linux-imx

// SPDX-License-Identifier: GPL-2.0

/*

 * trace_hwlatdetect.c - A simple Hardware Latency detector.

 *

 * Use this tracer to detect large system latencies induced by the behavior of

 * certain underlying system hardware or firmware, independent of Linux itself.

 * The code was developed originally to detect the presence of SMIs on Intel

 * and AMD systems, although there is no dependency upon x86 herein.

 *

 * The classical example usage of this tracer is in detecting the presence of

 * SMIs or System Management Interrupts on Intel and AMD systems. An SMI is a

 * somewhat special form of hardware interrupt spawned from earlier CPU debug

 * modes in which the (BIOS/EFI/etc.) firmware arranges for the South Bridge

 * LPC (or other device) to generate a special interrupt under certain

 * circumstances, for example, upon expiration of a special SMI timer device,

 * due to certain external thermal readings, on certain I/O address accesses,

 * and other situations. An SMI hits a special CPU pin, triggers a special

 * SMI mode (complete with special memory map), and the OS is unaware.

 *

 * Although certain hardware-inducing latencies are necessary (for example,

 * a modern system often requires an SMI handler for correct thermal control

 * and remote management) they can wreak havoc upon any OS-level performance

 * guarantees toward low-latency, especially when the OS is not even made

 * aware of the presence of these interrupts. For this reason, we need a

 * somewhat brute force mechanism to detect these interrupts. In this case,

 * we do it by hogging all of the CPU(s) for configurable timer intervals,

 * sampling the built-in CPU timer, looking for discontiguous readings.

 *

 * WARNING: This implementation necessarily introduces latencies. Therefore,

 *          you should NEVER use this tracer while running in a production

 *          environment requiring any kind of low-latency performance

 *          guarantee(s).

 *

 * Copyright (C) 2008-2009 Jon Masters, Red Hat, Inc. <jcm@redhat.com>

 * Copyright (C) 2013-2016 Steven Rostedt, Red Hat, Inc. <srostedt@redhat.com>

 *

 * Includes useful feedback from Clark Williams <clark@redhat.com>

 *

 */

#include <linux/kthread.h>

#include <linux/tracefs.h>

#include <linux/uaccess.h>

#include <linux/cpumask.h>

#include <linux/delay.h>

#include <linux/sched/clock.h>

#include "trace.h"

​

static struct trace_array   *hwlat_trace;

​

#define U64STR_SIZE     22          /* 20 digits max */

​

#define BANNER          "hwlat_detector: "

#define DEFAULT_SAMPLE_WINDOW   1000000         /* 1s */

#define DEFAULT_SAMPLE_WIDTH    500000          /* 0.5s */

#define DEFAULT_LAT_THRESHOLD   10          /* 10us */

​

/* sampling thread*/

static struct task_struct *hwlat_kthread;

​

static struct dentry *hwlat_sample_width;   /* sample width us */

static struct dentry *hwlat_sample_window;  /* sample window us */

​

/* Save the previous tracing_thresh value */