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