Using the Linux sysfs_notify call

I am trying to communicate asynchronously between a kernel driver and a user-space program (I know there are lots of questions here that ask for similar information, but I could find none that deal with sysfs_notify). I am leaving Vilhelm's edit here, but adding the source to both a simple driver utilizing sysfs and a user-space program to poll it. The driver works fine (I got most of it from the net; it is missing the credits, but I couldn't find them when I went back to add them). Unfortunately, the polling program does not work. It always returns success immediately. Interestingly, if I don't perform the two reads prior to the poll, the revents members are set to POLLERR | POLLIN instead of just POLLIN as seen in the program output. Program output: root@ubuntu:/home/wmulcahy/demo# ./readhello triggered Attribute file value: 74 (t) [0] revents[0]: 00000001 revents[1]: 00000001 Here is the driver: hello.c (you can see where I started out...) ``` #include <linux/module.h> #include <linux/kernel.h> #include <linux/init.h> #include <linux/fs.h> #include <linux/slab.h> struct my_attr { struct attribute attr; int value; }; static struct my_attr notify = { .attr.name="notify", .attr.mode = 0644, .value = 0, }; static struct my_attr trigger = { .attr.name="trigger", .attr.mode = 0644, .value = 0, }; static struct attribute * myattr[] = { &notify.attr, &trigger.attr, NULL }; static ssize_t show(struct kobject *kobj, struct attribute *attr, char *buf) { struct my_attr *a = container_of(attr, struct my_attr, attr); printk( "hello: show called (%s)\n", a->attr.name ); return scnprintf(buf, PAGE_SIZE, "%s: %d\n", a->attr.name, a->value); } static struct kobject *mykobj; static ssize_t store(struct kobject *kobj, struct attribute *attr, const char *buf, size_t len) { struct my_attr *a = container_of(attr, struct my_attr, attr); sscanf(buf, "%d", &a->value); notify.value = a->value; printk("sysfs_notify store %s = %d\n", a->attr.name, a->value); sysfs_notify(mykobj, NULL, "notify"); return sizeof(int); } static struct sysfs_ops myops = { .show = show, .store = store, }; static struct kobj_type mytype = { .sysfs_ops = &myops, .default_attrs = myattr, }; static struct kobject *mykobj; static int __init hello_module_init(void) { int err = -1; printk("Hello: init\n"); mykobj = kzalloc(sizeof(*mykobj), GFP_KERNEL); if (mykobj) { kobject_init(mykobj, &mytype); if (kobject_add(mykobj, NULL, "%s", "hello")) { err = -1; printk("Hello: kobject_add() failed\n"); kobject_put(mykobj); mykobj = NULL; } err = 0; } return err; } static void __exit hello_module_exit(void) { if (mykobj) { kobject_put(mykobj); kfree(mykobj); } printk("Hello: exit\n"); } module_init(hello_module_init); module_exit(hello_module_exit); MODULE_LICENSE("GPL"); ``` And here is the poll program: readhello.c ``` #include <stdint.h> #include <stdlib.h> #include <stdio.h> #include <string.h> #include <unistd.h> #include <fcntl.h> #include <sys/types.h> #include <sys/stat.h> #include <poll.h> #define TEST_SYSFS_TRIGGER "/sys/hello/trigger" #define TEST_SYSFS_NOTIFY "/sys/hello/notify" int main(int argc, char **argv) { int cnt, notifyFd, triggerFd, rv; char attrData[100]; struct pollfd ufds[2]; // Open a connection to the attribute file. if ((notifyFd = open(TEST_SYSFS_NOTIFY, O_RDWR)) < 0) { perror("Unable to open notify"); exit(1); } // Open a connection to the attribute file. if ((triggerFd = open(TEST_SYSFS_TRIGGER, O_RDWR)) < 0) { perror("Unable to open trigger"); exit(1); } ufds[0].fd = notifyFd; ufds[0].events = POLLIN; ufds[1].fd = triggerFd; ufds[1].events = POLLIN; // Someone suggested dummy reads before the poll() call cnt = read( notifyFd, attrData, 100 ); cnt = read( triggerFd, attrData, 100 ); ufds[0].revents = 0; ufds[1].revents = 0; if (( rv = poll( ufds, 2, 10000)) < 0 ) { perror("poll error"); } else if (rv == 0) { printf("Timeout occurred!\n"); } else if (ufds[0].revents & POLLIN) { printf("triggered\n"); cnt = read( notifyFd, attrData, 1 ); printf( "Attribute file value: %02X (%c) [%d]\n", attrData[0], attrData[0], cnt ); } printf( "revents[0]: %08X\n", ufds[0].revents ); printf( "revents[1]: %08X\n", ufds[1].revents ); close( triggerFd ); close( notifyFd ); } ``` Some upcoming sysfs enhancements. Internally, the patch adds a wait queue head to every kobject on the system; that queue is inserted into a poll table in response to a poll() call. The sysfs code has no way of knowing, however, when the value of any given sysfs attribute has changed, so the subsystem implementing a pollable attribute must make explicit calls to: ``` void sysfs_notify(struct kobject *kobj, char *dir, char *attr); ``` Thanks, Lee