HID: uhid: implement feature requests

HID standard allows sending a feature request to the device which is
answered by an HID report. uhid implements this by sending a UHID_FEATURE
event to user-space which then must answer with UHID_FEATURE_ANSWER. If it
doesn't do this in a timely manner, the request is discarded silently.

We serialize the feature requests, that is, there is always only a single
active feature-request sent to user-space, other requests have to wait.
HIDP and USB-HID do it the same way.

Because we discard feature-requests silently, we must make sure to match
a response to the corresponding request. We use sequence-IDs for this so
user-space must copy the ID from the request into the answer.
Feature-answers are ignored if they do not contain the same ID as the
currently pending feature request.

Internally, we must make sure that feature-requests are synchronized with
UHID_DESTROY and close() events. We must not dead-lock when closing the
HID device, either, so we have to use separate locks.

Signed-off-by: David Herrmann <[e-mail]>
Signed-off-by: Jiri Kosina <[e-mail]>

drivers/hid/uhid.c

···
 	__u8 head;
 	__u8 tail;
 	struct uhid_event *outq[UHID_BUFSIZE];
+
+	struct mutex report_lock;
+	wait_queue_head_t report_wait;
+	atomic_t report_done;
+	atomic_t report_id;
+	struct uhid_event report_buf;
 };
 
 static struct miscdevice uhid_misc;
···
 static int uhid_hid_get_raw(struct hid_device *hid, unsigned char rnum,
 			    __u8 *buf, size_t count, unsigned char rtype)
 {
-	return 0;
+	struct uhid_device *uhid = hid->driver_data;
+	__u8 report_type;
+	struct uhid_event *ev;
+	unsigned long flags;
+	int ret;
+	size_t len;
+	struct uhid_feature_answer_req *req;
+
+	if (!uhid->running)
+		return -EIO;
+
+	switch (rtype) {
+	case HID_FEATURE_REPORT:
+		report_type = UHID_FEATURE_REPORT;
+		break;
+	case HID_OUTPUT_REPORT:
+		report_type = UHID_OUTPUT_REPORT;
+		break;
+	case HID_INPUT_REPORT:
+		report_type = UHID_INPUT_REPORT;
+		break;
+	default:
+		return -EINVAL;
+	}
+
+	ret = mutex_lock_interruptible(&uhid->report_lock);
+	if (ret)
+		return ret;
+
+	ev = kzalloc(sizeof(*ev), GFP_KERNEL);
+	if (!ev) {
+		ret = -ENOMEM;
+		goto unlock;
+	}
+
+	spin_lock_irqsave(&uhid->qlock, flags);
+	ev->type = UHID_FEATURE;
+	ev->u.feature.id = atomic_inc_return(&uhid->report_id);
+	ev->u.feature.rnum = rnum;
+	ev->u.feature.rtype = report_type;
+
+	atomic_set(&uhid->report_done, 0);
+	uhid_queue(uhid, ev);
+	spin_unlock_irqrestore(&uhid->qlock, flags);
+
+	ret = wait_event_interruptible_timeout(uhid->report_wait,
+				atomic_read(&uhid->report_done), 5 * HZ);
+
+	/*
+	 * Make sure "uhid->running" is cleared on shutdown before
+	 * "uhid->report_done" is set.
+	 */
+	smp_rmb();
+	if (!ret || !uhid->running) {
+		ret = -EIO;
+	} else if (ret < 0) {
+		ret = -ERESTARTSYS;
+	} else {
+		spin_lock_irqsave(&uhid->qlock, flags);
+		req = &uhid->report_buf.u.feature_answer;
+
+		if (req->err) {
+			ret = -EIO;
+		} else {
+			ret = 0;
+			len = min(count,
+				min_t(size_t, req->size, UHID_DATA_MAX));
+			memcpy(buf, req->data, len);
+		}
+
+		spin_unlock_irqrestore(&uhid->qlock, flags);
+	}
+
+	atomic_set(&uhid->report_done, 1);
+
+unlock:
+	mutex_unlock(&uhid->report_lock);
+	return ret ? ret : len;
 }
 
 static int uhid_hid_output_raw(struct hid_device *hid, __u8 *buf, size_t count,
···
 	if (!uhid->running)
 		return -EINVAL;
 
+	/* clear "running" before setting "report_done" */
 	uhid->running = false;
+	smp_wmb();
+	atomic_set(&uhid->report_done, 1);
+	wake_up_interruptible(&uhid->report_wait);
 
 	hid_destroy_device(uhid->hid);
 	kfree(uhid->rd_data);
···
 	return 0;
 }
 
+static int uhid_dev_feature_answer(struct uhid_device *uhid,
+				   struct uhid_event *ev)
+{
+	unsigned long flags;
+
+	if (!uhid->running)
+		return -EINVAL;
+
+	spin_lock_irqsave(&uhid->qlock, flags);
+
+	/* id for old report; drop it silently */
+	if (atomic_read(&uhid->report_id) != ev->u.feature_answer.id)
+		goto unlock;
+	if (atomic_read(&uhid->report_done))
+		goto unlock;
+
+	memcpy(&uhid->report_buf, ev, sizeof(*ev));
+	atomic_set(&uhid->report_done, 1);
+	wake_up_interruptible(&uhid->report_wait);
+
+unlock:
+	spin_unlock_irqrestore(&uhid->qlock, flags);
+	return 0;
+}
+
 static int uhid_char_open(struct inode *inode, struct file *file)
 {
 	struct uhid_device *uhid;
···
 		return -ENOMEM;
 
 	mutex_init(&uhid->devlock);
+	mutex_init(&uhid->report_lock);
 	spin_lock_init(&uhid->qlock);
 	init_waitqueue_head(&uhid->waitq);
+	init_waitqueue_head(&uhid->report_wait);
 	uhid->running = false;
+	atomic_set(&uhid->report_done, 1);
 
 	file->private_data = uhid;
 	nonseekable_open(inode, file);
···
 		break;
 	case UHID_INPUT:
 		ret = uhid_dev_input(uhid, &uhid->input_buf);
+		break;
+	case UHID_FEATURE_ANSWER:
+		ret = uhid_dev_feature_answer(uhid, &uhid->input_buf);
 		break;
 	default:
 		ret = -EOPNOTSUPP;

include/linux/uhid.h

···
 	UHID_OUTPUT,
 	UHID_OUTPUT_EV,
 	UHID_INPUT,
+	UHID_FEATURE,
+	UHID_FEATURE_ANSWER,
 };
 
 struct uhid_create_req {
···
 	__s32 value;
 } __attribute__((__packed__));
 
+struct uhid_feature_req {
+	__u32 id;
+	__u8 rnum;
+	__u8 rtype;
+} __attribute__((__packed__));
+
+struct uhid_feature_answer_req {
+	__u32 id;
+	__u16 err;
+	__u16 size;
+	__u8 data[UHID_DATA_MAX];
+};
+
 struct uhid_event {
 	__u32 type;
 
···
 		struct uhid_input_req input;
 		struct uhid_output_req output;
 		struct uhid_output_ev_req output_ev;
+		struct uhid_feature_req feature;
+		struct uhid_feature_answer_req feature_answer;
 	} u;
 } __attribute__((__packed__));