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