLE_SLEEP_MAX_US 2000
+
+/* LMR limits */
+#define LMR_MAX_LANES 32
+#define LMR_MAX_RX_NUM 6
+#define LMR_MAX_TIMING_STEP 63
+#define LMR_MAX_VOLTAGE_STEP 127
+
+/* LMR PCIe generation numbers */
+#define LMR_GEN6 6
+#define LMR_GEN5 5
+#define LMR_GEN4 4
+
+/* LMR lane register stride */
+#define LMR_LANE_REG_STRIDE 4
+
+/* LMR receivers and directions */
+#define LMR_RX_LOCAL 0
+#define LMR_STEP_DIR_INCREASE 1
+#define LMR_STEP_DIR_DECREASE 0
+
+/* LMR step & direction encoding masks */
+#define LMR_STEPS_MASK 0x7F
+#define LMR_TIMING_STEP_MASK 0x3F
+#define LMR_TIMING_DIR_SHIFT 6
+#define LMR_VOLTAGE_STEP_MASK 0x7F
+#define LMR_VOLTAGE_DIR_SHIFT 7
+
+/* LMR capabilities report bit fields */
+#define LMR_CAP_MARGIN_HV BIT(0)
+#define LMR_CAP_MARGIN_EV_IV BIT(1)
+#define LMR_CAP_IND_LEFT_RIGHT_TIMING BIT(2)
+#define LMR_CAP_IND_UP_DOWN_VOLTAGE BIT(3)
+#define LMR_CAP_ERROR_SAMPLER BIT(4)
+#define LMR_CAP_SAMPLE_MULTIPLE_RX BIT(5)
+
+struct pci_margin_lane {
+ struct pci_margin_dev *mdev;
+ int lane;
+ u8 rx;
+ int timing_val;
+ int voltage_val;
+};
+
+struct pci_margin_dev {
+ struct pci_dev *dev;
+ u16 cap;
+ struct dentry *debugfs;
+ struct mutex lock;
+ int num_lanes;
+ struct pci_margin_lane *lanes;
+ bool enabled;
+};
+
+static int pci_lmr_run_cmd(struct pci_dev *dev, int lane, u8 rx, u8 type,
+ u8 usage, u8 payload, u16 *status_val)
+{
+ u16 lmr = dev->lmr_cap;
+ u16 ctrl_offset = lmr + PCI_LMR_LANE_CTRL + LMR_LANE_REG_STRIDE * lane;
+ u16 sts_offset = lmr + PCI_LMR_LANE_STS + LMR_LANE_REG_STRIDE * lane;
+ u16 ctrl, sts;
+ unsigned long timeout;
+
+ ctrl = FIELD_PREP(PCI_LMR_LANE_CTRL_RX_NUM, rx) |
+ FIELD_PREP(PCI_LMR_LANE_CTRL_MTYPE, type) |
+ FIELD_PREP(PCI_LMR_LANE_CTRL_USAGE, usage) |
+ FIELD_PREP(PCI_LMR_LANE_CTRL_PAYLOAD, payload);
+
+ pci_write_config_word(dev, ctrl_offset, ctrl);
+
+ timeout = jiffies + msecs_to_jiffies(LMR_CMD_TIMEOUT_MS);
+ while (1) {
+ pci_read_config_word(dev, sts_offset, &sts);
+ if (sts == 0xFFFF)
+ return -ENODEV;
+
+ if (FIELD_GET(PCI_LMR_LANE_STS_MTYPE, sts) == type) {
+ if (status_val)
+ *status_val = sts;
+ return 0;
+ }
+
+ if (time_after(jiffies, timeout))
+ break;
+
+ usleep_range(LMR_CMD_SLEEP_MIN_US, LMR_CMD_SLEEP_MAX_US);
+ }
+
+ return -ETIMEDOUT;
+}
+
+static int pci_lmr_run_cmd_locked(struct pci_margin_dev *mdev, int lane, u8 rx,
+ u8 type, u8 usage, u8 payload, u16
*status_val)
+{
+ int ret;
+
+ if (!mdev->enabled)
+ return -EACCES;
+
+ mutex_lock(&mdev->lock);
+ ret = pci_lmr_run_cmd(mdev->dev, lane, rx, type, usage, payload,
status_val);
+ mutex_unlock(&mdev->lock);
+
+ return ret;
+}
+
+#if IS_ENABLED(CONFIG_DEBUG_FS)
+
+static int margin_caps_show(struct seq_file *s, void *v)
+{
+ struct pci_margin_dev *mdev = s->private;
+ struct pci_dev *dev = mdev->dev;
+ u16 cap;
+
+ pci_read_config_word(dev, mdev->cap + PCI_LMR_PORT_CAP, &cap);
+ seq_printf(s, "Port Capabilities: %#06x\n", cap);
