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>

1 files changed, 7 insertions, 0 deletions

diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
index 05ef4b05a63..b1a477f8260 100644
--- a/drivers/hid/uhid.c
+++ b/drivers/hid/uhid.c
@@ -117,6 +117,13 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer,
 
 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;
 }