include/linux/fence.h

/*
 * Fence mechanism for dma-buf to allow for asynchronous dma access
 *
 * Copyright (C) 2012 Canonical Ltd
 * Copyright (C) 2012 Texas Instruments
 *
 * Authors:
 * Rob Clark <robdclark@gmail.com>
 * Maarten Lankhorst <maarten.lankhorst@canonical.com>
 *
 * This program is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 as published by
 * the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
 * more details.
 */

#ifndef __LINUX_FENCE_H
#define __LINUX_FENCE_H

#include <linux/err.h>
#include <linux/wait.h>
#include <linux/list.h>
#include <linux/bitops.h>
#include <linux/kref.h>
#include <linux/sched.h>
#include <linux/printk.h>
#include <linux/rcupdate.h>

struct fence;
struct fence_ops;
struct fence_cb;

/**
 * struct fence - software synchronization primitive
 * @refcount: refcount for this fence
 * @ops: fence_ops associated with this fence
 * @rcu: used for releasing fence with kfree_rcu
 * @cb_list: list of all callbacks to call
 * @lock: spin_lock_irqsave used for locking
 * @context: execution context this fence belongs to, returned by
 *           fence_context_alloc()
 * @seqno: the sequence number of this fence inside the execution context,
 * can be compared to decide which fence would be signaled later.
 * @flags: A mask of FENCE_FLAG_* defined below
 * @timestamp: Timestamp when the fence was signaled.
 * @status: Optional, only valid if < 0, must be set before calling
 * fence_signal, indicates that the fence has completed with an error.
 *
 * the flags member must be manipulated and read using the appropriate
 * atomic ops (bit_*), so taking the spinlock will not be needed most
 * of the time.
 *
 * FENCE_FLAG_SIGNALED_BIT - fence is already signaled
 * FENCE_FLAG_ENABLE_SIGNAL_BIT - enable_signaling might have been called*
 * FENCE_FLAG_USER_BITS - start of the unused bits, can be used by the
 * implementer of the fence for its own purposes. Can be used in different
 * ways by different fence implementers, so do not rely on this.
 *
 * Since atomic bitops are used, this is not guaranteed to be the case.
 * Particularly, if the bit was set, but fence_signal was called right
 * before this bit was set, it would have been able to set the
 * FENCE_FLAG_SIGNALED_BIT, before enable_signaling was called.
 * Adding a check for FENCE_FLAG_SIGNALED_BIT after setting
 * FENCE_FLAG_ENABLE_SIGNAL_BIT closes this race, and makes sure that
 * after fence_signal was called, any enable_signaling call will have either
 * been completed, or never called at all.
 */
struct fence {
	struct kref refcount;
	const struct fence_ops *ops;
	struct rcu_head rcu;
	struct list_head cb_list;
	spinlock_t *lock;
	u64 context;
	unsigned seqno;
	unsigned long flags;
	ktime_t timestamp;
	int status;
};

enum fence_flag_bits {
	FENCE_FLAG_SIGNALED_BIT,
	FENCE_FLAG_ENABLE_SIGNAL_BIT,
	FENCE_FLAG_USER_BITS, /* must always be last member */
};

typedef void (*fence_func_t)(struct fence *fence, struct fence_cb *cb);

/**
 * struct fence_cb - callback for fence_add_callback
 * @node: used by fence_add_callback to append this struct to fence::cb_list
 * @func: fence_func_t to call
 *
 * This struct will be initialized by fence_add_callback, additional
 * data can be passed along by embedding fence_cb in another struct.
 */
struct fence_cb {
	struct list_head node;
	fence_func_t func;
};

/**
 * struct fence_ops - operations implemented for fence
 * @get_driver_name: returns the driver name.
 * @get_timeline_name: return the name of the context this fence belongs to.
 * @enable_signaling: enable software signaling of fence.
 * @disable_signaling: disable software signaling of fence (optional).
 * @signaled: [optional] peek whether the fence is signaled, can be null.
 * @wait: custom wait implementation, or fence_default_wait.
 * @release: [optional] called on destruction of fence, can be null
 * @fill_driver_data: [optional] callback to fill in free-form debug info
 * Returns amount of bytes filled, or -errno.
 * @fence_value_str: [optional] fills in the value of the fence as a string
 * @timeline_value_str: [optional] fills in the current value of the timeline
 * as a string
 *
 * Notes on enable_signaling:
 * For fence implementations that have the capability for hw->hw
 * signaling, they can implement this op to enable the necessary
 * irqs, or insert commands into cmdstream, etc.  This is called
 * in the first wait() or add_callback() path to let the fence
 * implementation know that there is another driver waiting on
 * the signal (ie. hw->sw case).
 *
 * This function can be called called from atomic context, but not
 * from irq context, so normal spinlocks can be used.
 *
 * A return value of false indicates the fence already passed,
 * or some failure occurred that made it impossible to enable
 * signaling. True indicates successful enabling.
 *
 * fence->status may be set in enable_signaling, but only when false is
 * returned.
 *
 * Calling fence_signal before enable_signaling is called allows
 * for a tiny race window in which enable_signaling is called during,
 * before, or after fence_signal. To fight this, it is recommended
 * that before enable_signaling returns true an extra reference is
 * taken on the fence, to be released when the fence is signaled.
 * This will mean fence_signal will still be called twice, but
 * the second time will be a noop since it was already signaled.
 *
 * Notes on signaled:
 * May set fence->status if returning true.
 *
 * Notes on wait:
 * Must not be NULL, set to fence_default_wait for default implementation.
 * the fence_default_wait implementation should work for any fence, as long
 * as enable_signaling works correctly.
 *
 * Must return -ERESTARTSYS if the wait is intr = true and the wait was
 * interrupted, and remaining jiffies if fence has signaled, or 0 if wait
 * timed out. Can also return other error values on custom implementations,
 * which should be treated as if the fence is signaled. For example a hardware
 * lockup could be reported like that.
 *
 * Notes on release:
 * Can be NULL, this function allows additional commands to run on
 * destruction of the fence. Can be called from irq context.
 * If pointer is set to NULL, kfree will get called instead.
 */

