poll sketch

kernel_module/debugfs.c

@@ -14,7 +14,7 @@ Requires `CONFIG_DEBUG_FS=y`.
 
 MODULE_LICENSE("GPL");
 
-static struct dentry *dir = 0;
+static struct dentry *dir;
 static u32 value = 42;
 
 int init_module(void)

kernel_module/fops.c

@@ -5,6 +5,8 @@ Usage:
 
 	/fops.sh
 
+The buffer can be written and read from. If data overflows, data is thrown away.
+
 No, there ain't no official docs:
 http://stackoverflow.com/questions/15213932/what-are-the-struct-file-operations-arguments
 
@@ -25,7 +27,7 @@ Here we use debugfs.
 
 MODULE_LICENSE("GPL");
 
-static struct dentry *dir = 0;
+static struct dentry *dir;
 
 static char data[] = {'a', 'b', 'c', 'd'};
 

kernel_module/poll.c

@@ -0,0 +1,75 @@
+/*
+Basic poll file_operation example.
+
+Waits for a second, give jiffies to user, wait for a second...
+
+usleep_range
+*/
+
+#include <asm/uaccess.h> /* copy_from_user, copy_to_user */
+#include <linux/debugfs.h>
+#include <linux/delay.h> /* usleep_range */
+#include <linux/errno.h> /* EFAULT */
+#include <linux/fs.h>
+#include <linux/jiffies.h>
+#include <linux/kernel.h> /* min */
+#include <linux/kthread.h>
+#include <linux/module.h>
+#include <linux/poll.h>
+#include <linux/printk.h> /* printk */
+
+MODULE_LICENSE("GPL");
+
+static struct dentry *dir;
+static struct task_struct *kthread;
+static wait_queue_head_t waitqueue;
+
+static ssize_t read(struct file *file, char __user *buf, size_t len, loff_t *off)
+{
+	ssize_t ret;
+	char s[1024];
+
+	ret = snprintf(s, sizeof(s), "%llu", (unsigned long long)jiffies);
+	if (copy_to_user(buf, s, ret)) {
+		ret = -EFAULT;
+	}
+	return ret;
+}
+
+unsigned int poll(struct file *filp, struct poll_table_struct *wait)
+{
+	/*TODO*/
+	/*wait_event_interruptible(waitqueue, (dev->rp != dev->wp));*/
+	pr_info("poll_wait before\n");
+	poll_wait(filp, &waitqueue, wait);
+	pr_info("poll_wait after\n");
+	return POLLIN;
+}
+
+static int kthread_func(void *data)
+{
+	while (!kthread_should_stop()) {
+		usleep_range(1000000, 1000001);
+		wake_up_interruptible(&waitqueue);
+	}
+	return 0;
+}
+
+static const struct file_operations fops = {
+	.read = read,
+	.poll = poll
+};
+
+int init_module(void)
+{
+	dir = debugfs_create_dir("kernel_module_cheat_poll", 0);
+    debugfs_create_file("f", 0666, dir, NULL, &fops);
+	init_waitqueue_head(&waitqueue);
+	kthread = kthread_create(kthread_func, NULL, "mykthread");
+	return 0;
+}
+
+void cleanup_module(void)
+{
+	debugfs_remove_recursive(dir);
+}

kernel_module/test/poll.c

@@ -0,0 +1,37 @@
+#define _XOPEN_SOURCE 700
+#include <fcntl.h> /* creat, O_CREAT */
+#include <poll.h> /* poll */
+#include <stdio.h> /* printf, puts, snprintf */
+#include <stdlib.h> /* EXIT_FAILURE, EXIT_SUCCESS */
+#include <unistd.h> /* read */
+
+int main(int argc, char **argv) {
+    char buf[1024];
+    int fd, n;
+    short revents;
+    struct pollfd pfd;
+
+    fd = open(argv[1], O_RDONLY | O_NONBLOCK);
+    pfd.fd = fd;
+    pfd.events = POLLIN;
+    while (1) {
+        puts("loop");
+        poll(&pfd, 1, -1);
+        revents = pfd.revents;
+        if (revents & POLLIN) {
+            n = read(pfd.fd, buf, sizeof(buf));
+            printf("POLLIN n=%d buf=%.*s\n", n, n, buf);
+        }
+        if (revents & POLLHUP) {
+            printf("POLLHUP\n");
+            close(pfd.fd);
+            break;
+        }
+        if (revents & POLLNVAL) {
+            printf("POLLNVAL\n");
+        }
+        if (revents & POLLERR) {
+            printf("POLLERR\n");
+        }
+    }
+}