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diff --git a/kernel/Documentation/hwspinlock.txt b/kernel/Documentation/hwspinlock.txt new file mode 100644 index 000000000..62f7d4ea6 --- /dev/null +++ b/kernel/Documentation/hwspinlock.txt @@ -0,0 +1,307 @@ +Hardware Spinlock Framework + +1. Introduction + +Hardware spinlock modules provide hardware assistance for synchronization +and mutual exclusion between heterogeneous processors and those not operating +under a single, shared operating system. + +For example, OMAP4 has dual Cortex-A9, dual Cortex-M3 and a C64x+ DSP, +each of which is running a different Operating System (the master, A9, +is usually running Linux and the slave processors, the M3 and the DSP, +are running some flavor of RTOS). + +A generic hwspinlock framework allows platform-independent drivers to use +the hwspinlock device in order to access data structures that are shared +between remote processors, that otherwise have no alternative mechanism +to accomplish synchronization and mutual exclusion operations. + +This is necessary, for example, for Inter-processor communications: +on OMAP4, cpu-intensive multimedia tasks are offloaded by the host to the +remote M3 and/or C64x+ slave processors (by an IPC subsystem called Syslink). + +To achieve fast message-based communications, a minimal kernel support +is needed to deliver messages arriving from a remote processor to the +appropriate user process. + +This communication is based on simple data structures that is shared between +the remote processors, and access to it is synchronized using the hwspinlock +module (remote processor directly places new messages in this shared data +structure). + +A common hwspinlock interface makes it possible to have generic, platform- +independent, drivers. + +2. User API + + struct hwspinlock *hwspin_lock_request(void); + - dynamically assign an hwspinlock and return its address, or NULL + in case an unused hwspinlock isn't available. Users of this + API will usually want to communicate the lock's id to the remote core + before it can be used to achieve synchronization. + Should be called from a process context (might sleep). + + struct hwspinlock *hwspin_lock_request_specific(unsigned int id); + - assign a specific hwspinlock id and return its address, or NULL + if that hwspinlock is already in use. Usually board code will + be calling this function in order to reserve specific hwspinlock + ids for predefined purposes. + Should be called from a process context (might sleep). + + int hwspin_lock_free(struct hwspinlock *hwlock); + - free a previously-assigned hwspinlock; returns 0 on success, or an + appropriate error code on failure (e.g. -EINVAL if the hwspinlock + is already free). + Should be called from a process context (might sleep). + + int hwspin_lock_timeout(struct hwspinlock *hwlock, unsigned int timeout); + - lock a previously-assigned hwspinlock with a timeout limit (specified in + msecs). If the hwspinlock is already taken, the function will busy loop + waiting for it to be released, but give up when the timeout elapses. + Upon a successful return from this function, preemption is disabled so + the caller must not sleep, and is advised to release the hwspinlock as + soon as possible, in order to minimize remote cores polling on the + hardware interconnect. + Returns 0 when successful and an appropriate error code otherwise (most + notably -ETIMEDOUT if the hwspinlock is still busy after timeout msecs). + The function will never sleep. + + int hwspin_lock_timeout_irq(struct hwspinlock *hwlock, unsigned int timeout); + - lock a previously-assigned hwspinlock with a timeout limit (specified in + msecs). If the hwspinlock is already taken, the function will busy loop + waiting for it to be released, but give up when the timeout elapses. + Upon a successful return from this function, preemption and the local + interrupts are disabled, so the caller must not sleep, and is advised to + release the hwspinlock as soon as possible. + Returns 0 when successful and an appropriate error code otherwise (most + notably -ETIMEDOUT if the hwspinlock is still busy after timeout msecs). + The function will never sleep. + + int hwspin_lock_timeout_irqsave(struct hwspinlock *hwlock, unsigned int to, + unsigned long *flags); + - lock a previously-assigned hwspinlock with a timeout limit (specified in + msecs). If the hwspinlock is already taken, the function will busy loop + waiting for it to be released, but give up when the timeout elapses. + Upon a successful return from this function, preemption is disabled, + local interrupts are disabled and their previous state is saved at the + given flags placeholder. The caller must not sleep, and is advised to + release the hwspinlock as soon as possible. + Returns 0 when successful and an appropriate error code otherwise (most + notably -ETIMEDOUT if the hwspinlock is still busy after timeout msecs). + The function will never sleep. + + int hwspin_trylock(struct hwspinlock *hwlock); + - attempt to lock a previously-assigned hwspinlock, but immediately fail if + it is already taken. + Upon a successful return from this function, preemption is disabled so + caller must not sleep, and is advised to release the hwspinlock as soon as + possible, in order to minimize remote cores polling on the hardware + interconnect. + Returns 0 on success and an appropriate error code otherwise (most + notably -EBUSY if the hwspinlock was already taken). + The function will never sleep. + + int hwspin_trylock_irq(struct hwspinlock *hwlock); + - attempt to lock a previously-assigned hwspinlock, but immediately fail if + it is already taken. + Upon a successful return from this function, preemption and the local + interrupts are disabled so caller must not sleep, and is advised to + release the hwspinlock as soon as possible. + Returns 0 on success and an appropriate error code otherwise (most + notably -EBUSY if the hwspinlock was already taken). + The function will never sleep. + + int hwspin_trylock_irqsave(struct hwspinlock *hwlock, unsigned long *flags); + - attempt to lock a previously-assigned hwspinlock, but immediately fail if + it is already taken. + Upon a successful return from this function, preemption is disabled, + the local interrupts