drivers/gpu/drm/drm_modeset_lock.c - kernel/msm-4.19 - Gitiles

 /*
  * Copyright (C) 2014 Red Hat
  * Author: Rob Clark <robdclark@gmail.com>
  *
  * Permission is hereby granted, free of charge, to any person obtaining a
  * copy of this software and associated documentation files (the "Software"),
  * to deal in the Software without restriction, including without limitation
  * the rights to use, copy, modify, merge, publish, distribute, sublicense,
  * and/or sell copies of the Software, and to permit persons to whom the
  * Software is furnished to do so, subject to the following conditions:
  *
  * The above copyright notice and this permission notice shall be included in
  * all copies or substantial portions of the Software.
  *
  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
  * THE COPYRIGHT HOLDER(S) OR AUTHOR(S) BE LIABLE FOR ANY CLAIM, DAMAGES OR
  * OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
  * ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
  * OTHER DEALINGS IN THE SOFTWARE.
  */

 #include <drm/drmP.h>
 #include <drm/drm_crtc.h>
 #include <drm/drm_modeset_lock.h>

 /**
  * DOC: kms locking
  *
  * As KMS moves toward more fine grained locking, and atomic ioctl where
  * userspace can indirectly control locking order, it becomes necessary
  * to use &ww_mutex and acquire-contexts to avoid deadlocks.  But because
  * the locking is more distributed around the driver code, we want a bit
  * of extra utility/tracking out of our acquire-ctx.  This is provided
  * by &struct drm_modeset_lock and &struct drm_modeset_acquire_ctx.
  *
  * For basic principles of &ww_mutex, see: Documentation/locking/ww-mutex-design.txt
  *
  * The basic usage pattern is to::
  *
  *     drm_modeset_acquire_init(&ctx)
  *     retry:
  *     foreach (lock in random_ordered_set_of_locks) {
  *         ret = drm_modeset_lock(lock, &ctx)
  *         if (ret == -EDEADLK) {
  *             drm_modeset_backoff(&ctx);
  *             goto retry;
  *         }
  *     }
  *     ... do stuff ...
  *     drm_modeset_drop_locks(&ctx);
  *     drm_modeset_acquire_fini(&ctx);
  *
  * On top of of these per-object locks using &ww_mutex there's also an overall
  * &drm_mode_config.mutex, for protecting everything else. Mostly this means
  * probe state of connectors, and preventing hotplug add/removal of connectors.
  *
  * Finally there's a bunch of dedicated locks to protect drm core internal
  * lists and lookup data structures.
  */

 static DEFINE_WW_CLASS(crtc_ww_class);

 /**
  * drm_modeset_lock_all - take all modeset locks
  * @dev: DRM device
  *
  * This function takes all modeset locks, suitable where a more fine-grained
  * scheme isn't (yet) implemented. Locks must be dropped by calling the
  * drm_modeset_unlock_all() function.
  *
  * This function is deprecated. It allocates a lock acquisition context and
  * stores it in &drm_device.mode_config. This facilitate conversion of
  * existing code because it removes the need to manually deal with the
  * acquisition context, but it is also brittle because the context is global
  * and care must be taken not to nest calls. New code should use the
  * drm_modeset_lock_all_ctx() function and pass in the context explicitly.
  */
 void drm_modeset_lock_all(struct drm_device *dev)
 {
 	struct drm_mode_config *config = &dev->mode_config;
 	struct drm_modeset_acquire_ctx *ctx;
 	int ret;

 	ctx = kzalloc(sizeof(*ctx), GFP_KERNEL);
 	if (WARN_ON(!ctx))
 		return;

 	mutex_lock(&config->mutex);

 	drm_modeset_acquire_init(ctx, 0);

 retry:
 	ret = drm_modeset_lock_all_ctx(dev, ctx);
 	if (ret < 0) {
 		if (ret == -EDEADLK) {
 			drm_modeset_backoff(ctx);
 			goto retry;
 		}

 		drm_modeset_acquire_fini(ctx);
 		kfree(ctx);
 		return;
 	}

 	WARN_ON(config->acquire_ctx);

