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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 EST.

1	===========================
2	Hardware Spinlock Framework
3	===========================
4	
5	Introduction
6	============
7	
8	Hardware spinlock modules provide hardware assistance for synchronization
9	and mutual exclusion between heterogeneous processors and those not operating
10	under a single, shared operating system.
11	
12	For example, OMAP4 has dual Cortex-A9, dual Cortex-M3 and a C64x+ DSP,
13	each of which is running a different Operating System (the master, A9,
14	is usually running Linux and the slave processors, the M3 and the DSP,
15	are running some flavor of RTOS).
16	
17	A generic hwspinlock framework allows platform-independent drivers to use
18	the hwspinlock device in order to access data structures that are shared
19	between remote processors, that otherwise have no alternative mechanism
20	to accomplish synchronization and mutual exclusion operations.
21	
22	This is necessary, for example, for Inter-processor communications:
23	on OMAP4, cpu-intensive multimedia tasks are offloaded by the host to the
24	remote M3 and/or C64x+ slave processors (by an IPC subsystem called Syslink).
25	
26	To achieve fast message-based communications, a minimal kernel support
27	is needed to deliver messages arriving from a remote processor to the
28	appropriate user process.
29	
30	This communication is based on simple data structures that is shared between
31	the remote processors, and access to it is synchronized using the hwspinlock
32	module (remote processor directly places new messages in this shared data
33	structure).
34	
35	A common hwspinlock interface makes it possible to have generic, platform-
36	independent, drivers.
37	
38	User API
39	========
40	
41	::
42	
43	  struct hwspinlock *hwspin_lock_request(void);
44	
45	Dynamically assign an hwspinlock and return its address, or NULL
46	in case an unused hwspinlock isn't available. Users of this
47	API will usually want to communicate the lock's id to the remote core
48	before it can be used to achieve synchronization.
49	
50	Should be called from a process context (might sleep).
51	
52	::
53	
54	  struct hwspinlock *hwspin_lock_request_specific(unsigned int id);
55	
56	Assign a specific hwspinlock id and return its address, or NULL
57	if that hwspinlock is already in use. Usually board code will
58	be calling this function in order to reserve specific hwspinlock
59	ids for predefined purposes.
60	
61	Should be called from a process context (might sleep).
62	
63	::
64	
65	  int of_hwspin_lock_get_id(struct device_node *np, int index);
66	
67	Retrieve the global lock id for an OF phandle-based specific lock.
68	This function provides a means for DT users of a hwspinlock module
69	to get the global lock id of a specific hwspinlock, so that it can
70	be requested using the normal hwspin_lock_request_specific() API.
71	
72	The function returns a lock id number on success, -EPROBE_DEFER if
73	the hwspinlock device is not yet registered with the core, or other
74	error values.
75	
76	Should be called from a process context (might sleep).
77	
78	::
79	
80	  int hwspin_lock_free(struct hwspinlock *hwlock);
81	
82	Free a previously-assigned hwspinlock; returns 0 on success, or an
83	appropriate error code on failure (e.g. -EINVAL if the hwspinlock
84	is already free).
85	
86	Should be called from a process context (might sleep).
87	
88	::
89	
90	  int hwspin_lock_timeout(struct hwspinlock *hwlock, unsigned int timeout);
91	
92	Lock a previously-assigned hwspinlock with a timeout limit (specified in
93	msecs). If the hwspinlock is already taken, the function will busy loop
94	waiting for it to be released, but give up when the timeout elapses.
95	Upon a successful return from this function, preemption is disabled so
96	the caller must not sleep, and is advised to release the hwspinlock as
97	soon as possible, in order to minimize remote cores polling on the
98	hardware interconnect.
99	
100	Returns 0 when successful and an appropriate error code otherwise (most
101	notably -ETIMEDOUT if the hwspinlock is still busy after timeout msecs).
102	The function will never sleep.
103	
104	::
105	
106	  int hwspin_lock_timeout_irq(struct hwspinlock *hwlock, unsigned int timeout);
107	
108	Lock a previously-assigned hwspinlock with a timeout limit (specified in
109	msecs). If the hwspinlock is already taken, the function will busy loop
110	waiting for it to be released, but give up when the timeout elapses.
111	Upon a successful return from this function, preemption and the local
112	interrupts are disabled, so the caller must not sleep, and is advised to
113	release the hwspinlock as soon as possible.
114	
115	Returns 0 when successful and an appropriate error code otherwise (most
116	notably -ETIMEDOUT if the hwspinlock is still busy after timeout msecs).
