drivers: hwspinlock: add framework

Add a platform-independent hwspinlock framework.

Hardware spinlock devices are needed, e.g., in order to access data
that is shared between remote processors, that otherwise have no
alternative mechanism to accomplish synchronization and mutual exclusion
operations.

Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>
Cc: Hari Kanigeri <h-kanigeri2@ti.com>
Cc: Benoit Cousson <b-cousson@ti.com>
Cc: Kevin Hilman <khilman@ti.com>
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Paul Walmsley <paul@pwsan.com>
Cc: Russell King <linux@arm.linux.org.uk>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Tony Lindgren <tony@atomide.com>

1 files changed, 293 insertions, 0 deletions

diff --git a/Documentation/hwspinlock.txt b/Documentation/hwspinlock.txt
new file mode 100644
index 000000000000..7dcd1a4e726c
--- /dev/null
+++ b/Documentation/hwspinlock.txt
@@ -0,0 +1,293 @@
+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.