[PATCH 5/5] PCI/FLIT: Ratelimit flit error logging

[Yazen Ghannam]

A high rate of flit errors can flood the kernel log with error counter
and error log messages. Model the throttling on the AER capability: give
each device its own ratelimit_state, initialized in pci_flit_init() with
the default interval and burst, rather than a single static per-call-site
limiter shared across all ports.

Take one ratelimit decision per interrupt and use it to gate the console
output, so a storm on one port cannot suppress logging on another. Leave
the flit_error trace event unthrottled so RAS tooling still records every
erroneous flit while the log is limited.

Expose the interval and burst through per-device sysfs attributes
(flit_ratelimit_interval_ms, flit_ratelimit_burst), gated on CAP_SYS_ADMIN,
so the limit can be tuned or disabled at runtime.

Signed-off-by: Yazen Ghannam <yazen.ghannam@xxxxxxx>
Assisted-by: Claude:claude-opus-4-8
---
 drivers/pci/pci-sysfs.c |   3 +
 drivers/pci/pci.h       |   1 +
 drivers/pci/pcie/flit.c | 119 ++++++++++++++++++++++++++++++++++++++--
 include/linux/pci.h     |   2 +
 4 files changed, 121 insertions(+), 4 deletions(-)

diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
index e985a3854f8d..e85953c249f2 100644
--- a/drivers/pci/pci-sysfs.c
+++ b/drivers/pci/pci-sysfs.c
@@ -1932,6 +1932,9 @@ const struct attribute_group *pci_dev_attr_groups[] = {
 	&aer_stats_attr_group,
 	&aer_attr_group,
 #endif
+#ifdef CONFIG_PCIE_FLIT
+	&flit_attr_group,
+#endif
 #ifdef CONFIG_PCIEASPM
 	&aspm_ctrl_attr_group,
 #endif
diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
index 31d04366b980..28ff4243d125 100644
--- a/drivers/pci/pci.h
+++ b/drivers/pci/pci.h
@@ -896,6 +896,7 @@ static inline bool pci_dpc_recovered(struct pci_dev *pdev) { return false; }
 
 #ifdef CONFIG_PCIE_FLIT
 void pci_flit_init(struct pci_dev *pdev);
+extern const struct attribute_group flit_attr_group;
 #else
 static inline void pci_flit_init(struct pci_dev *pdev) { }
 #endif
diff --git a/drivers/pci/pcie/flit.c b/drivers/pci/pcie/flit.c
index 83cf53f39291..7cb5aff7c552 100644
--- a/drivers/pci/pcie/flit.c
+++ b/drivers/pci/pcie/flit.c
@@ -12,9 +12,12 @@
 #define pr_fmt(fmt) "Flit: " fmt
 #define dev_fmt pr_fmt
 
+#include <linux/capability.h>
 #include <linux/init.h>
 #include <linux/interrupt.h>
 #include <linux/pci.h>
+#include <linux/ratelimit.h>
+#include <linux/sysfs.h>
 #include <ras/ras_event.h>
 #include "portdrv.h"
 #include "../pci.h"
@@ -30,10 +33,106 @@ void pci_flit_init(struct pci_dev *pdev)
 		return;
 
