Documentation/locking/hwspinlock.rst - third_party/kernel - Git at Google


            
            ===========================

            
            Hardware Spinlock Framework

            
            ===========================


            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.


            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 of_hwspin_lock_get_id(struct device_node *np, int index);


            Retrieve the global lock id for an OF phandle-based specific lock.

            
            This function provides a means for DT users of a hwspinlock module

            
            to get the global lock id of a specific hwspinlock, so that it can

            
            be requested using the normal hwspin_lock_request_specific() API.


            The function returns a lock id number on success, -EPROBE_DEFER if

            
            the hwspinlock device is not yet registered with the core, or other

            
            error values.


            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_lock_timeout_raw(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.


            Caution: User must protect the routine of getting hardware lock with mutex

            
            or spinlock to avoid dead-lock, that will let user can do some time-consuming

            
            or sleepable operations under the hardware lock.


            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_in_atomic(struct hwspinlock *hwlock, unsigned int to);


            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.


            This function shall be called only from an atomic context and the timeout

            
            value shall not exceed a few msecs.


            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.


            ::


              int hwspin_trylock_raw(struct hwspinlock *hwlock);


            Attempt to lock a previously-assigned hwspinlock, but immediately fail if

            
            it is already taken.


            Caution: User must protect the routine of getting hardware lock with mutex

            
            or spinlock to avoid dead-lock, that will let user can do some time-consuming

            
            or sleepable operations under the hardware lock.


            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_in_atomic(struct hwspinlock *hwlock);


            Attempt to lock a previously-assigned hwspinlock, but immediately fail if

            
            it is already taken.


            This function shall be called only from an atomic context.


            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.


            ::


              void hwspin_unlock_raw(struct hwspinlock *hwlock);


            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).

            
            This function will never sleep.


            ::


              void hwspin_unlock_in_atomic(struct hwspinlock *hwlock);


            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).

            
            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.


            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;

            
            	}


            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).


            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.


            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
	===========================

	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.

	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 of_hwspin_lock_get_id(struct device_node *np, int index);

	Retrieve the global lock id for an OF phandle-based specific lock.
	This function provides a means for DT users of a hwspinlock module
	to get the global lock id of a specific hwspinlock, so that it can
	be requested using the normal hwspin_lock_request_specific() API.

	The function returns a lock id number on success, -EPROBE_DEFER if
	the hwspinlock device is not yet registered with the core, or other
	error values.

	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_lock_timeout_raw(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.

	Caution: User must protect the routine of getting hardware lock with mutex
	or spinlock to avoid dead-lock, that will let user can do some time-consuming
	or sleepable operations under the hardware lock.

	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_in_atomic(struct hwspinlock *hwlock, unsigned int to);

	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.

	This function shall be called only from an atomic context and the timeout
	value shall not exceed a few msecs.

	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.

	::

	int hwspin_trylock_raw(struct hwspinlock *hwlock);

	Attempt to lock a previously-assigned hwspinlock, but immediately fail if
	it is already taken.

	Caution: User must protect the routine of getting hardware lock with mutex
	or spinlock to avoid dead-lock, that will let user can do some time-consuming
	or sleepable operations under the hardware lock.

	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_in_atomic(struct hwspinlock *hwlock);

	Attempt to lock a previously-assigned hwspinlock, but immediately fail if
	it is already taken.

	This function shall be called only from an atomic context.

	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.

	::

	void hwspin_unlock_raw(struct hwspinlock *hwlock);

	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).
	This function will never sleep.

	::

	void hwspin_unlock_in_atomic(struct hwspinlock *hwlock);

	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).
	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.

	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;
	}


	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).

	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.

	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.