Documentation/hwspinlock.txt - fp2-dev/kernel/msm - Gitiles

 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.
      Can be called from an atomic context (this function will not sleep) but
      not from within interrupt context.

   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.
      Can be called from an atomic context (this function will not sleep) but
      not from within interrupt context.

   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).
      Can be called from an atomic context (this function will not sleep) but
      not from within interrupt context.

   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 *hwlock);
    - to be called from the underlying platform-specific implementation, in
      order to register a new hwspinlock instance. Can be called from an atomic
      context (this function will not sleep) but not from within interrupt
      context. Returns 0 on success, or appropriate error code on failure.

   struct hwspinlock *hwspin_lock_unregister(unsigned int id);
    - to be called from the underlying vendor-specific implementation, in order
      to unregister an existing (and unused) hwspinlock instance.
      Can be called from an atomic context (will not sleep) but not from
      within interrupt context.
      Returns the address of hwspinlock on success, or NULL on error (e.g.
      if the hwspinlock is sill in use).

 5. struct hwspinlock

 This struct represents an hwspinlock instance. It is registered by the
 underlying hwspinlock implementation using the hwspin_lock_register() API.

 /**
  * struct hwspinlock - vendor-specific hwspinlock implementation
  *
  * @dev: underlying device, will be used with runtime PM api
  * @ops: vendor-specific hwspinlock handlers
  * @id: a global, unique, system-wide, index of the lock.
  * @lock: initialized and used by hwspinlock core
  * @owner: underlying implementation module, used to maintain module ref count
  */
 struct hwspinlock {
 	struct device *dev;
 	const struct hwspinlock_ops *ops;
 	int id;
 	spinlock_t lock;
 	struct module *owner;
 };

 The underlying implementation is responsible to assign the dev, ops, id and
 owner members. The lock member, OTOH, is initialized and used by the hwspinlock
 core.

 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.
	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.
	Can be called from an atomic context (this function will not sleep) but
	not from within interrupt context.

	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.
	Can be called from an atomic context (this function will not sleep) but
	not from within interrupt context.

	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).
	Can be called from an atomic context (this function will not sleep) but
	not from within interrupt context.

	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 *hwlock);
	- to be called from the underlying platform-specific implementation, in
	order to register a new hwspinlock instance. Can be called from an atomic
	context (this function will not sleep) but not from within interrupt
	context. Returns 0 on success, or appropriate error code on failure.

	struct hwspinlock *hwspin_lock_unregister(unsigned int id);
	- to be called from the underlying vendor-specific implementation, in order
	to unregister an existing (and unused) hwspinlock instance.
	Can be called from an atomic context (will not sleep) but not from
	within interrupt context.
	Returns the address of hwspinlock on success, or NULL on error (e.g.
	if the hwspinlock is sill in use).

	5. struct hwspinlock

	This struct represents an hwspinlock instance. It is registered by the
	underlying hwspinlock implementation using the hwspin_lock_register() API.

	/**
	* struct hwspinlock - vendor-specific hwspinlock implementation
	*
	* @dev: underlying device, will be used with runtime PM api
	* @ops: vendor-specific hwspinlock handlers
	* @id: a global, unique, system-wide, index of the lock.
	* @lock: initialized and used by hwspinlock core
	* @owner: underlying implementation module, used to maintain module ref count
	*/
	struct hwspinlock {
	struct device *dev;
	const struct hwspinlock_ops *ops;
	int id;
	spinlock_t lock;
	struct module *owner;
	};

	The underlying implementation is responsible to assign the dev, ops, id and
	owner members. The lock member, OTOH, is initialized and used by the hwspinlock
	core.

	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.