struct fence_ops {
	const char * (*get_driver_name)(struct fence *fence);
	const char * (*get_timeline_name)(struct fence *fence);
	bool (*enable_signaling)(struct fence *fence);
	void (*disable_signaling)(struct fence *fence);
	bool (*signaled)(struct fence *fence);
	signed long (*wait)(struct fence *fence, bool intr, signed long timeout);
	void (*release)(struct fence *fence);

	int (*fill_driver_data)(struct fence *fence, void *data, int size);
	void (*fence_value_str)(struct fence *fence, char *str, int size);
	void (*timeline_value_str)(struct fence *fence, char *str, int size);
};

void fence_init(struct fence *fence, const struct fence_ops *ops,
		spinlock_t *lock, u64 context, unsigned seqno);

void fence_release(struct kref *kref);
void fence_free(struct fence *fence);

/**
 * fence_put - decreases refcount of the fence
 * @fence:	[in]	fence to reduce refcount of
 */
static inline void fence_put(struct fence *fence)
{
	if (fence)
		kref_put(&fence->refcount, fence_release);
}

/**
 * fence_get - increases refcount of the fence
 * @fence:	[in]	fence to increase refcount of
 *
 * Returns the same fence, with refcount increased by 1.
 */
static inline struct fence *fence_get(struct fence *fence)
{
	if (fence)
		kref_get(&fence->refcount);
	return fence;
}

/**
 * fence_get_rcu - get a fence from a reservation_object_list with rcu read lock
 * @fence:	[in]	fence to increase refcount of
 *
 * Function returns NULL if no refcount could be obtained, or the fence.
 */
static inline struct fence *fence_get_rcu(struct fence *fence)
{
	if (kref_get_unless_zero(&fence->refcount))
		return fence;
	else
		return NULL;
}

/**
 * fence_get_rcu_safe  - acquire a reference to an RCU tracked fence
 * @fence:	[in]	pointer to fence to increase refcount of
 *
 * Function returns NULL if no refcount could be obtained, or the fence.
 * This function handles acquiring a reference to a fence that may be
 * reallocated within the RCU grace period (such as with SLAB_DESTROY_BY_RCU),
 * so long as the caller is using RCU on the pointer to the fence.
 *
 * An alternative mechanism is to employ a seqlock to protect a bunch of
 * fences, such as used by struct reservation_object. When using a seqlock,
 * the seqlock must be taken before and checked after a reference to the
 * fence is acquired (as shown here).
 *
 * The caller is required to hold the RCU read lock.
 */
static inline struct fence *fence_get_rcu_safe(struct fence * __rcu *fencep)
{
	do {
		struct fence *fence;

		fence = rcu_dereference(*fencep);
		if (!fence || !fence_get_rcu(fence))
			return NULL;

		/* The atomic_inc_not_zero() inside fence_get_rcu()
		 * provides a full memory barrier upon success (such as now).
		 * This is paired with the write barrier from assigning
		 * to the __rcu protected fence pointer so that if that
		 * pointer still matches the current fence, we know we
		 * have successfully acquire a reference to it. If it no
		 * longer matches, we are holding a reference to some other
		 * reallocated pointer. This is possible if the allocator
		 * is using a freelist like SLAB_DESTROY_BY_RCU where the
		 * fence remains valid for the RCU grace period, but it
		 * may be reallocated. When using such allocators, we are
		 * responsible for ensuring the reference we get is to
		 * the right fence, as below.
		 */
		if (fence == rcu_access_pointer(*fencep))
			return rcu_pointer_handoff(fence);

		fence_put(fence);
	} while (1);
}

int fence_signal(struct fence *fence);
int fence_signal_locked(struct fence *fence);
signed long fence_default_wait(struct fence *fence, bool intr, signed long timeout);
int fence_add_callback(struct fence *fence, struct fence_cb *cb,
		       fence_func_t func);
bool fence_remove_callback(struct fence *fence, struct fence_cb *cb);
void fence_enable_sw_signaling(struct fence *fence);