+ seq_printf(s, " Margining Ready: %s\n",
+ (cap & PCI_LMR_PORT_CAP_MARGIN_READY) ? "yes" : "no");
+ seq_printf(s, " SW Ready Required: %s\n",
+ (cap & PCI_LMR_PORT_CAP_SW_READY) ? "yes" : "no");
+ return 0;
+}
+DEFINE_SHOW_ATTRIBUTE(margin_caps);
+
+static int margin_port_status_show(struct seq_file *s, void *v)
+{
+ struct pci_margin_dev *mdev = s->private;
+ struct pci_dev *dev = mdev->dev;
+ u16 sts;
+
+ pci_read_config_word(dev, mdev->cap + PCI_LMR_PORT_STS, &sts);
+ seq_printf(s, "Port Status: %#06x\n", sts);
+ seq_printf(s, " Margining Ready: %s\n",
+ (sts & PCI_LMR_PORT_STS_MARGIN_READY) ? "yes" : "no");
+ seq_printf(s, " SW Ready: %s\n", (sts & PCI_LMR_PORT_STS_SW_READY) ?
"yes" : "no");
+ return 0;
+}
+DEFINE_SHOW_ATTRIBUTE(margin_port_status);
+
+static int margin_enable_show(struct seq_file *s, void *v)
+{
+ struct pci_margin_dev *mdev = s->private;
+
+ seq_printf(s, "%d\n", mdev->enabled);
+ return 0;
+}
+
+static ssize_t margin_enable_write(struct file *file, const char __user
*user_buf,
+ size_t count, loff_t *ppos)
+{
+ struct pci_margin_dev *mdev = ((struct seq_file
*)file->private_data)->private;
+ struct pci_dev *dev = mdev->dev;
+ unsigned long timeout;
+ bool enable;
+ int ret, i;
+ u16 sts, cap;
+
+ ret = kstrtobool_from_user(user_buf, count, &enable);
+ if (ret)
+ return ret;
+
+ mutex_lock(&mdev->lock);
+
+ if (mdev->enabled == enable)
+ goto out;
+
+ if (enable) {
+ pci_read_config_word(dev, mdev->cap + PCI_LMR_PORT_CAP, &cap);
+
+ pci_read_config_word(dev, mdev->cap + PCI_LMR_PORT_STS, &sts);
+ sts |= PCI_LMR_PORT_STS_SW_READY;
+ pci_write_config_word(dev, mdev->cap + PCI_LMR_PORT_STS, sts);
+
+ timeout = jiffies + msecs_to_jiffies(LMR_ENABLE_TIMEOUT_MS);
+ while (1) {
+ pci_read_config_word(dev, mdev->cap + PCI_LMR_PORT_STS,
&sts);
+ if (sts == 0xFFFF) {
+ ret = -ENODEV;
+ goto out;
+ }
+ if (sts & PCI_LMR_PORT_STS_MARGIN_READY)
+ break;
+ if (time_after(jiffies, timeout)) {
+ ret = -ETIMEDOUT;
+ sts &= ~PCI_LMR_PORT_STS_SW_READY;
+ pci_write_config_word(dev, mdev->cap +
PCI_LMR_PORT_STS, sts);
+ goto out;
+ }
+ usleep_range(LMR_ENABLE_SLEEP_MIN_US,
LMR_ENABLE_SLEEP_MAX_US);
+ }
+ mdev->enabled = true;
+ } else {
+ pci_read_config_word(dev, mdev->cap + PCI_LMR_PORT_STS, &sts);
+ sts &= ~PCI_LMR_PORT_STS_SW_READY;
+ pci_write_config_word(dev, mdev->cap + PCI_LMR_PORT_STS, sts);
+
+ for (i = 0; i < mdev->num_lanes; i++) {
+ mdev->lanes[i].timing_val = 0;
+ mdev->lanes[i].voltage_val = 0;
+ }
+ mdev->enabled = false;
+ }
+
+out:
+ mutex_unlock(&mdev->lock);
+ return ret ? ret : count;
+}
+
+static int margin_enable_open(struct inode *inode, struct file *file)
+{
+ return single_open(file, margin_enable_show, inode->i_private);
+}
+
+static const struct file_operations margin_enable_fops = {
+ .open = margin_enable_open,
+ .read = seq_read,
+ .write = margin_enable_write,
+ .llseek = seq_lseek,
+ .release = single_release,
+};
+
