HID: uhid: allow poll()'ing on uhid devices
As long as the internal buffer is not empty, we return POLLIN to
user-space.
uhid->head and uhid->tail are no atomics so the comparison may return
inexact results. However, this doesn't matter here as user-space would
need to poll() in two threads simultaneously to trigger this. And in this
case it doesn't matter if a cached result is returned or the exact new
result as user-space does not know which thread returns first from poll()
and the following read(). So it is safe to compare the values without
locking.
Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
index 05ef4b0..b1a477f 100644
--- a/drivers/hid/uhid.c
+++ b/drivers/hid/uhid.c
@@ -117,6 +117,13 @@
static unsigned int uhid_char_poll(struct file *file, poll_table *wait)
{
+ struct uhid_device *uhid = file->private_data;
+
+ poll_wait(file, &uhid->waitq, wait);
+
+ if (uhid->head != uhid->tail)
+ return POLLIN | POLLRDNORM;
+
return 0;
}