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1Hardware Spinlock Framework
2
31. Introduction
4
5Hardware spinlock modules provide hardware assistance for synchronization
6and mutual exclusion between heterogeneous processors and those not operating
7under a single, shared operating system.
8
9For example, OMAP4 has dual Cortex-A9, dual Cortex-M3 and a C64x+ DSP,
10each of which is running a different Operating System (the master, A9,
11is usually running Linux and the slave processors, the M3 and the DSP,
12are running some flavor of RTOS).
13
14A generic hwspinlock framework allows platform-independent drivers to use
15the hwspinlock device in order to access data structures that are shared
16between remote processors, that otherwise have no alternative mechanism
17to accomplish synchronization and mutual exclusion operations.
18
19This is necessary, for example, for Inter-processor communications:
20on OMAP4, cpu-intensive multimedia tasks are offloaded by the host to the
21remote M3 and/or C64x+ slave processors (by an IPC subsystem called Syslink).
22
23To achieve fast message-based communications, a minimal kernel support
24is needed to deliver messages arriving from a remote processor to the
25appropriate user process.
26
27This communication is based on simple data structures that is shared between
28the remote processors, and access to it is synchronized using the hwspinlock
29module (remote processor directly places new messages in this shared data
30structure).
31
32A common hwspinlock interface makes it possible to have generic, platform-
33independent, drivers.
34
352. User API
36
37 struct hwspinlock *hwspin_lock_request(void);
38 - dynamically assign an hwspinlock and return its address, or NULL
39 in case an unused hwspinlock isn't available. Users of this
40 API will usually want to communicate the lock's id to the remote core
41 before it can be used to achieve synchronization.
42 Can be called from an atomic context (this function will not sleep) but
43 not from within interrupt context.
44
45 struct hwspinlock *hwspin_lock_request_specific(unsigned int id);
46 - assign a specific hwspinlock id and return its address, or NULL
47 if that hwspinlock is already in use. Usually board code will
48 be calling this function in order to reserve specific hwspinlock
49 ids for predefined purposes.
50 Can be called from an atomic context (this function will not sleep) but
51 not from within interrupt context.
52
53 int hwspin_lock_free(struct hwspinlock *hwlock);
54 - free a previously-assigned hwspinlock; returns 0 on success, or an
55 appropriate error code on failure (e.g. -EINVAL if the hwspinlock
56 is already free).
57 Can be called from an atomic context (this function will not sleep) but
58 not from within interrupt context.
59
60 int hwspin_lock_timeout(struct hwspinlock *hwlock, unsigned int timeout);
61 - lock a previously-assigned hwspinlock with a timeout limit (specified in
62 msecs). If the hwspinlock is already taken, the function will busy loop
63 waiting for it to be released, but give up when the timeout elapses.
64 Upon a successful return from this function, preemption is disabled so
65 the caller must not sleep, and is advised to release the hwspinlock as
66 soon as possible, in order to minimize remote cores polling on the
67 hardware interconnect.
68 Returns 0 when successful and an appropriate error code otherwise (most
69 notably -ETIMEDOUT if the hwspinlock is still busy after timeout msecs).
70 The function will never sleep.
71
72 int hwspin_lock_timeout_irq(struct hwspinlock *hwlock, unsigned int timeout);
73 - lock a previously-assigned hwspinlock with a timeout limit (specified in
74 msecs). If the hwspinlock is already taken, the function will busy loop
75 waiting for it to be released, but give up when the timeout elapses.
76 Upon a successful return from this function, preemption and the local
77 interrupts are disabled, so the caller must not sleep, and is advised to
78 release the hwspinlock as soon as possible.
79 Returns 0 when successful and an appropriate error code otherwise (most
80 notably -ETIMEDOUT if the hwspinlock is still busy after timeout msecs).
81 The function will never sleep.
82
83 int hwspin_lock_timeout_irqsave(struct hwspinlock *hwlock, unsigned int to,
84 unsigned long *flags);
85 - lock a previously-assigned hwspinlock with a timeout limit (specified in
86 msecs). If the hwspinlock is already taken, the function will busy loop
87 waiting for it to be released, but give up when the timeout elapses.