 	/*
 	 * We hold the locks now, so it is safe to stash the acquisition
 	 * context for drm_modeset_unlock_all().
 	 */
 	config->acquire_ctx = ctx;

 	drm_warn_on_modeset_not_all_locked(dev);
 }
 EXPORT_SYMBOL(drm_modeset_lock_all);

 /**
  * drm_modeset_unlock_all - drop all modeset locks
  * @dev: DRM device
  *
  * This function drops all modeset locks taken by a previous call to the
  * drm_modeset_lock_all() function.
  *
  * This function is deprecated. It uses the lock acquisition context stored
  * in &drm_device.mode_config. This facilitates conversion of existing
  * code because it removes the need to manually deal with the acquisition
  * context, but it is also brittle because the context is global and care must
  * be taken not to nest calls. New code should pass the acquisition context
  * directly to the drm_modeset_drop_locks() function.
  */
 void drm_modeset_unlock_all(struct drm_device *dev)
 {
 	struct drm_mode_config *config = &dev->mode_config;
 	struct drm_modeset_acquire_ctx *ctx = config->acquire_ctx;

 	if (WARN_ON(!ctx))
 		return;

 	config->acquire_ctx = NULL;
 	drm_modeset_drop_locks(ctx);
 	drm_modeset_acquire_fini(ctx);

 	kfree(ctx);

 	mutex_unlock(&dev->mode_config.mutex);
 }
 EXPORT_SYMBOL(drm_modeset_unlock_all);

 /**
  * drm_modeset_lock_crtc - lock crtc with hidden acquire ctx for a plane update
  * @crtc: DRM CRTC
  * @plane: DRM plane to be updated on @crtc
  *
  * This function locks the given crtc and plane (which should be either the
  * primary or cursor plane) using a hidden acquire context. This is necessary so
  * that drivers internally using the atomic interfaces can grab further locks
  * with the lock acquire context.
  *
  * Note that @plane can be NULL, e.g. when the cursor support hasn't yet been
  * converted to universal planes yet.
  */
 void drm_modeset_lock_crtc(struct drm_crtc *crtc,
 			   struct drm_plane *plane)
 {
 	struct drm_modeset_acquire_ctx *ctx;
 	int ret;

 	ctx = kzalloc(sizeof(*ctx), GFP_KERNEL);
 	if (WARN_ON(!ctx))
 		return;

 	drm_modeset_acquire_init(ctx, 0);

 retry:
 	ret = drm_modeset_lock(&crtc->mutex, ctx);
 	if (ret)
 		goto fail;

 	if (plane) {
 		ret = drm_modeset_lock(&plane->mutex, ctx);
 		if (ret)
 			goto fail;

 		if (plane->crtc) {
 			ret = drm_modeset_lock(&plane->crtc->mutex, ctx);
 			if (ret)
 				goto fail;
 		}
 	}

 	WARN_ON(crtc->acquire_ctx);

 	/* now we hold the locks, so now that it is safe, stash the
 	 * ctx for drm_modeset_unlock_crtc():
 	 */
 	crtc->acquire_ctx = ctx;

 	return;

 fail:
 	if (ret == -EDEADLK) {
 		drm_modeset_backoff(ctx);
 		goto retry;
 	}
 }
 EXPORT_SYMBOL(drm_modeset_lock_crtc);

 /**
  * drm_modeset_legacy_acquire_ctx - find acquire ctx for legacy ioctls
  * @crtc: drm crtc
  *
  * Legacy ioctl operations like cursor updates or page flips only have per-crtc
  * locking, and store the acquire ctx in the corresponding crtc. All other
  * legacy operations take all locks and use a global acquire context. This
  * function grabs the right one.
  */
 struct drm_modeset_acquire_ctx *
 drm_modeset_legacy_acquire_ctx(struct drm_crtc *crtc)
 {
 	if (crtc->acquire_ctx)
 		return crtc->acquire_ctx;

 	WARN_ON(!crtc->dev->mode_config.acquire_ctx);

 	return crtc->dev->mode_config.acquire_ctx;
 }
 EXPORT_SYMBOL(drm_modeset_legacy_acquire_ctx);

