On Tue, 18 Aug 2026, Priyank Rathod wrote:
> 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>
Please sort these alphabetically and add the includes for things you're
actually using in this file.
> +
> +#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
Please use GENMASK() and FIELD_PREP/GET() and drop SHIFT defines.
> +
> +/* 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)
Add include for BIT()
> +
> +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;
Please document what the lock protects.
> + 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)
PCI_POSSIBLE_ERROR()
> + 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);
guard() + direct return + drop ret variable.
> +
> + 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");
str_yes_no()
> + seq_printf(s, " SW Ready Required: %s\n",
> + (cap & PCI_LMR_PORT_CAP_SW_READY) ? "yes" : "no");
ditto.
> + 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");
ditto x 2.
> + 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);
Using guard() will make the flow simpler in this function as you can drop
the label and do direct returns.
> +
> + 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) {
PCI_POSSIBLE_ERROR()
> + 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;
This would be look simpler if you add a struct seq_file local variable.
private_data is void * so you can rely on implicit cast when assigning
that to the local var.
> + u8 rx;
> + int ret;
> +
> + ret = kstrtou8_from_user(user_buf, count, 0, &rx);
> + if (ret)
> + return ret;
> +
> + if (rx > LMR_MAX_RX_NUM)
There could be off-by-one problem here (is rx == LMR_MAX_RX_NUM okay or
not, I'm not sure?).
> + 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");
str_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;
Why isn't the field directly extracted?
If the second step remains, you should use FIELD_GET() for it as well.
> + 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);
Is this the only variation between this and the previous function, why not
create a helper that takes TIMING/VOLTAGE as arg to avoid copying most of
it?
> + 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);
guard()
> +
> + 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);
FIELD_PREP(), you might want to calculate the value into a local variable
first though to keep this call shorter.
> + 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);
guard()
> +
> + 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);
FIELD_PREP(). Similar to above, instead of embedding calculation here,
consider calculating the value first.
> + 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));
scnprinf() so that snprintf() could be eventually removed (you don't use
return value so the difference won't matter).
> + 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);
scnprintf()
> + 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;
The logic in that check seems odd. Why does this function continue of
speed == PCI_SPEED_UNKNOWN. Did you mean to use || instead?
Also, shouldn't this be checked right after pcie_get_speed_cap() call.
> +
> + dev->lmr_cap = lmr;
> +
> + mdev = kzalloc_obj(*mdev, GFP_KERNEL);
Add include.
> + 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);
Consider using __counted_by() and flex array inside struct pci_margin_dev
so you don't need to do the extra alloc.
> + 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);
Perhaps abstract this mapping into a macro so it looks just a call here.
> + 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,
>
--
i.