+static int margin_lane_receiver_show(struct seq_file *s, void *v)
+{
+ struct pci_margin_lane *plane = s->private;
+
+ seq_printf(s, "%d\n", plane->rx);
+ return 0;
+}
+
+static ssize_t margin_lane_receiver_write(struct file *file, const char __user
*user_buf,
+ size_t count, loff_t *ppos)
+{
+ struct pci_margin_lane *plane = ((struct seq_file
*)file->private_data)->private;
+ u8 rx;
+ int ret;
+
+ ret = kstrtou8_from_user(user_buf, count, 0, &rx);
+ if (ret)
+ return ret;
+
+ if (rx > LMR_MAX_RX_NUM)
+ return -EINVAL;
+
+ plane->rx = rx;
+ return count;
+}
+
+static int margin_lane_receiver_open(struct inode *inode, struct file *file)
+{
+ return single_open(file, margin_lane_receiver_show, inode->i_private);
+}
+
+static const struct file_operations margin_lane_receiver_fops = {
+ .open = margin_lane_receiver_open,
+ .read = seq_read,
+ .write = margin_lane_receiver_write,
+ .llseek = seq_lseek,
+ .release = single_release,
+};
+
+static int margin_lane_caps_show(struct seq_file *s, void *v)
+{
+ struct pci_margin_lane *plane = s->private;
+ struct pci_margin_dev *mdev = plane->mdev;
+ u16 sts;
+ int ret;
+ u8 val;
+
+ ret = pci_lmr_run_cmd_locked(mdev, plane->lane, plane->rx,
+ LMR_TYPE_REPORT_CAPS, 0, 0, &sts);
+ if (ret)
+ return ret;
+
+ val = FIELD_GET(PCI_LMR_LANE_STS_PAYLOAD, sts);
+ seq_printf(s, "Lane %d Rx %d Capabilities: %#02x\n", plane->lane,
plane->rx, val);
+ seq_printf(s, " Margining H/V: %s\n", (val & LMR_CAP_MARGIN_HV) ?
"both" : "either");
+ seq_printf(s, " Margining eV/iV: %s\n",
+ (val & LMR_CAP_MARGIN_EV_IV) ? "both (close & open)" : "one
(close only)");
+ seq_printf(s, " Left/Right: %s\n", (val &
LMR_CAP_IND_LEFT_RIGHT_TIMING) ? "both" : "one");
+ seq_printf(s, " Up/Down: %s\n", (val & LMR_CAP_IND_UP_DOWN_VOLTAGE) ?
"both" : "one");
+ seq_printf(s, " Error Sampler: %s\n",
+ (val & LMR_CAP_ERROR_SAMPLER) ? "yes" : "no (main sampler)");
+ seq_printf(s, " Sample Multiple Receivers: %s\n",
+ (val & LMR_CAP_SAMPLE_MULTIPLE_RX) ? "yes" : "no");
+
+ return 0;
+}
+DEFINE_SHOW_ATTRIBUTE(margin_lane_caps);
+
+static int margin_lane_timing_steps_show(struct seq_file *s, void *v)
+{
+ struct pci_margin_lane *plane = s->private;
+ struct pci_margin_dev *mdev = plane->mdev;
+ u16 sts;
+ int ret;
+ u8 val;
+
+ ret = pci_lmr_run_cmd_locked(mdev, plane->lane, plane->rx,
+ LMR_TYPE_REPORT_TIMING_STEPS, 0, 0, &sts);
+ if (ret)
+ return ret;
+
+ val = FIELD_GET(PCI_LMR_LANE_STS_PAYLOAD, sts) & LMR_STEPS_MASK;
+ seq_printf(s, "%d\n", val);
+ return 0;
+}
+DEFINE_SHOW_ATTRIBUTE(margin_lane_timing_steps);
+
+static int margin_lane_voltage_steps_show(struct seq_file *s, void *v)
+{
+ struct pci_margin_lane *plane = s->private;
+ struct pci_margin_dev *mdev = plane->mdev;
+ u16 sts;
+ int ret;
+ u8 val;
+
+ ret = pci_lmr_run_cmd_locked(mdev, plane->lane, plane->rx,
+ LMR_TYPE_REPORT_VOLTAGE_STEPS, 0, 0, &sts);
+ if (ret)
+ return ret;
+
+ val = FIELD_GET(PCI_LMR_LANE_STS_PAYLOAD, sts) & LMR_STEPS_MASK;
