user_events: Add sample code for typical usage

Add sample code for user_events typical usage to show how to register
and monitor status, as well as to write out data.

Link: https://lkml.kernel.org/r/[e-mail]

Acked-by: Masami Hiramatsu <[e-mail]>
Signed-off-by: Beau Belgrave <[e-mail]>
Signed-off-by: Steven Rostedt (Google) <[e-mail]>

samples/user_events/Makefile

···
+# SPDX-License-Identifier: GPL-2.0
+CFLAGS += -Wl,-no-as-needed -Wall -I../../usr/include
+
+example: example.o
+example.o: example.c

samples/user_events/example.c

···
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (c) 2021, Microsoft Corporation.
+ *
+ * Authors:
+ *   Beau Belgrave <[e-mail]>
+ */
+
+#include <errno.h>
+#include <sys/ioctl.h>
+#include <sys/mman.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <linux/user_events.h>
+
+/* Assumes debugfs is mounted */
+const char *data_file = "/sys/kernel/debug/tracing/user_events_data";
+const char *status_file = "/sys/kernel/debug/tracing/user_events_status";
+
+static int event_status(char **status)
+{
+	int fd = open(status_file, O_RDONLY);
+
+	*status = mmap(NULL, sysconf(_SC_PAGESIZE), PROT_READ,
+		       MAP_SHARED, fd, 0);
+
+	close(fd);
+
+	if (*status == MAP_FAILED)
+		return -1;
+
+	return 0;
+}
+
+static int event_reg(int fd, const char *command, int *status, int *write)
+{
+	struct user_reg reg = {0};
+
+	reg.size = sizeof(reg);
+	reg.name_args = (__u64)command;
+
+	if (ioctl(fd, DIAG_IOCSREG, &reg) == -1)
+		return -1;
+
+	*status = reg.status_index;
+	*write = reg.write_index;
+
+	return 0;
+}
+
+int main(int argc, char **argv)
+{
+	int data_fd, status, write;
+	char *status_page;
+	struct iovec io[2];
+	__u32 count = 0;
+
+	if (event_status(&status_page) == -1)
+		return errno;
+
+	data_fd = open(data_file, O_RDWR);
+
+	if (event_reg(data_fd, "test u32 count", &status, &write) == -1)
+		return errno;
+
+	/* Setup iovec */
+	io[0].iov_base = &write;
+	io[0].iov_len = sizeof(write);
+	io[1].iov_base = &count;
+	io[1].iov_len = sizeof(count);
+
+ask:
+	printf("Press enter to check status...\n");
+	getchar();
+
+	/* Check if anyone is listening */
+	if (status_page[status]) {
+		/* Yep, trace out our data */
+		writev(data_fd, (const struct iovec *)io, 2);
+
+		/* Increase the count */
+		count++;
+
+		printf("Something was attached, wrote data\n");
+	}
+
+	goto ask;
+
+	return 0;
+}