117	The function will never sleep.
118	
119	::
120	
121	  int hwspin_lock_timeout_irqsave(struct hwspinlock *hwlock, unsigned int to,
122					  unsigned long *flags);
123	
124	Lock a previously-assigned hwspinlock with a timeout limit (specified in
125	msecs). If the hwspinlock is already taken, the function will busy loop
126	waiting for it to be released, but give up when the timeout elapses.
127	Upon a successful return from this function, preemption is disabled,
128	local interrupts are disabled and their previous state is saved at the
129	given flags placeholder. The caller must not sleep, and is advised to
130	release the hwspinlock as soon as possible.
131	
132	Returns 0 when successful and an appropriate error code otherwise (most
133	notably -ETIMEDOUT if the hwspinlock is still busy after timeout msecs).
134	
135	The function will never sleep.
136	
137	::
138	
139	  int hwspin_trylock(struct hwspinlock *hwlock);
140	
141	
142	Attempt to lock a previously-assigned hwspinlock, but immediately fail if
143	it is already taken.
144	
145	Upon a successful return from this function, preemption is disabled so
146	caller must not sleep, and is advised to release the hwspinlock as soon as
147	possible, in order to minimize remote cores polling on the hardware
148	interconnect.
149	
150	Returns 0 on success and an appropriate error code otherwise (most
151	notably -EBUSY if the hwspinlock was already taken).
152	The function will never sleep.
153	
154	::
155	
156	  int hwspin_trylock_irq(struct hwspinlock *hwlock);
157	
158	
159	Attempt to lock a previously-assigned hwspinlock, but immediately fail if
160	it is already taken.
161	
162	Upon a successful return from this function, preemption and the local
163	interrupts are disabled so caller must not sleep, and is advised to
164	release the hwspinlock as soon as possible.
165	
166	Returns 0 on success and an appropriate error code otherwise (most
167	notably -EBUSY if the hwspinlock was already taken).
168	
169	The function will never sleep.
170	
171	::
172	
173	  int hwspin_trylock_irqsave(struct hwspinlock *hwlock, unsigned long *flags);
174	
175	Attempt to lock a previously-assigned hwspinlock, but immediately fail if
176	it is already taken.
177	
178	Upon a successful return from this function, preemption is disabled,
179	the local interrupts are disabled and their previous state is saved
180	at the given flags placeholder. The caller must not sleep, and is advised
181	to release the hwspinlock as soon as possible.
182	
183	Returns 0 on success and an appropriate error code otherwise (most
184	notably -EBUSY if the hwspinlock was already taken).
185	The function will never sleep.
186	
187	::
188	
189	  void hwspin_unlock(struct hwspinlock *hwlock);
190	
191	Unlock a previously-locked hwspinlock. Always succeed, and can be called
192	from any context (the function never sleeps).
193	
194	.. note::
195	
196	  code should **never** unlock an hwspinlock which is already unlocked
197	  (there is no protection against this).
198	
199	::
200	
201	  void hwspin_unlock_irq(struct hwspinlock *hwlock);
202	
203	Unlock a previously-locked hwspinlock and enable local interrupts.
204	The caller should **never** unlock an hwspinlock which is already unlocked.
205	
206	Doing so is considered a bug (there is no protection against this).
207	Upon a successful return from this function, preemption and local
208	interrupts are enabled. This function will never sleep.
209	
210	::
211	
212	  void
213	  hwspin_unlock_irqrestore(struct hwspinlock *hwlock, unsigned long *flags);
214	
215	Unlock a previously-locked hwspinlock.
216	
217	The caller should **never** unlock an hwspinlock which is already unlocked.
218	Doing so is considered a bug (there is no protection against this).
219	Upon a successful return from this function, preemption is reenabled,
220	and the state of the local interrupts is restored to the state saved at
221	the given flags. This function will never sleep.
222	
223	::
224	
225	  int hwspin_lock_get_id(struct hwspinlock *hwlock);
226	
227	Retrieve id number of a given hwspinlock. This is needed when an
228	hwspinlock is dynamically assigned: before it can be used to achieve
229	mutual exclusion with a remote cpu, the id number should be communicated
230	to the remote task with which we want to synchronize.
231	
232	Returns the hwspinlock id number, or -EINVAL if hwlock is null.
233	
234	Typical usage
235	=============
236	
237	::
238	
239		#include <linux/hwspinlock.h>
240		#include <linux/err.h>
241	
242		int hwspinlock_example1(void)
243		{
244			struct hwspinlock *hwlock;
245			int ret;
246	
247			/* dynamically assign a hwspinlock */
248			hwlock = hwspin_lock_request();
249			if (!hwlock)
250				...
251	
252			id = hwspin_lock_get_id(hwlock);
253			/* probably need to communicate id to a remote processor now */
254	
255			/* take the lock, spin for 1 sec if it's already taken */
256			ret = hwspin_lock_timeout(hwlock, 1000);
257			if (ret)
258				...