88 Upon a successful return from this function, preemption is disabled,
89 local interrupts are disabled and their previous state is saved at the
90 given flags placeholder. The caller must not sleep, and is advised to
91 release the hwspinlock as soon as possible.
92 Returns 0 when successful and an appropriate error code otherwise (most
93 notably -ETIMEDOUT if the hwspinlock is still busy after timeout msecs).
94 The function will never sleep.
95
96 int hwspin_trylock(struct hwspinlock *hwlock);
97 - attempt to lock a previously-assigned hwspinlock, but immediately fail if
98 it is already taken.
99 Upon a successful return from this function, preemption is disabled so
100 caller must not sleep, and is advised to release the hwspinlock as soon as
101 possible, in order to minimize remote cores polling on the hardware
102 interconnect.
103 Returns 0 on success and an appropriate error code otherwise (most
104 notably -EBUSY if the hwspinlock was already taken).
105 The function will never sleep.
106
107 int hwspin_trylock_irq(struct hwspinlock *hwlock);
108 - attempt to lock a previously-assigned hwspinlock, but immediately fail if
109 it is already taken.
110 Upon a successful return from this function, preemption and the local
111 interrupts are disabled so caller must not sleep, and is advised to
112 release the hwspinlock as soon as possible.
113 Returns 0 on success and an appropriate error code otherwise (most
114 notably -EBUSY if the hwspinlock was already taken).
115 The function will never sleep.
116
117 int hwspin_trylock_irqsave(struct hwspinlock *hwlock, unsigned long *flags);
118 - attempt to lock a previously-assigned hwspinlock, but immediately fail if
119 it is already taken.
120 Upon a successful return from this function, preemption is disabled,
121 the local interrupts are disabled and their previous state is saved
122 at the given flags placeholder. The caller must not sleep, and is advised
123 to release the hwspinlock as soon as possible.
124 Returns 0 on success and an appropriate error code otherwise (most
125 notably -EBUSY if the hwspinlock was already taken).
126 The function will never sleep.
127
128 void hwspin_unlock(struct hwspinlock *hwlock);
129 - unlock a previously-locked hwspinlock. Always succeed, and can be called
130 from any context (the function never sleeps). Note: code should _never_
131 unlock an hwspinlock which is already unlocked (there is no protection
132 against this).
133
134 void hwspin_unlock_irq(struct hwspinlock *hwlock);
135 - unlock a previously-locked hwspinlock and enable local interrupts.
136 The caller should _never_ unlock an hwspinlock which is already unlocked.
137 Doing so is considered a bug (there is no protection against this).
138 Upon a successful return from this function, preemption and local
139 interrupts are enabled. This function will never sleep.
140
141 void
142 hwspin_unlock_irqrestore(struct hwspinlock *hwlock, unsigned long *flags);
143 - unlock a previously-locked hwspinlock.
144 The caller should _never_ unlock an hwspinlock which is already unlocked.
145 Doing so is considered a bug (there is no protection against this).
146 Upon a successful return from this function, preemption is reenabled,
147 and the state of the local interrupts is restored to the state saved at
148 the given flags. This function will never sleep.
149
150 int hwspin_lock_get_id(struct hwspinlock *hwlock);
151 - retrieve id number of a given hwspinlock. This is needed when an
152 hwspinlock is dynamically assigned: before it can be used to achieve
153 mutual exclusion with a remote cpu, the id number should be communicated
154 to the remote task with which we want to synchronize.
155 Returns the hwspinlock id number, or -EINVAL if hwlock is null.
156
1573. Typical usage
158
159#include <linux/hwspinlock.h>
160#include <linux/err.h>
161
162int hwspinlock_example1(void)
163{
164 struct hwspinlock *hwlock;
165 int ret;
166
167 /* dynamically assign a hwspinlock */
168 hwlock = hwspin_lock_request();
169 if (!hwlock)
170 ...