 /**
  * drm_modeset_unlock_crtc - drop crtc lock
  * @crtc: drm crtc
  *
  * This drops the crtc lock acquire with drm_modeset_lock_crtc() and all other
  * locks acquired through the hidden context.
  */
 void drm_modeset_unlock_crtc(struct drm_crtc *crtc)
 {
 	struct drm_modeset_acquire_ctx *ctx = crtc->acquire_ctx;

 	if (WARN_ON(!ctx))
 		return;

 	crtc->acquire_ctx = NULL;
 	drm_modeset_drop_locks(ctx);
 	drm_modeset_acquire_fini(ctx);

 	kfree(ctx);
 }
 EXPORT_SYMBOL(drm_modeset_unlock_crtc);

 /**
  * drm_warn_on_modeset_not_all_locked - check that all modeset locks are locked
  * @dev: device
  *
  * Useful as a debug assert.
  */
 void drm_warn_on_modeset_not_all_locked(struct drm_device *dev)
 {
 	struct drm_crtc *crtc;

 	/* Locking is currently fubar in the panic handler. */
 	if (oops_in_progress)
 		return;

 	drm_for_each_crtc(crtc, dev)
 		WARN_ON(!drm_modeset_is_locked(&crtc->mutex));

 	WARN_ON(!drm_modeset_is_locked(&dev->mode_config.connection_mutex));
 	WARN_ON(!mutex_is_locked(&dev->mode_config.mutex));
 }
 EXPORT_SYMBOL(drm_warn_on_modeset_not_all_locked);

 /**
  * drm_modeset_acquire_init - initialize acquire context
  * @ctx: the acquire context
  * @flags: for future
  */
 void drm_modeset_acquire_init(struct drm_modeset_acquire_ctx *ctx,
 		uint32_t flags)
 {
 	memset(ctx, 0, sizeof(*ctx));
 	ww_acquire_init(&ctx->ww_ctx, &crtc_ww_class);
 	INIT_LIST_HEAD(&ctx->locked);
 }
 EXPORT_SYMBOL(drm_modeset_acquire_init);

 /**
  * drm_modeset_acquire_fini - cleanup acquire context
  * @ctx: the acquire context
  */
 void drm_modeset_acquire_fini(struct drm_modeset_acquire_ctx *ctx)
 {
 	ww_acquire_fini(&ctx->ww_ctx);
 }
 EXPORT_SYMBOL(drm_modeset_acquire_fini);

 /**
  * drm_modeset_drop_locks - drop all locks
  * @ctx: the acquire context
  *
  * Drop all locks currently held against this acquire context.
  */
 void drm_modeset_drop_locks(struct drm_modeset_acquire_ctx *ctx)
 {
 	WARN_ON(ctx->contended);
 	while (!list_empty(&ctx->locked)) {
 		struct drm_modeset_lock *lock;

 		lock = list_first_entry(&ctx->locked,
 				struct drm_modeset_lock, head);

 		drm_modeset_unlock(lock);
 	}
 }
 EXPORT_SYMBOL(drm_modeset_drop_locks);

 static inline int modeset_lock(struct drm_modeset_lock *lock,
 		struct drm_modeset_acquire_ctx *ctx,
 		bool interruptible, bool slow)
 {
 	int ret;

 	WARN_ON(ctx->contended);

 	if (ctx->trylock_only) {
 		lockdep_assert_held(&ctx->ww_ctx);

 		if (!ww_mutex_trylock(&lock->mutex))
 			return -EBUSY;
 		else
 			return 0;
 	} else if (interruptible && slow) {
 		ret = ww_mutex_lock_slow_interruptible(&lock->mutex, &ctx->ww_ctx);
 	} else if (interruptible) {
 		ret = ww_mutex_lock_interruptible(&lock->mutex, &ctx->ww_ctx);
 	} else if (slow) {
 		ww_mutex_lock_slow(&lock->mutex, &ctx->ww_ctx);
 		ret = 0;
 	} else {
 		ret = ww_mutex_lock(&lock->mutex, &ctx->ww_ctx);
 	}
 	if (!ret) {
 		WARN_ON(!list_empty(&lock->head));
 		list_add(&lock->head, &ctx->locked);
 	} else if (ret == -EALREADY) {
 		/* we already hold the lock.. this is fine.  For atomic
 		 * we will need to be able to drm_modeset_lock() things
 		 * without having to keep track of what is already locked
 		 * or not.
 		 */
 		ret = 0;
 	} else if (ret == -EDEADLK) {
 		ctx->contended = lock;
 	}

