pcmcia: add locking documentation

Tested-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>

Documentation/pcmcia/locking.txt

+This file explains the locking and exclusion scheme used in the PCCARD
+and PCMCIA subsystems.
+
+
+A) Overview, Locking Hierarchy:
+===============================
+
+pcmcia_socket_list_rwsem	- protects only the list of sockets
+- skt_mutex			- serializes card insert / ejection
+  - ops_mutex			- serializes socket operation
+
+
+B) Exclusion
+============
+
+The following functions and callbacks to struct pcmcia_socket must
+be called with "skt_mutex" held:
+
+	socket_detect_change()
+	send_event()
+	socket_reset()
+	socket_shutdown()
+	socket_setup()
+	socket_remove()
+	socket_insert()
+	socket_early_resume()
+	socket_late_resume()
+	socket_resume()
+	socket_suspend()
+
+	struct pcmcia_callback	*callback
+
+The following functions and callbacks to struct pcmcia_socket must
+be called with "ops_mutex" held:
+
+	socket_reset()
+	socket_setup()
+
+	struct pccard_operations *ops
+
+Note that send_event() and struct pcmcia_callback *callback must not be
+called with "ops_mutex" held.
+
+
+C) Protection
+=============
+
+1. Global Data:
+---------------
+struct list_head	pcmcia_socket_list;
+
+protected by pcmcia_socket_list_rwsem;
+
+
+2. Per-Socket Data:
+-------------------
+The resource_ops are on their own to provide proper locking.
+
+The "main" struct pcmcia_socket is protected as follows (read-only fields
+or single-use fields not mentioned):
+
+- by pcmcia_socket_list_rwsem:
+	struct list_head	socket_list;
+
+- by thread_lock:
+	unsigned int		thread_events;
+
+- by skt_mutex:
+	u_int			suspended_state;
+	void			(*tune_bridge);
+	struct pcmcia_callback	*callback;
+	int			resume_status;
+
+- by ops_mutex:
+	socket_state_t		socket;
+	u_int			state;
+	u_short			lock_count;
+	pccard_mem_map		cis_mem;
+	void __iomem 		*cis_virt;
+	struct { }		irq;
+	io_window_t		io[];
+	pccard_mem_map		win[];
+	struct list_head	cis_cache;
+	size_t			fake_cis_len;
+	u8			*fake_cis;
+	u_int			irq_mask;
+	void 			(*zoom_video);
+	int 			(*power_hook);
+	u8			resource...;
+	struct list_head	devices_list;
+	u8			device_count;
+	struct 			pcmcia_state;