171
172 id = hwspin_lock_get_id(hwlock);
173 /* probably need to communicate id to a remote processor now */
174
175 /* take the lock, spin for 1 sec if it's already taken */
176 ret = hwspin_lock_timeout(hwlock, 1000);
177 if (ret)
178 ...
179
180 /*
181 * we took the lock, do our thing now, but do NOT sleep
182 */
183
184 /* release the lock */
185 hwspin_unlock(hwlock);
186
187 /* free the lock */
188 ret = hwspin_lock_free(hwlock);
189 if (ret)
190 ...
191
192 return ret;
193}
194
195int hwspinlock_example2(void)
196{
197 struct hwspinlock *hwlock;
198 int ret;
199
200 /*
201 * assign a specific hwspinlock id - this should be called early
202 * by board init code.
203 */
204 hwlock = hwspin_lock_request_specific(PREDEFINED_LOCK_ID);
205 if (!hwlock)
206 ...
207
208 /* try to take it, but don't spin on it */
209 ret = hwspin_trylock(hwlock);
210 if (!ret) {
211 pr_info("lock is already taken\n");
212 return -EBUSY;
213 }
214
215 /*
216 * we took the lock, do our thing now, but do NOT sleep
217 */
218
219 /* release the lock */
220 hwspin_unlock(hwlock);
221
222 /* free the lock */
223 ret = hwspin_lock_free(hwlock);
224 if (ret)
225 ...
226
227 return ret;
228}
229
230
2314. API for implementors
232
233 int hwspin_lock_register(struct hwspinlock *hwlock);
234 - to be called from the underlying platform-specific implementation, in
235 order to register a new hwspinlock instance. Can be called from an atomic
236 context (this function will not sleep) but not from within interrupt
237 context. Returns 0 on success, or appropriate error code on failure.
238
239 struct hwspinlock *hwspin_lock_unregister(unsigned int id);
240 - to be called from the underlying vendor-specific implementation, in order
241 to unregister an existing (and unused) hwspinlock instance.
242 Can be called from an atomic context (will not sleep) but not from
243 within interrupt context.
244 Returns the address of hwspinlock on success, or NULL on error (e.g.
245 if the hwspinlock is sill in use).
246
2475. struct hwspinlock
248
249This struct represents an hwspinlock instance. It is registered by the
250underlying hwspinlock implementation using the hwspin_lock_register() API.
251
252/**
253 * struct hwspinlock - vendor-specific hwspinlock implementation
254 *
255 * @dev: underlying device, will be used with runtime PM api
256 * @ops: vendor-specific hwspinlock handlers
257 * @id: a global, unique, system-wide, index of the lock.
258 * @lock: initialized and used by hwspinlock core
259 * @owner: underlying implementation module, used to maintain module ref count
260 */
261struct hwspinlock {
262 struct device *dev;
263 const struct hwspinlock_ops *ops;
264 int id;
265 spinlock_t lock;
266 struct module *owner;
267};
268
269The underlying implementation is responsible to assign the dev, ops, id and
270owner members. The lock member, OTOH, is initialized and used by the hwspinlock
271core.
272
2736. Implementation callbacks
274
275There are three possible callbacks defined in 'struct hwspinlock_ops':
276
277struct hwspinlock_ops {
278 int (*trylock)(struct hwspinlock *lock);
279 void (*unlock)(struct hwspinlock *lock);
280 void (*relax)(struct hwspinlock *lock);
281};
282
283The first two callbacks are mandatory:
284
285The ->trylock() callback should make a single attempt to take the lock, and
286return 0 on failure and 1 on success. This callback may _not_ sleep.
287
288The ->unlock() callback releases the lock. It always succeed, and it, too,
289may _not_ sleep.
290
291The ->relax() callback is optional. It is called by hwspinlock core while
292spinning on a lock, and can be used by the underlying implementation to force
293a delay between two successive invocations of ->trylock(). It may _not_ sleep.