 	return ret;
 }

 static int modeset_backoff(struct drm_modeset_acquire_ctx *ctx,
 		bool interruptible)
 {
 	struct drm_modeset_lock *contended = ctx->contended;

 	ctx->contended = NULL;

 	if (WARN_ON(!contended))
 		return 0;

 	drm_modeset_drop_locks(ctx);

 	return modeset_lock(contended, ctx, interruptible, true);
 }

 /**
  * drm_modeset_backoff - deadlock avoidance backoff
  * @ctx: the acquire context
  *
  * If deadlock is detected (ie. drm_modeset_lock() returns -EDEADLK),
  * you must call this function to drop all currently held locks and
  * block until the contended lock becomes available.
  */
 void drm_modeset_backoff(struct drm_modeset_acquire_ctx *ctx)
 {
 	modeset_backoff(ctx, false);
 }
 EXPORT_SYMBOL(drm_modeset_backoff);

 /**
  * drm_modeset_backoff_interruptible - deadlock avoidance backoff
  * @ctx: the acquire context
  *
  * Interruptible version of drm_modeset_backoff()
  */
 int drm_modeset_backoff_interruptible(struct drm_modeset_acquire_ctx *ctx)
 {
 	return modeset_backoff(ctx, true);
 }
 EXPORT_SYMBOL(drm_modeset_backoff_interruptible);

 /**
  * drm_modeset_lock_init - initialize lock
  * @lock: lock to init
  */
 void drm_modeset_lock_init(struct drm_modeset_lock *lock)
 {
 	ww_mutex_init(&lock->mutex, &crtc_ww_class);
 	INIT_LIST_HEAD(&lock->head);
 }
 EXPORT_SYMBOL(drm_modeset_lock_init);

 /**
  * drm_modeset_lock - take modeset lock
  * @lock: lock to take
  * @ctx: acquire ctx
  *
  * If ctx is not NULL, then its ww acquire context is used and the
  * lock will be tracked by the context and can be released by calling
  * drm_modeset_drop_locks().  If -EDEADLK is returned, this means a
  * deadlock scenario has been detected and it is an error to attempt
  * to take any more locks without first calling drm_modeset_backoff().
  */
 int drm_modeset_lock(struct drm_modeset_lock *lock,
 		struct drm_modeset_acquire_ctx *ctx)
 {
 	if (ctx)
 		return modeset_lock(lock, ctx, false, false);

 	ww_mutex_lock(&lock->mutex, NULL);
 	return 0;
 }
 EXPORT_SYMBOL(drm_modeset_lock);

 /**
  * drm_modeset_lock_interruptible - take modeset lock
  * @lock: lock to take
  * @ctx: acquire ctx
  *
  * Interruptible version of drm_modeset_lock()
  */
 int drm_modeset_lock_interruptible(struct drm_modeset_lock *lock,
 		struct drm_modeset_acquire_ctx *ctx)
 {
 	if (ctx)
 		return modeset_lock(lock, ctx, true, false);

 	return ww_mutex_lock_interruptible(&lock->mutex, NULL);
 }
 EXPORT_SYMBOL(drm_modeset_lock_interruptible);

 /**
  * drm_modeset_unlock - drop modeset lock
  * @lock: lock to release
  */
 void drm_modeset_unlock(struct drm_modeset_lock *lock)
 {
 	list_del_init(&lock->head);
 	ww_mutex_unlock(&lock->mutex);
 }
 EXPORT_SYMBOL(drm_modeset_unlock);

