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@xxxxxxxxxxxxxx>
---
drivers/hid/uhid.c | 46 +++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 45 insertions(+), 1 deletion(-)
diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
index b1a477f..9386082 100644
--- 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,
--
1.7.10.4
--
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