+
+ seq_printf(s, "%d\n", val);
+ return 0;
+}
+DEFINE_SHOW_ATTRIBUTE(margin_lane_voltage_steps);
+
+static ssize_t margin_lane_timing_write(struct file *file, const char __user
*user_buf,
+ size_t count, loff_t *ppos)
+{
+ struct pci_margin_lane *plane = ((struct seq_file
*)file->private_data)->private;
+ struct pci_margin_dev *mdev = plane->mdev;
+ int val, ret;
+ u8 step, dir;
+ u16 sts;
+ u8 caps;
+
+ ret = kstrtoint_from_user(user_buf, count, 0, &val);
+ if (ret)
+ return ret;
+
+ if (!mdev->enabled)
+ return -EACCES;
+
+ mutex_lock(&mdev->lock);
+
+ if (val == 0) {
+ ret = pci_lmr_run_cmd(mdev->dev, plane->lane, plane->rx,
+ LMR_TYPE_DEMARGIN, 0, 0, &sts);
+ if (ret)
+ goto out;
+ plane->timing_val = 0;
+ plane->voltage_val = 0;
+ goto out;
+ }
+
+ ret = pci_lmr_run_cmd(mdev->dev, plane->lane, plane->rx,
LMR_TYPE_REPORT_CAPS, 0, 0, &sts);
+ if (ret)
+ goto out;
+ caps = FIELD_GET(PCI_LMR_LANE_STS_PAYLOAD, sts);
+
+ if (val < 0) {
+ step = -val;
+ dir = LMR_STEP_DIR_DECREASE;
+ } else {
+ step = val;
+ dir = LMR_STEP_DIR_INCREASE;
+ if (!(caps & LMR_CAP_IND_LEFT_RIGHT_TIMING)) {
+ ret = -EINVAL;
+ goto out;
+ }
+ }
+
+ if (step > LMR_MAX_TIMING_STEP) {
+ ret = -EINVAL;
+ goto out;
+ }
+
+ ret = pci_lmr_run_cmd(mdev->dev, plane->lane, plane->rx,
+ LMR_TYPE_TIMING, 0,
+ (step & LMR_TIMING_STEP_MASK) | (dir <<
LMR_TIMING_DIR_SHIFT), &sts);
+ if (ret)
+ goto out;
+
+ plane->timing_val = val;
+
+out:
+ mutex_unlock(&mdev->lock);
+ return ret ? ret : count;
+}
+
+static int margin_lane_timing_show(struct seq_file *s, void *v)
+{
+ struct pci_margin_lane *plane = s->private;
+
+ seq_printf(s, "%d\n", plane->timing_val);
+ return 0;
+}
+
+static int margin_lane_timing_open(struct inode *inode, struct file *file)
+{
+ return single_open(file, margin_lane_timing_show, inode->i_private);
+}
+
+static const struct file_operations margin_lane_timing_fops = {
+ .open = margin_lane_timing_open,
+ .read = seq_read,
+ .write = margin_lane_timing_write,
+ .llseek = seq_lseek,
+ .release = single_release,
+};
+
+static ssize_t margin_lane_voltage_write(struct file *file, const char __user
*user_buf,
+ size_t count, loff_t *ppos)
+{
+ struct pci_margin_lane *plane = ((struct seq_file
*)file->private_data)->private;
+ struct pci_margin_dev *mdev = plane->mdev;
+ int val, ret;
+ u8 step, dir;
+ u16 sts;
+ u8 caps;
+
+ ret = kstrtoint_from_user(user_buf, count, 0, &val);
+ if (ret)
+ return ret;
+
+ if (!mdev->enabled)
+ return -EACCES;
+
+ mutex_lock(&mdev->lock);
+
+ if (val == 0) {
+ ret = pci_lmr_run_cmd(mdev->dev, plane->lane, plane->rx,
+ LMR_TYPE_DEMARGIN, 0, 0, &sts);
+ if (ret)
+ goto out;
+ plane->timing_val = 0;
+ plane->voltage_val = 0;
+ goto out;
+ }
+
+ ret = pci_lmr_run_cmd(mdev->dev, plane->lane, plane->rx,
LMR_TYPE_REPORT_CAPS, 0, 0, &sts);
+ if (ret)
+ goto out;
+ caps = FIELD_GET(PCI_LMR_LANE_STS_PAYLOAD, sts);
+
+ if (val < 0) {
+ step = -val;