 	pdev->flit_cap = pci_find_ext_capability(pdev, PCI_EXT_CAP_ID_FLIT);
-	if (pdev->flit_cap)
-		pci_dbg(pdev, "Flit Mode Error Logging Capability present.\n");
+	if (!pdev->flit_cap)
+		return;
+
+	ratelimit_state_init(&pdev->flit_ratelimit, DEFAULT_RATELIMIT_INTERVAL,
+			     DEFAULT_RATELIMIT_BURST);
+
+	pci_dbg(pdev, "Flit Mode Error Logging Capability present.\n");
+}
+
+/*
+ * Ratelimit interval
+ * <=0: disabled with ratelimit.interval = 0
+ * >0: enabled with ratelimit.interval in ms
+ */
+static ssize_t flit_ratelimit_interval_ms_show(struct device *dev,
+					       struct device_attribute *attr,
+					       char *buf)
+{
+	struct pci_dev *pdev = to_pci_dev(dev);
+
+	return sysfs_emit(buf, "%d\n",
+			  jiffies_to_msecs(pdev->flit_ratelimit.interval));
+}
+
+static ssize_t flit_ratelimit_interval_ms_store(struct device *dev,
+						struct device_attribute *attr,
+						const char *buf, size_t count)
+{
+	struct pci_dev *pdev = to_pci_dev(dev);
+	int interval;
+
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+
+	if (kstrtoint(buf, 0, &interval) < 0)
+		return -EINVAL;
+
+	if (interval <= 0)
+		interval = 0;
+	else
+		interval = msecs_to_jiffies(interval);
+
+	pdev->flit_ratelimit.interval = interval;
+
+	return count;
+}
+static DEVICE_ATTR_RW(flit_ratelimit_interval_ms);
+
+static ssize_t flit_ratelimit_burst_show(struct device *dev,
+					 struct device_attribute *attr,
+					 char *buf)
+{
+	struct pci_dev *pdev = to_pci_dev(dev);
+
+	return sysfs_emit(buf, "%d\n", pdev->flit_ratelimit.burst);
+}
+
+static ssize_t flit_ratelimit_burst_store(struct device *dev,
+					  struct device_attribute *attr,
+					  const char *buf, size_t count)
+{
+	struct pci_dev *pdev = to_pci_dev(dev);
+	int burst;
+
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+
+	if (kstrtoint(buf, 0, &burst) < 0)
+		return -EINVAL;
+
+	pdev->flit_ratelimit.burst = burst;
+
+	return count;
+}
+static DEVICE_ATTR_RW(flit_ratelimit_burst);
+
+static struct attribute *flit_attrs[] = {
+	&dev_attr_flit_ratelimit_interval_ms.attr,
+	&dev_attr_flit_ratelimit_burst.attr,
+	NULL
+};
+
+static umode_t flit_attrs_are_visible(struct kobject *kobj,
+				      struct attribute *a, int n)
+{
+	struct device *dev = kobj_to_dev(kobj);
+	struct pci_dev *pdev = to_pci_dev(dev);
+
+	if (!pdev->flit_cap)
+		return 0;
+
+	return a->mode;
 }
 
+const struct attribute_group flit_attr_group = {
+	.name = "flit",
+	.attrs = flit_attrs,
+	.is_visible = flit_attrs_are_visible,
+};
+
 static void flit_cntr_enable(struct pci_dev *pdev)
 {
 	u16 flit = pdev->flit_cap;
@@ -70,12 +169,22 @@ static irqreturn_t flit_isr(int irq, void *context)
 	u16 flit = pdev->flit_cap;
 	u16 cntr_ctrl, cntr_sta;
 	u32 err_log1, err_log2;
+	bool print;
 
 	/* Read and log Counter and Error Log Registers. */
 	pci_read_config_word(pdev, flit + PCI_FLIT_ERR_CNTR_CTRL, &cntr_ctrl);
 	pci_read_config_word(pdev, flit + PCI_FLIT_ERR_CNTR_STA, &cntr_sta);
 
-	pci_info(pdev, HW_ERR "Counter Control: 0x%04x Counter Status: 0x%04x\n", cntr_ctrl, cntr_sta);
+	/*
+	 * Take a single ratelimit decision per interrupt and use it to gate
+	 * the console output.  The trace event is always emitted so RAS
+	 * tooling records every erroneous flit even while the log is throttled.
+	 */
+	print = __ratelimit(&pdev->flit_ratelimit);
+
+	if (print)
+		pci_info(pdev, HW_ERR "Counter Control: 0x%04x Counter Status: 0x%04x\n",
+			 cntr_ctrl, cntr_sta);
 
 	do {
 		pci_read_config_dword(pdev, flit + PCI_FLIT_ERR_LOG1, &err_log1);
@@ -84,7 +193,9 @@ static irqreturn_t flit_isr(int irq, void *context)
 			break;
 
 		pci_read_config_dword(pdev, flit + PCI_FLIT_ERR_LOG2, &err_log2);
-		pci_info(pdev, HW_ERR "  Error Log1: 0x%08x Error Log2: 0x%08x\n", err_log1, err_log2);
+		if (print)
+			pci_info(pdev, HW_ERR "  Error Log1: 0x%08x Error Log2: 0x%08x\n",
+				 err_log1, err_log2);
 		trace_flit_event(pci_name(pdev), cntr_ctrl, cntr_sta, err_log1, err_log2);
 
 		pci_write_config_dword(pdev, flit + PCI_FLIT_ERR_LOG1, err_log1);
diff --git a/include/linux/pci.h b/include/linux/pci.h
index 4e4d1d9eb7fe..01df7c84a658 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -40,6 +40,7 @@
 #include <linux/io.h>
 #include <linux/resource_ext.h>
 #include <linux/msi_api.h>
+#include <linux/ratelimit_types.h>
 #include <uapi/linux/pci.h>
 
 #include <linux/pci_ids.h>
@@ -595,6 +596,7 @@ struct pci_dev {
 
 #ifdef CONFIG_PCIE_FLIT
 	u16		flit_cap;	/* Flit Logging Capabilities */
+	struct ratelimit_state flit_ratelimit;	/* Flit error log ratelimit */
 #endif
 };
 
-- 
2.53.0