 /**
  * drm_modeset_lock_all_ctx - take all modeset locks
  * @dev: DRM device
  * @ctx: lock acquisition context
  *
  * This function takes all modeset locks, suitable where a more fine-grained
  * scheme isn't (yet) implemented.
  *
  * Unlike drm_modeset_lock_all(), it doesn't take the &drm_mode_config.mutex
  * since that lock isn't required for modeset state changes. Callers which
  * need to grab that lock too need to do so outside of the acquire context
  * @ctx.
  *
  * Locks acquired with this function should be released by calling the
  * drm_modeset_drop_locks() function on @ctx.
  *
  * Returns: 0 on success or a negative error-code on failure.
  */
 int drm_modeset_lock_all_ctx(struct drm_device *dev,
 			     struct drm_modeset_acquire_ctx *ctx)
 {
 	struct drm_crtc *crtc;
 	struct drm_plane *plane;
 	int ret;

 	ret = drm_modeset_lock(&dev->mode_config.connection_mutex, ctx);
 	if (ret)
 		return ret;

 	drm_for_each_crtc(crtc, dev) {
 		ret = drm_modeset_lock(&crtc->mutex, ctx);
 		if (ret)
 			return ret;
 	}

 	drm_for_each_plane(plane, dev) {
 		ret = drm_modeset_lock(&plane->mutex, ctx);
 		if (ret)
 			return ret;
 	}

 	return 0;
 }
 EXPORT_SYMBOL(drm_modeset_lock_all_ctx);
	/*
	* Copyright (C) 2014 Red Hat
	* Author: Rob Clark <robdclark@gmail.com>
	*
	* Permission is hereby granted, free of charge, to any person obtaining a
	* copy of this software and associated documentation files (the "Software"),
	* to deal in the Software without restriction, including without limitation
	* the rights to use, copy, modify, merge, publish, distribute, sublicense,
	* and/or sell copies of the Software, and to permit persons to whom the
	* Software is furnished to do so, subject to the following conditions:
	*
	* The above copyright notice and this permission notice shall be included in
	* all copies or substantial portions of the Software.
	*
	* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
	* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
	* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
	* THE COPYRIGHT HOLDER(S) OR AUTHOR(S) BE LIABLE FOR ANY CLAIM, DAMAGES OR
	* OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
	* ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
	* OTHER DEALINGS IN THE SOFTWARE.
	*/

	#include <drm/drmP.h>
	#include <drm/drm_crtc.h>
	#include <drm/drm_modeset_lock.h>

	/**
	* DOC: kms locking
	*
	* As KMS moves toward more fine grained locking, and atomic ioctl where
	* userspace can indirectly control locking order, it becomes necessary
	* to use &ww_mutex and acquire-contexts to avoid deadlocks. But because
	* the locking is more distributed around the driver code, we want a bit
	* of extra utility/tracking out of our acquire-ctx. This is provided
	* by &struct drm_modeset_lock and &struct drm_modeset_acquire_ctx.
	*
	* For basic principles of &ww_mutex, see: Documentation/locking/ww-mutex-design.txt
	*
	* The basic usage pattern is to::
	*
	* drm_modeset_acquire_init(&ctx)
	* retry:
	* foreach (lock in random_ordered_set_of_locks) {
	* ret = drm_modeset_lock(lock, &ctx)
	* if (ret == -EDEADLK) {
	* drm_modeset_backoff(&ctx);
	* goto retry;
	* }
	* }
	* ... do stuff ...
	* drm_modeset_drop_locks(&ctx);
	* drm_modeset_acquire_fini(&ctx);
	*
	* On top of of these per-object locks using &ww_mutex there's also an overall
	* &drm_mode_config.mutex, for protecting everything else. Mostly this means
	* probe state of connectors, and preventing hotplug add/removal of connectors.
	*
	* Finally there's a bunch of dedicated locks to protect drm core internal
	* lists and lookup data structures.
	*/

	static DEFINE_WW_CLASS(crtc_ww_class);