/**
 * fence_is_signaled_locked - Return an indication if the fence is signaled yet.
 * @fence:	[in]	the fence to check
 *
 * Returns true if the fence was already signaled, false if not. Since this
 * function doesn't enable signaling, it is not guaranteed to ever return
 * true if fence_add_callback, fence_wait or fence_enable_sw_signaling
 * haven't been called before.
 *
 * This function requires fence->lock to be held.
 */
static inline bool
fence_is_signaled_locked(struct fence *fence)
{
	if (test_bit(FENCE_FLAG_SIGNALED_BIT, &fence->flags))
		return true;

	if (fence->ops->signaled && fence->ops->signaled(fence)) {
		fence_signal_locked(fence);
		return true;
	}

	return false;
}

/**
 * fence_is_signaled - Return an indication if the fence is signaled yet.
 * @fence:	[in]	the fence to check
 *
 * Returns true if the fence was already signaled, false if not. Since this
 * function doesn't enable signaling, it is not guaranteed to ever return
 * true if fence_add_callback, fence_wait or fence_enable_sw_signaling
 * haven't been called before.
 *
 * It's recommended for seqno fences to call fence_signal when the
 * operation is complete, it makes it possible to prevent issues from
 * wraparound between time of issue and time of use by checking the return
 * value of this function before calling hardware-specific wait instructions.
 */
static inline bool
fence_is_signaled(struct fence *fence)
{
	if (test_bit(FENCE_FLAG_SIGNALED_BIT, &fence->flags))
		return true;

	if (fence->ops->signaled && fence->ops->signaled(fence)) {
		fence_signal(fence);
		return true;
	}

	return false;
}

/**
 * fence_is_later - return if f1 is chronologically later than f2
 * @f1:	[in]	the first fence from the same context
 * @f2:	[in]	the second fence from the same context
 *
 * Returns true if f1 is chronologically later than f2. Both fences must be
 * from the same context, since a seqno is not re-used across contexts.
 */
static inline bool fence_is_later(struct fence *f1, struct fence *f2)
{
	if (WARN_ON(f1->context != f2->context))
		return false;

	return (int)(f1->seqno - f2->seqno) > 0;
}

/**
 * fence_later - return the chronologically later fence
 * @f1:	[in]	the first fence from the same context
 * @f2:	[in]	the second fence from the same context
 *
 * Returns NULL if both fences are signaled, otherwise the fence that would be
 * signaled last. Both fences must be from the same context, since a seqno is
 * not re-used across contexts.
 */
static inline struct fence *fence_later(struct fence *f1, struct fence *f2)
{
	if (WARN_ON(f1->context != f2->context))
		return NULL;

	/*
	 * can't check just FENCE_FLAG_SIGNALED_BIT here, it may never have been
	 * set if enable_signaling wasn't called, and enabling that here is
	 * overkill.
	 */
	if (fence_is_later(f1, f2))
		return fence_is_signaled(f1) ? NULL : f1;
	else
		return fence_is_signaled(f2) ? NULL : f2;
}

signed long fence_wait_timeout(struct fence *, bool intr, signed long timeout);
signed long fence_wait_any_timeout(struct fence **fences, uint32_t count,
				   bool intr, signed long timeout);

/**
 * fence_wait - sleep until the fence gets signaled
 * @fence:	[in]	the fence to wait on
 * @intr:	[in]	if true, do an interruptible wait
 *
 * This function will return -ERESTARTSYS if interrupted by a signal,
 * or 0 if the fence was signaled. Other error values may be
 * returned on custom implementations.
 *
 * Performs a synchronous wait on this fence. It is assumed the caller
 * directly or indirectly holds a reference to the fence, otherwise the
 * fence might be freed before return, resulting in undefined behavior.
 */
static inline signed long fence_wait(struct fence *fence, bool intr)
{
	signed long ret;

	/* Since fence_wait_timeout cannot timeout with
	 * MAX_SCHEDULE_TIMEOUT, only valid return values are
	 * -ERESTARTSYS and MAX_SCHEDULE_TIMEOUT.
	 */
	ret = fence_wait_timeout(fence, intr, MAX_SCHEDULE_TIMEOUT);

	return ret < 0 ? ret : 0;
}

u64 fence_context_alloc(unsigned num);

#define FENCE_TRACE(f, fmt, args...) \
	do {								\
		struct fence *__ff = (f);				\
		if (IS_ENABLED(CONFIG_FENCE_TRACE))			\
			pr_info("f %llu#%u: " fmt,			\
				__ff->context, __ff->seqno, ##args);	\
	} while (0)

#define FENCE_WARN(f, fmt, args...) \
	do {								\
		struct fence *__ff = (f);				\
		pr_warn("f %llu#%u: " fmt, __ff->context, __ff->seqno,	\
			 ##args);					\
	} while (0)

#define FENCE_ERR(f, fmt, args...) \
	do {								\
		struct fence *__ff = (f);				\
		pr_err("f %llu#%u: " fmt, __ff->context, __ff->seqno,	\
			##args);					\
	} while (0)

#endif /* __LINUX_FENCE_H */