+ dir = 0;
+ } else {
+ step = val;
+ dir = 1;
+ if (!(caps & LMR_CAP_IND_UP_DOWN_VOLTAGE)) {
+ ret = -EINVAL;
+ goto out;
+ }
+ }
+
+ if (step > LMR_MAX_VOLTAGE_STEP) {
+ ret = -EINVAL;
+ goto out;
+ }
+
+ ret = pci_lmr_run_cmd(mdev->dev, plane->lane, plane->rx,
+ LMR_TYPE_VOLTAGE, 0,
+ (step & LMR_VOLTAGE_STEP_MASK) |
+ (dir << LMR_VOLTAGE_DIR_SHIFT), &sts);
+ if (ret)
+ goto out;
+
+ plane->voltage_val = val;
+
+out:
+ mutex_unlock(&mdev->lock);
+ return ret ? ret : count;
+}
+
+static int margin_lane_voltage_show(struct seq_file *s, void *v)
+{
+ struct pci_margin_lane *plane = s->private;
+
+ seq_printf(s, "%d\n", plane->voltage_val);
+ return 0;
+}
+
+static int margin_lane_voltage_open(struct inode *inode, struct file *file)
+{
+ return single_open(file, margin_lane_voltage_show, inode->i_private);
+}
+
+static const struct file_operations margin_lane_voltage_fops = {
+ .open = margin_lane_voltage_open,
+ .read = seq_read,
+ .write = margin_lane_voltage_write,
+ .llseek = seq_lseek,
+ .release = single_release,
+};
+
+static void pci_margin_debugfs_init(struct pci_margin_dev *mdev)
+{
+ struct pci_dev *dev = mdev->dev;
+ char dirname[64];
+ int i;
+
+ snprintf(dirname, sizeof(dirname), "pcie_lmr_%s", dev_name(&dev->dev));
+ mdev->debugfs = debugfs_create_dir(dirname, NULL);
+ if (!mdev->debugfs)
+ return;
+
+ debugfs_create_file("capabilities", 0444, mdev->debugfs, mdev,
&margin_caps_fops);
+ debugfs_create_file("port_status", 0444, mdev->debugfs, mdev,
&margin_port_status_fops);
+ debugfs_create_file("enable", 0644, mdev->debugfs, mdev,
&margin_enable_fops);
+
+ for (i = 0; i < mdev->num_lanes; i++) {
+ struct pci_margin_lane *plane = &mdev->lanes[i];
+ struct dentry *lane_dir;
+ char lane_name[16];
+
+ snprintf(lane_name, sizeof(lane_name), "lane%d", i);
+ lane_dir = debugfs_create_dir(lane_name, mdev->debugfs);
+ if (!lane_dir)
+ continue;
+
+ debugfs_create_file("receiver", 0644, lane_dir, plane,
&margin_lane_receiver_fops);
+ debugfs_create_file("caps", 0444, lane_dir, plane,
&margin_lane_caps_fops);
+ debugfs_create_file("num_timing_steps", 0444, lane_dir, plane,
+ &margin_lane_timing_steps_fops);
+ debugfs_create_file("num_voltage_steps", 0444, lane_dir, plane,
+ &margin_lane_voltage_steps_fops);
+ debugfs_create_file("margin_timing", 0644, lane_dir, plane,
+ &margin_lane_timing_fops);
+ debugfs_create_file("margin_voltage", 0644, lane_dir, plane,
+ &margin_lane_voltage_fops);
+ }
+}
+
+static void pci_margin_debugfs_remove(struct pci_margin_dev *mdev)
+{
+ debugfs_remove_recursive(mdev->debugfs);
+}
+
+#else
+static inline void pci_margin_debugfs_init(struct pci_margin_dev *mdev) { }
+static inline void pci_margin_debugfs_remove(struct pci_margin_dev *mdev) { }
+#endif
+
+void pci_lmr_init(struct pci_dev *dev)
+{
+ struct pci_margin_dev *mdev;
+ enum pci_bus_speed speed;
+ u16 lmr;
+ u16 lnkcap;
+ int i;
+
+ if (!pci_is_pcie(dev))
+ return;
