linux/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 *hwlock);
 231   - to be called from the underlying platform-specific implementation, in
 232     order to register a new hwspinlock instance. Should be called from
 233     a process context (this function might sleep).
 234     Returns 0 on success, or appropriate error code on failure.
 235
 236  struct hwspinlock *hwspin_lock_unregister(unsigned int id);
 237   - to be called from the underlying vendor-specific implementation, in order
 238     to unregister an existing (and unused) hwspinlock instance.
 239     Should be called from a process context (this function might sleep).
 240     Returns the address of hwspinlock on success, or NULL on error (e.g.
 241     if the hwspinlock is sill in use).
 242
 2435. struct hwspinlock
 244
 245This struct represents an hwspinlock instance. It is registered by the
 246underlying hwspinlock implementation using the hwspin_lock_register() API.
 247
 248/**
 249 * struct hwspinlock - vendor-specific hwspinlock implementation
 250 *
 251 * @dev: underlying device, will be used with runtime PM api
 252 * @ops: vendor-specific hwspinlock handlers
 253 * @id: a global, unique, system-wide, index of the lock.
 254 * @lock: initialized and used by hwspinlock core
 255 * @owner: underlying implementation module, used to maintain module ref count
 256 */
 257struct hwspinlock {
 258        struct device *dev;
 259        const struct hwspinlock_ops *ops;
 260        int id;
 261        spinlock_t lock;
 262        struct module *owner;
 263};
 264
 265The underlying implementation is responsible to assign the dev, ops, id and
 266owner members. The lock member, OTOH, is initialized and used by the hwspinlock
 267core.
 268
 2696. Implementation callbacks
 270
 271There are three possible callbacks defined in 'struct hwspinlock_ops':
 272
 273struct hwspinlock_ops {
 274        int (*trylock)(struct hwspinlock *lock);
 275        void (*unlock)(struct hwspinlock *lock);
 276        void (*relax)(struct hwspinlock *lock);
 277};
 278
 279The first two callbacks are mandatory:
 280
 281The ->trylock() callback should make a single attempt to take the lock, and
 282return 0 on failure and 1 on success. This callback may _not_ sleep.
 283
 284The ->unlock() callback releases the lock. It always succeed, and it, too,
 285may _not_ sleep.
 286
 287The ->relax() callback is optional. It is called by hwspinlock core while
 288spinning on a lock, and can be used by the underlying implementation to force
 289a delay between two successive invocations of ->trylock(). It may _not_ sleep.
 290
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