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