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 <redacted>
---
 drivers/hid/uhid.c |    7 +++++++
 1 file changed, 7 insertions(+)

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 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;
 }
 

-- 
1.7.10.4