259	
260			/*
261			* we took the lock, do our thing now, but do NOT sleep
262			*/
263	
264			/* release the lock */
265			hwspin_unlock(hwlock);
266	
267			/* free the lock */
268			ret = hwspin_lock_free(hwlock);
269			if (ret)
270				...
271	
272			return ret;
273		}
274	
275		int hwspinlock_example2(void)
276		{
277			struct hwspinlock *hwlock;
278			int ret;
279	
280			/*
281			* assign a specific hwspinlock id - this should be called early
282			* by board init code.
283			*/
284			hwlock = hwspin_lock_request_specific(PREDEFINED_LOCK_ID);
285			if (!hwlock)
286				...
287	
288			/* try to take it, but don't spin on it */
289			ret = hwspin_trylock(hwlock);
290			if (!ret) {
291				pr_info("lock is already taken\n");
292				return -EBUSY;
293			}
294	
295			/*
296			* we took the lock, do our thing now, but do NOT sleep
297			*/
298	
299			/* release the lock */
300			hwspin_unlock(hwlock);
301	
302			/* free the lock */
303			ret = hwspin_lock_free(hwlock);
304			if (ret)
305				...
306	
307			return ret;
308		}
309	
310	
311	API for implementors
312	====================
313	
314	::
315	
316	  int hwspin_lock_register(struct hwspinlock_device *bank, struct device *dev,
317			const struct hwspinlock_ops *ops, int base_id, int num_locks);
318	
319	To be called from the underlying platform-specific implementation, in
320	order to register a new hwspinlock device (which is usually a bank of
321	numerous locks). Should be called from a process context (this function
322	might sleep).
323	
324	Returns 0 on success, or appropriate error code on failure.
325	
326	::
327	
328	  int hwspin_lock_unregister(struct hwspinlock_device *bank);
329	
330	To be called from the underlying vendor-specific implementation, in order
331	to unregister an hwspinlock device (which is usually a bank of numerous
332	locks).
333	
334	Should be called from a process context (this function might sleep).
335	
336	Returns the address of hwspinlock on success, or NULL on error (e.g.
337	if the hwspinlock is still in use).
338	
339	Important structs
340	=================
341	
342	struct hwspinlock_device is a device which usually contains a bank
343	of hardware locks. It is registered by the underlying hwspinlock
344	implementation using the hwspin_lock_register() API.
345	
346	::
347	
348		/**
349		* struct hwspinlock_device - a device which usually spans numerous hwspinlocks
350		* @dev: underlying device, will be used to invoke runtime PM api
351		* @ops: platform-specific hwspinlock handlers
352		* @base_id: id index of the first lock in this device
353		* @num_locks: number of locks in this device
354		* @lock: dynamically allocated array of 'struct hwspinlock'
355		*/
356		struct hwspinlock_device {
357			struct device *dev;
358			const struct hwspinlock_ops *ops;
359			int base_id;
360			int num_locks;
361			struct hwspinlock lock[0];
362		};
363	
364	struct hwspinlock_device contains an array of hwspinlock structs, each
365	of which represents a single hardware lock::
366	
367		/**
368		* struct hwspinlock - this struct represents a single hwspinlock instance
369		* @bank: the hwspinlock_device structure which owns this lock
370		* @lock: initialized and used by hwspinlock core
371		* @priv: private data, owned by the underlying platform-specific hwspinlock drv
372		*/
373		struct hwspinlock {
374			struct hwspinlock_device *bank;
375			spinlock_t lock;
376			void *priv;
377		};
378	
379	When registering a bank of locks, the hwspinlock driver only needs to
380	set the priv members of the locks. The rest of the members are set and
381	initialized by the hwspinlock core itself.
382	
383	Implementation callbacks
384	========================
385	
386	There are three possible callbacks defined in 'struct hwspinlock_ops'::
387	
388		struct hwspinlock_ops {
389			int (*trylock)(struct hwspinlock *lock);
390			void (*unlock)(struct hwspinlock *lock);
391			void (*relax)(struct hwspinlock *lock);
392		};
393	
394	The first two callbacks are mandatory:
395	
396	The ->trylock() callback should make a single attempt to take the lock, and
397	return 0 on failure and 1 on success. This callback may **not** sleep.
398	
399	The ->unlock() callback releases the lock. It always succeed, and it, too,
400	may **not** sleep.
401	
402	The ->relax() callback is optional. It is called by hwspinlock core while
403	spinning on a lock, and can be used by the underlying implementation to force
404	a delay between two successive invocations of ->trylock(). It may **not** sleep.
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