are disabled and their previous state is saved + at the given flags placeholder. The caller must not sleep, and is advised + to release the hwspinlock as soon as possible. + Returns 0 on success and an appropriate error code otherwise (most + notably -EBUSY if the hwspinlock was already taken). + The function will never sleep. + + void hwspin_unlock(struct hwspinlock *hwlock); + - unlock a previously-locked hwspinlock. Always succeed, and can be called + from any context (the function never sleeps). Note: code should _never_ + unlock an hwspinlock which is already unlocked (there is no protection + against this). + + void hwspin_unlock_irq(struct hwspinlock *hwlock); + - unlock a previously-locked hwspinlock and enable local interrupts. + The caller should _never_ unlock an hwspinlock which is already unlocked. + Doing so is considered a bug (there is no protection against this). + Upon a successful return from this function, preemption and local + interrupts are enabled. This function will never sleep. + + void + hwspin_unlock_irqrestore(struct hwspinlock *hwlock, unsigned long *flags); + - unlock a previously-locked hwspinlock. + The caller should _never_ unlock an hwspinlock which is already unlocked. + Doing so is considered a bug (there is no protection against this). + Upon a successful return from this function, preemption is reenabled, + and the state of the local interrupts is restored to the state saved at + the given flags. This function will never sleep. + + int hwspin_lock_get_id(struct hwspinlock *hwlock); + - retrieve id number of a given hwspinlock. This is needed when an + hwspinlock is dynamically assigned: before it can be used to achieve + mutual exclusion with a remote cpu, the id number should be communicated + to the remote task with which we want to synchronize. + Returns the hwspinlock id number, or -EINVAL if hwlock is null. + +3. Typical usage + +#include <linux/hwspinlock.h> +#include <linux/err.h> + +int hwspinlock_example1(void) +{ + struct hwspinlock *hwlock; + int ret; + + /* dynamically assign a hwspinlock */ + hwlock = hwspin_lock_request(); + if (!hwlock) + ... + + id = hwspin_lock_get_id(hwlock); + /* probably need to communicate id to a remote processor now */ + + /* take the lock, spin for 1 sec if it's already taken */ + ret = hwspin_lock_timeout(hwlock, 1000); + if (ret) + ... + + /* + * we took the lock, do our thing now, but do NOT sleep + */ + + /* release the lock */ + hwspin_unlock(hwlock); + + /* free the lock */ + ret = hwspin_lock_free(hwlock); + if (ret) + ... + + return ret; +} + +int hwspinlock_example2(void) +{ + struct hwspinlock *hwlock; + int ret; + + /* + * assign a specific hwspinlock id - this should be called early + * by board init code. + */ + hwlock = hwspin_lock_request_specific(PREDEFINED_LOCK_ID); + if (!hwlock) + ... + + /* try to take it, but don't spin on it */ + ret = hwspin_trylock(hwlock); + if (!ret) { + pr_info("lock is already taken\n"); + return -EBUSY; + } + + /* + * we took the lock, do our thing now, but do NOT sleep + */ + + /* release the lock */ + hwspin_unlock(hwlock); + + /* free the lock */ + ret = hwspin_lock_free(hwlock); + if (ret) + ... + + return ret; +} + + +4. API for implementors + + int hwspin_lock_register(struct hwspinlock_device *bank, struct device *dev, + const struct hwspinlock_ops *ops, int base_id, int num_locks); + - to be called from the underlying platform-specific implementation, in + order to register a new hwspinlock device (which is usually a bank of + numerous locks). Should be called from a process context (this function + might sleep). + Returns 0 on success, or appropriate error code on failure. + + int hwspin_lock_unregister(struct hwspinlock_device *bank); + - to be called from the underlying vendor-specific implementation, in order + to unregister an hwspinlock device (which is usually a bank of numerous + locks). + Should be called from a process context (this function might sleep). + Returns the address of hwspinlock on success, or NULL on error (e.g. + if the hwspinlock is still in use). + +5. Important structs + +struct hwspinlock_device is a device which usually contains a bank +of hardware locks. It is registered by the underlying hwspinlock +implementation using the hwspin_lock_register() API. + +/** + * struct hwspinlock_device - a device which usually spans numerous hwspinlocks + * @dev: underlying device, will be used to invoke runtime PM api + * @ops: platform-specific hwspinlock handlers + * @base_id: id index of the first lock in this device + * @num_locks: number of locks in this device + * @lock: dynamically allocated array of 'struct hwspinlock' + */ +struct hwspinlock_device { + struct device *dev; + const struct hwspinlock_ops *ops; + int base_id; + int num_locks; + struct hwspinlock lock[0]; +}; + +struct hwspinlock_device contains an array of hwspinlock structs, each +of which represents a single hardware lock: + +/** + * struct hwspinlock - this struct represents a single hwspinlock instance + * @bank: the hwspinlock_device structure which owns this lock + * @lock: initialized and used by hwspinlock core + * @priv: private data, owned by the underlying platform-specific hwspinlock drv + */ +struct hwspinlock { + struct hwspinlock_device *bank; + spinlock_t lock; + void *priv; +}; + +When registering a bank of locks, the hwspinlock driver only needs to +set the priv members of the locks. The rest of the members are set and +initialized by the hwspinlock core itself. + +6. Implementation callbacks + +There are three possible callbacks defined in 'struct hwspinlock_ops': + +struct hwspinlock_ops { + int (*trylock)(struct hwspinlock *lock); + void (*unlock)(struct hwspinlock *lock); + void (*relax)(struct hwspinlock *lock); +}; + +The first two callbacks are mandatory: + +The ->trylock() callback should make a single attempt to take the lock, and +return 0 on failure and 1 on success. This callback may _not_ sleep. + +The ->unlock() callback releases the lock. It always succeed, and it, too, +may _not_ sleep. + +The ->relax() callback is optional. It is called by hwspinlock core while +spinning on a lock, and can be used by the underlying implementation to force +a delay between two successive invocations of ->trylock(). It may _not_ sleep. |