+
+ speed = pcie_get_speed_cap(dev);
+
+ lmr = pci_find_ext_capability(dev, PCI_EXT_CAP_ID_LMR);
+ if (!lmr) {
+ if (speed >= PCIE_SPEED_64_0GT)
+ pci_warn(dev,
+ "Missing Lane Margining at Receiver Capability
(mandatory for Gen6+)\n");
+ else if (speed >= PCIE_SPEED_16_0GT)
+ pci_dbg(dev,
+ "Optional Lane Margining at Receiver Capability
not found\n");
+ return;
+ }
+
+ if (speed < PCIE_SPEED_16_0GT && speed != PCI_SPEED_UNKNOWN)
+ return;
+
+ dev->lmr_cap = lmr;
+
+ mdev = kzalloc_obj(*mdev, GFP_KERNEL);
+ if (!mdev)
+ return;
+
+ mdev->dev = dev;
+ mdev->cap = lmr;
+ mutex_init(&mdev->lock);
+
+ pcie_capability_read_word(dev, PCI_EXP_LNKCAP, &lnkcap);
+ mdev->num_lanes = FIELD_GET(PCI_EXP_LNKCAP_MLW, lnkcap);
+
+ if (mdev->num_lanes == 0 || mdev->num_lanes > LMR_MAX_LANES) {
+ pci_warn(dev, "Invalid link width %d for LMR\n",
mdev->num_lanes);
+ goto err_free_mdev;
+ }
+
+ mdev->lanes = kcalloc(mdev->num_lanes, sizeof(*mdev->lanes),
GFP_KERNEL);
+ if (!mdev->lanes)
+ goto err_free_mdev;
+
+ for (i = 0; i < mdev->num_lanes; i++) {
+ mdev->lanes[i].mdev = mdev;
+ mdev->lanes[i].lane = i;
+ mdev->lanes[i].rx = LMR_RX_LOCAL;
+ }
+
+ pci_margin_debugfs_init(mdev);
+
+ dev->lmr = mdev;
+
+ pci_info(dev, "Lane Margining at Receiver (Gen%u) Capability
detected\n",
+ speed >= PCIE_SPEED_64_0GT ? LMR_GEN6 :
+ speed >= PCIE_SPEED_32_0GT ? LMR_GEN5 :
+ LMR_GEN4);
+ return;
+
+err_free_mdev:
+ mutex_destroy(&mdev->lock);
+ kfree(mdev);
+}
+
+void pci_lmr_exit(struct pci_dev *dev)
+{
+ struct pci_margin_dev *mdev = dev->lmr;
+
+ if (!mdev)
+ return;
+
+ pci_margin_debugfs_remove(mdev);
+ mutex_destroy(&mdev->lock);
+ kfree(mdev->lanes);
+ kfree(mdev);
+ dev->lmr = NULL;
+}
diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
index dd0abbc63e18..352b95568ebf 100644
--- a/drivers/pci/probe.c
+++ b/drivers/pci/probe.c
@@ -2666,6 +2666,7 @@ static void pci_init_capabilities(struct pci_dev *dev)
pci_pasid_init(dev); /* Process Address Space ID */
pci_acs_init(dev); /* Access Control Services */
pci_ptm_init(dev); /* Precision Time Measurement */
+ pci_lmr_init(dev); /* Lane Margining at Receiver */
pci_aer_init(dev); /* Advanced Error Reporting */
pci_dpc_init(dev); /* Downstream Port Containment */
pci_rcec_init(dev); /* Root Complex Event Collector */
diff --git a/drivers/pci/remove.c b/drivers/pci/remove.c
index d8bffa21498a..6fba29040e44 100644
--- a/drivers/pci/remove.c
+++ b/drivers/pci/remove.c
@@ -36,6 +36,7 @@ static void pci_destroy_dev(struct pci_dev *dev)
pci_doe_sysfs_teardown(dev);
pci_npem_remove(dev);
+ pci_lmr_exit(dev);
/*
* While device is in D0 drop the device from TSM link operations
diff --git a/include/linux/pci.h b/include/linux/pci.h
index 64b308b6e61c..ef1275f4c5b6 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -349,6 +349,8 @@ struct rcec_ea;
* number resources to allow for hierarchy expansion.
* @is_pciehp: PCIe Hot-Plug Capable bridge.
