Root/Documentation/hwspinlock.txt

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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     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
1543. Typical usage
155
156#include <linux/hwspinlock.h>
157#include <linux/err.h>
158
159int 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
192int 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
2284. 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 sill in use).
245
2465. Important structs
247
248struct hwspinlock_device is a device which usually contains a bank
249of hardware locks. It is registered by the underlying hwspinlock
250implementation 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 */
260struct 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
268struct hwspinlock_device contains an array of hwspinlock structs, each
269of 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 */
277struct hwspinlock {
278    struct hwspinlock_device *bank;
279    spinlock_t lock;
280    void *priv;
281};
282
283When registering a bank of locks, the hwspinlock driver only needs to
284set the priv members of the locks. The rest of the members are set and
285initialized by the hwspinlock core itself.
286
2876. Implementation callbacks
288
289There are three possible callbacks defined in 'struct hwspinlock_ops':
290
291struct hwspinlock_ops {
292    int (*trylock)(struct hwspinlock *lock);
293    void (*unlock)(struct hwspinlock *lock);
294    void (*relax)(struct hwspinlock *lock);
295};
296
297The first two callbacks are mandatory:
298
299The ->trylock() callback should make a single attempt to take the lock, and
300return 0 on failure and 1 on success. This callback may _not_ sleep.
301
302The ->unlock() callback releases the lock. It always succeed, and it, too,
303may _not_ sleep.
304
305The ->relax() callback is optional. It is called by hwspinlock core while
306spinning on a lock, and can be used by the underlying implementation to force
307a delay between two successive invocations of ->trylock(). It may _not_ sleep.
308

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