Using the Linux sysfs_notify call - linux

Question

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

Answer 1

The blocking poll is from the user side. User code can simply tell the kernel which attributes it's interested in, then block in a poll() until one of them has changed.
The sysfs_notify() is a kernel-side call that releases the user-space poll(). After you adjust your kernel attribute value, just call sysfs_notify() to allow any user-space applications to respond to their outstanding poll().
Think of the poll() as "subscribing" to notices of a change in an attribute of interest, and sysfs_notify() as "publishing" the change to any subscribers.