*/
+struct pci_margin_dev;
+
struct pci_dev {
struct list_head bus_list; /* Node in per-bus list */
struct pci_bus *bus; /* Bus this device is on */
@@ -528,6 +530,10 @@ struct pci_dev {
atomic_t ptm_enable_cnt;
u8 ptm_granularity;
#endif
+#ifdef CONFIG_PCIE_LMR
+ u16 lmr_cap; /* Lane Margining Capability */
+ struct pci_margin_dev *lmr;
+#endif
#ifdef CONFIG_PCI_MSI
void __iomem *msix_base;
raw_spinlock_t msi_lock;
diff --git a/include/uapi/linux/pci_regs.h b/include/uapi/linux/pci_regs.h
index facaa324bd86..a13c09f8d6df 100644
--- a/include/uapi/linux/pci_regs.h
+++ b/include/uapi/linux/pci_regs.h
@@ -757,6 +757,7 @@
#define PCI_EXT_CAP_ID_VF_REBAR 0x24 /* VF Resizable BAR */
#define PCI_EXT_CAP_ID_DLF 0x25 /* Data Link Feature */
#define PCI_EXT_CAP_ID_PL_16GT 0x26 /* Physical Layer 16.0 GT/s */
+#define PCI_EXT_CAP_ID_LMR 0x27 /* Lane Margining at Receiver */
#define PCI_EXT_CAP_ID_NPEM 0x29 /* Native PCIe Enclosure Management */
#define PCI_EXT_CAP_ID_PL_32GT 0x2A /* Physical Layer 32.0 GT/s */
#define PCI_EXT_CAP_ID_DOE 0x2E /* Data Object Exchange */
@@ -1181,6 +1182,24 @@
#define PCI_PL_16GT_LE_CTRL_USP_TX_PRESET_MASK 0x000000F0
#define PCI_PL_16GT_LE_CTRL_USP_TX_PRESET_SHIFT 4
+/* Lane Margining at Receiver */
+#define PCI_LMR_PORT_CAP 0x04 /* Margining Port Capabilities
*/
+#define PCI_LMR_PORT_CAP_MARGIN_READY 0x0001 /* Margining Ready */
+#define PCI_LMR_PORT_CAP_SW_READY 0x0002 /* Margining SW Ready */
+#define PCI_LMR_PORT_STS 0x06 /* Margining Port Status */
+#define PCI_LMR_PORT_STS_MARGIN_READY 0x0001 /* Margining Ready */
+#define PCI_LMR_PORT_STS_SW_READY 0x0002 /* Margining SW Ready */
+#define PCI_LMR_LANE_CTRL 0x08 /* Margining Lane Control */
+#define PCI_LMR_LANE_CTRL_RX_NUM 0x0007 /* Receiver Number */
+#define PCI_LMR_LANE_CTRL_MTYPE 0x0038 /* Margining Type */
+#define PCI_LMR_LANE_CTRL_USAGE 0x0040 /* Margining Usage Model */
+#define PCI_LMR_LANE_CTRL_PAYLOAD 0xFF00 /* Margining Payload */
+#define PCI_LMR_LANE_STS 0x0A /* Margining Lane Status */
+#define PCI_LMR_LANE_STS_RX_NUM 0x0007 /* Receiver Number */
+#define PCI_LMR_LANE_STS_MTYPE 0x0038 /* Margining Type */
+#define PCI_LMR_LANE_STS_USAGE 0x0040 /* Margining Usage
Model */
+#define PCI_LMR_LANE_STS_PAYLOAD 0xFF00 /* Margining Payload */
+
/* Physical Layer 32.0 GT/s */
#define PCI_PL_32GT_LE_CTRL 0x20 /* Lane Equalization Control Register */
diff --git a/tools/testing/selftests/Makefile b/tools/testing/selftests/Makefile
index 8a4b6ddc68df..6990d999388a 100644
--- a/tools/testing/selftests/Makefile
+++ b/tools/testing/selftests/Makefile
@@ -91,6 +91,7 @@ TARGETS += net/tcp_ao
TARGETS += nolibc
TARGETS += pci_endpoint
TARGETS += pcie_bwctrl
+TARGETS += pcie_lmt
TARGETS += perf_events
TARGETS += pidfd
TARGETS += pid_namespace
diff --git a/tools/testing/selftests/pcie_lmt/Makefile
b/tools/testing/selftests/pcie_lmt/Makefile
new file mode 100644
index 000000000000..36ac85937d78
--- /dev/null
+++ b/tools/testing/selftests/pcie_lmt/Makefile
@@ -0,0 +1,3 @@
+# SPDX-License-Identifier: GPL-2.0
+TEST_PROGS = pcie_lmt.sh
+include ../lib.mk
diff --git a/tools/testing/selftests/pcie_lmt/pcie_lmt.sh
b/tools/testing/selftests/pcie_lmt/pcie_lmt.sh
new file mode 100755
index 000000000000..d57a6e8f3a23
--- /dev/null
+++ b/tools/testing/selftests/pcie_lmt/pcie_lmt.sh
@@ -0,0 +1,105 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+#
+# Copyright (C) 2026 Google LLC
+# Author: Priyank Rathod <[email protected]>
+#
+# Kselftest for PCIe Lane Margining at Receiver (LMR / LMT)
+# Tests the debugfs interface exposed by drivers/pci/pcie/margin.c
+# (/sys/kernel/debug/pci/pcie_lmr_<pci_dev_name>/)
+
+set -e
+
+TESTNAME="pcie_lmt"
+
+# Kselftest framework requirement - SKIP code is 4.