	/**
	* drm_modeset_lock_all - take all modeset locks
	* @dev: DRM device
	*
	* This function takes all modeset locks, suitable where a more fine-grained
	* scheme isn't (yet) implemented. Locks must be dropped by calling the
	* drm_modeset_unlock_all() function.
	*
	* This function is deprecated. It allocates a lock acquisition context and
	* stores it in &drm_device.mode_config. This facilitate conversion of
	* existing code because it removes the need to manually deal with the
	* acquisition context, but it is also brittle because the context is global
	* and care must be taken not to nest calls. New code should use the
	* drm_modeset_lock_all_ctx() function and pass in the context explicitly.
	*/
	void drm_modeset_lock_all(struct drm_device *dev)
	{
	struct drm_mode_config *config = &dev->mode_config;
	struct drm_modeset_acquire_ctx *ctx;
	int ret;

	ctx = kzalloc(sizeof(*ctx), GFP_KERNEL);
	if (WARN_ON(!ctx))
	return;

	mutex_lock(&config->mutex);

	drm_modeset_acquire_init(ctx, 0);

	retry:
	ret = drm_modeset_lock_all_ctx(dev, ctx);
	if (ret < 0) {
	if (ret == -EDEADLK) {
	drm_modeset_backoff(ctx);
	goto retry;
	}

	drm_modeset_acquire_fini(ctx);
	kfree(ctx);
	return;
	}

	WARN_ON(config->acquire_ctx);

	/*
	* We hold the locks now, so it is safe to stash the acquisition
	* context for drm_modeset_unlock_all().
	*/
	config->acquire_ctx = ctx;

	drm_warn_on_modeset_not_all_locked(dev);
	}
	EXPORT_SYMBOL(drm_modeset_lock_all);

	/**
	* drm_modeset_unlock_all - drop all modeset locks
	* @dev: DRM device
	*
	* This function drops all modeset locks taken by a previous call to the
	* drm_modeset_lock_all() function.
	*
	* This function is deprecated. It uses the lock acquisition context stored
	* in &drm_device.mode_config. This facilitates conversion of existing
	* code because it removes the need to manually deal with the acquisition
	* context, but it is also brittle because the context is global and care must
	* be taken not to nest calls. New code should pass the acquisition context
	* directly to the drm_modeset_drop_locks() function.
	*/
	void drm_modeset_unlock_all(struct drm_device *dev)
	{
	struct drm_mode_config *config = &dev->mode_config;
	struct drm_modeset_acquire_ctx *ctx = config->acquire_ctx;

	if (WARN_ON(!ctx))
	return;

	config->acquire_ctx = NULL;
	drm_modeset_drop_locks(ctx);
	drm_modeset_acquire_fini(ctx);

	kfree(ctx);

	mutex_unlock(&dev->mode_config.mutex);
	}
	EXPORT_SYMBOL(drm_modeset_unlock_all);

	/**
	* drm_modeset_lock_crtc - lock crtc with hidden acquire ctx for a plane update
	* @crtc: DRM CRTC
	* @plane: DRM plane to be updated on @crtc
	*
	* This function locks the given crtc and plane (which should be either the
	* primary or cursor plane) using a hidden acquire context. This is necessary so
	* that drivers internally using the atomic interfaces can grab further locks
	* with the lock acquire context.
	*
	* Note that @plane can be NULL, e.g. when the cursor support hasn't yet been
	* converted to universal planes yet.
	*/
	void drm_modeset_lock_crtc(struct drm_crtc *crtc,
	struct drm_plane *plane)
	{
	struct drm_modeset_acquire_ctx *ctx;
	int ret;

	ctx = kzalloc(sizeof(*ctx), GFP_KERNEL);
	if (WARN_ON(!ctx))
	return;

	drm_modeset_acquire_init(ctx, 0);

	retry:
	ret = drm_modeset_lock(&crtc->mutex, ctx);
	if (ret)
	goto fail;

	if (plane) {
	ret = drm_modeset_lock(&plane->mutex, ctx);
	if (ret)
	goto fail;

	if (plane->crtc) {
	ret = drm_modeset_lock(&plane->crtc->mutex, ctx);
	if (ret)
	goto fail;
	}
	}

	WARN_ON(crtc->acquire_ctx);

	/* now we hold the locks, so now that it is safe, stash the
	* ctx for drm_modeset_unlock_crtc():
	*/
	crtc->acquire_ctx = ctx;

	return;

	fail:
	if (ret == -EDEADLK) {
	drm_modeset_backoff(ctx);
	goto retry;
	}
	}
	EXPORT_SYMBOL(drm_modeset_lock_crtc);

