HID: uhid: add internal message buffer

When receiving messages from the HID subsystem, we need to process them
and store them in an internal buffer so user-space can read() on the char
device to retrieve the messages.

This adds a static buffer for 32 messages to each uhid device. Each
message is dynamically allocated so the uhid_device structure does not get
too big.

uhid_queue() adds a message to the buffer. If the buffer is full, the
message is discarded. uhid_queue_event() is an helper for messages without
payload.

This also adds a public header: uhid.h. It contains the declarations for
the user-space API. It is built around "struct uhid_event" which contains
a type field which specifies the event type and each event can then add a
variable-length payload. For now, there is only a dummy event but later
patches will add new event types and payloads.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/hid/uhid.c

@@ -25,16 +25,81 @@
 #include <linux/wait.h>
 
 #define UHID_NAME	"uhid"
+#define UHID_BUFSIZE	32
+
+struct uhid_device {
+	struct hid_device *hid;
+
+	wait_queue_head_t waitq;
+	spinlock_t qlock;
+	__u8 head;
+	__u8 tail;
+	struct uhid_event *outq[UHID_BUFSIZE];
+};
 
 static struct miscdevice uhid_misc;
 
+static void uhid_queue(struct uhid_device *uhid, struct uhid_event *ev)
+{
+	__u8 newhead;
+
+	newhead = (uhid->head + 1) % UHID_BUFSIZE;
+
+	if (newhead != uhid->tail) {
+		uhid->outq[uhid->head] = ev;
+		uhid->head = newhead;
+		wake_up_interruptible(&uhid->waitq);
+	} else {
+		hid_warn(uhid->hid, "Output queue is full\n");
+		kfree(ev);
+	}
+}
+
+static int uhid_queue_event(struct uhid_device *uhid, __u32 event)
+{
+	unsigned long flags;
+	struct uhid_event *ev;
+
+	ev = kzalloc(sizeof(*ev), GFP_KERNEL);
+	if (!ev)
+		return -ENOMEM;
+
+	ev->type = event;
+
+	spin_lock_irqsave(&uhid->qlock, flags);
+	uhid_queue(uhid, ev);
+	spin_unlock_irqrestore(&uhid->qlock, flags);
+
+	return 0;
+}
+
 static int uhid_char_open(struct inode *inode, struct file *file)
 {
+	struct uhid_device *uhid;
+
+	uhid = kzalloc(sizeof(*uhid), GFP_KERNEL);
+	if (!uhid)
+		return -ENOMEM;
+
+	spin_lock_init(&uhid->qlock);
+	init_waitqueue_head(&uhid->waitq);
+
+	file->private_data = uhid;
+	nonseekable_open(inode, file);
+
 	return 0;
 }
 
 static int uhid_char_release(struct inode *inode, struct file *file)
 {
+	struct uhid_device *uhid = file->private_data;
+	unsigned int i;
+
+	for (i = 0; i < UHID_BUFSIZE; ++i)
+		kfree(uhid->outq[i]);
+
+	kfree(uhid);
+
 	return 0;
 }
 

include/linux/Kbuild

@@ -373,6 +373,7 @@ header-y += tty.h
 header-y += types.h
 header-y += udf_fs_i.h
 header-y += udp.h
+header-y += uhid.h
 header-y += uinput.h
 header-y += uio.h
 header-y += ultrasound.h

include/linux/uhid.h

@@ -0,0 +1,33 @@
+#ifndef __UHID_H_
+#define __UHID_H_
+
+/*
+ * User-space I/O driver support for HID subsystem
+ * Copyright (c) 2012 David Herrmann
+ */
+
+/*
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the Free
+ * Software Foundation; either version 2 of the License, or (at your option)
+ * any later version.
+ */
+
+/*
+ * Public header for user-space communication. We try to keep every structure
+ * aligned but to be safe we also use __attribute__((__packed__)). Therefore,
+ * the communication should be ABI compatible even between architectures.
+ */
+
+#include <linux/input.h>
+#include <linux/types.h>
+
+enum uhid_event_type {
+	UHID_DUMMY,
+};
+
+struct uhid_event {
+	__u32 type;
+} __attribute__((__packed__));
+
+#endif /* __UHID_H_ */