HID: uhid: implement read() on uhid devices

User-space can use read() to get a single event from uhid devices. read()
does never return multiple events. This allows us to extend the event
structure and still keep backwards compatibility.

If user-space wants to get multiple events in one syscall, they should use
the readv()/writev() syscalls which are supported by uhid.

This introduces a new lock which helps us synchronizing simultaneous reads
from user-space. We also correctly return -EINVAL/-EFAULT only on errors
and retry the read() when some other thread captured the event faster than
we did.

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 b1a477f..9386082 100644 (file)
--- a/drivers/hid/uhid.c
+++ b/drivers/hid/uhid.c
@@ -28,6 +28,7 @@
 #define UHID_BUFSIZE   32
 
 struct uhid_device {
+       struct mutex devlock;
        struct hid_device *hid;
 
        wait_queue_head_t waitq;
@@ -81,6 +82,7 @@ static int uhid_char_open(struct inode *inode, struct file *file)
        if (!uhid)
                return -ENOMEM;
 
+       mutex_init(&uhid->devlock);
        spin_lock_init(&uhid->qlock);
        init_waitqueue_head(&uhid->waitq);
 
@@ -106,7 +108,49 @@ static int uhid_char_release(struct inode *inode, struct file *file)
 static ssize_t uhid_char_read(struct file *file, char __user *buffer,
                                size_t count, loff_t *ppos)
 {
-       return 0;
+       struct uhid_device *uhid = file->private_data;
+       int ret;
+       unsigned long flags;
+       size_t len;
+
+       /* they need at least the "type" member of uhid_event */
+       if (count < sizeof(__u32))
+               return -EINVAL;
+
+try_again:
+       if (file->f_flags & O_NONBLOCK) {
+               if (uhid->head == uhid->tail)
+                       return -EAGAIN;
+       } else {
+               ret = wait_event_interruptible(uhid->waitq,
+                                               uhid->head != uhid->tail);
+               if (ret)
+                       return ret;
+       }
+
+       ret = mutex_lock_interruptible(&uhid->devlock);
+       if (ret)
+               return ret;
+
+       if (uhid->head == uhid->tail) {
+               mutex_unlock(&uhid->devlock);
+               goto try_again;
+       } else {
+               len = min(count, sizeof(**uhid->outq));
+               if (copy_to_user(buffer, &uhid->outq[uhid->tail], len)) {
+                       ret = -EFAULT;
+               } else {
+                       kfree(uhid->outq[uhid->tail]);
+                       uhid->outq[uhid->tail] = NULL;
+
+                       spin_lock_irqsave(&uhid->qlock, flags);
+                       uhid->tail = (uhid->tail + 1) % UHID_BUFSIZE;
+                       spin_unlock_irqrestore(&uhid->qlock, flags);
+               }
+       }
+
+       mutex_unlock(&uhid->devlock);
+       return ret ? ret : len;
 }
 
 static ssize_t uhid_char_write(struct file *file, const char __user *buffer,