+ksft_skip=4
+retval=0
+skipmsg="skip all tests:"
+
+if [ $UID != 0 ]; then
+ echo "$skipmsg must be run as root" >&2
+ exit $ksft_skip
+fi
+
+DEBUGFS=$(mount -t debugfs | head -1 | awk '{ print $3 }')
+if [ -z "$DEBUGFS" ]; then
+ if [ -d "/sys/kernel/debug" ]; then
+ DEBUGFS="/sys/kernel/debug"
+ else
+ echo "$skipmsg debugfs is not mounted" >&2
+ exit $ksft_skip
+ fi
+fi
+
+if [ ! -d "$DEBUGFS/pci" ]; then
+ echo "$skipmsg debugfs pci directory ($DEBUGFS/pci) not found" >&2
+ exit $ksft_skip
+fi
+
+LMR_DEVS=$(ls -d $DEBUGFS/pci/pcie_lmr_* 2>/dev/null || true)
+if [ -z "$LMR_DEVS" ]; then
+ echo "$skipmsg no PCIe LMR devices found in $DEBUGFS/pci/" >&2
+ exit $ksft_skip
+fi
+
+cleanup_dev()
+{
+ local dev="$1"
+ echo 0 > "$dev/enable" 2>/dev/null || true
+}
+
+echo "$TESTNAME: testing PCIe LMR debugfs entries"
+
+for dev in $LMR_DEVS; do
+ dev_name=$(basename "$dev")
+ echo "$TESTNAME: probing device $dev_name"
+
+ if [ ! -r "$dev/capabilities" ] || [ ! -r "$dev/port_status" ] ||
+ [ ! -r "$dev/enable" ] || [ ! -w "$dev/enable" ]; then
+ echo "$TESTNAME: $dev_name missing mandatory root attributes"
+ retval=1
+ continue
+ fi
+
+ caps=$(cat "$dev/capabilities")
+ status=$(cat "$dev/port_status")
+ echo " $dev_name: capabilities read OK"
+ echo " $dev_name: port_status read OK"
+
+ trap 'cleanup_dev "$dev"' EXIT
+
+ if ! echo 1 > "$dev/enable" 2>/dev/null; then
+ echo " $dev_name: margining not ready by hardware (skipping
active lanes)"
+ continue
+ fi
+
+ echo " $dev_name: margining enabled OK"
+
+ for lane_dir in $(ls -d "$dev"/lane* 2>/dev/null || true); do
+ lane=$(basename "$lane_dir")
+ echo " $dev_name: testing $lane"
+
+ # Test setting receiver (Rx 0 is always local receiver)
+ echo 0 > "$lane_dir/receiver"
+ cat "$lane_dir/caps" > /dev/null
+ cat "$lane_dir/num_timing_steps" > /dev/null
+ cat "$lane_dir/num_voltage_steps" > /dev/null
+
+ # Test resetting timing and voltage margin
+ echo 0 > "$lane_dir/margin_timing"
+ echo 0 > "$lane_dir/margin_voltage"
+ done
+
+ echo 0 > "$dev/enable"
+ trap - EXIT
+ echo " $dev_name: margining disabled OK"
+done
+
+if [ $retval -eq 0 ]; then
+ echo "$TESTNAME [PASS]"
+else
+ echo "$TESTNAME [FAIL]"
+fi
+
+exit $retval
---
base-commit: 0f23d56f17fdfc7db69d51f64c8b91bbab947aa9
change-id: 20260818-pcie-lmt-3044d586aaec
Best regards,
--
Priyank Rathod <[email protected]>