	/**
	* drm_modeset_legacy_acquire_ctx - find acquire ctx for legacy ioctls
	* @crtc: drm crtc
	*
	* Legacy ioctl operations like cursor updates or page flips only have per-crtc
	* locking, and store the acquire ctx in the corresponding crtc. All other
	* legacy operations take all locks and use a global acquire context. This
	* function grabs the right one.
	*/
	struct drm_modeset_acquire_ctx *
	drm_modeset_legacy_acquire_ctx(struct drm_crtc *crtc)
	{
	if (crtc->acquire_ctx)
	return crtc->acquire_ctx;

	WARN_ON(!crtc->dev->mode_config.acquire_ctx);

	return crtc->dev->mode_config.acquire_ctx;
	}
	EXPORT_SYMBOL(drm_modeset_legacy_acquire_ctx);

	/**
	* drm_modeset_unlock_crtc - drop crtc lock
	* @crtc: drm crtc
	*
	* This drops the crtc lock acquire with drm_modeset_lock_crtc() and all other
	* locks acquired through the hidden context.
	*/
	void drm_modeset_unlock_crtc(struct drm_crtc *crtc)
	{
	struct drm_modeset_acquire_ctx *ctx = crtc->acquire_ctx;

	if (WARN_ON(!ctx))
	return;

	crtc->acquire_ctx = NULL;
	drm_modeset_drop_locks(ctx);
	drm_modeset_acquire_fini(ctx);

	kfree(ctx);
	}
	EXPORT_SYMBOL(drm_modeset_unlock_crtc);

	/**
	* drm_warn_on_modeset_not_all_locked - check that all modeset locks are locked
	* @dev: device
	*
	* Useful as a debug assert.
	*/
	void drm_warn_on_modeset_not_all_locked(struct drm_device *dev)
	{
	struct drm_crtc *crtc;

	/* Locking is currently fubar in the panic handler. */
	if (oops_in_progress)
	return;

	drm_for_each_crtc(crtc, dev)
	WARN_ON(!drm_modeset_is_locked(&crtc->mutex));

	WARN_ON(!drm_modeset_is_locked(&dev->mode_config.connection_mutex));
	WARN_ON(!mutex_is_locked(&dev->mode_config.mutex));
	}
	EXPORT_SYMBOL(drm_warn_on_modeset_not_all_locked);

	/**
	* drm_modeset_acquire_init - initialize acquire context
	* @ctx: the acquire context
	* @flags: for future
	*/
	void drm_modeset_acquire_init(struct drm_modeset_acquire_ctx *ctx,
	uint32_t flags)
	{
	memset(ctx, 0, sizeof(*ctx));
	ww_acquire_init(&ctx->ww_ctx, &crtc_ww_class);
	INIT_LIST_HEAD(&ctx->locked);
	}
	EXPORT_SYMBOL(drm_modeset_acquire_init);

	/**
	* drm_modeset_acquire_fini - cleanup acquire context
	* @ctx: the acquire context
	*/
	void drm_modeset_acquire_fini(struct drm_modeset_acquire_ctx *ctx)
	{
	ww_acquire_fini(&ctx->ww_ctx);
	}
	EXPORT_SYMBOL(drm_modeset_acquire_fini);

	/**
	* drm_modeset_drop_locks - drop all locks
	* @ctx: the acquire context
	*
	* Drop all locks currently held against this acquire context.
	*/
	void drm_modeset_drop_locks(struct drm_modeset_acquire_ctx *ctx)
	{
	WARN_ON(ctx->contended);
	while (!list_empty(&ctx->locked)) {
	struct drm_modeset_lock *lock;

	lock = list_first_entry(&ctx->locked,
	struct drm_modeset_lock, head);

	drm_modeset_unlock(lock);
	}
	}
	EXPORT_SYMBOL(drm_modeset_drop_locks);

	static inline int modeset_lock(struct drm_modeset_lock *lock,
	struct drm_modeset_acquire_ctx *ctx,
	bool interruptible, bool slow)
	{
	int ret;

	WARN_ON(ctx->contended);

	if (ctx->trylock_only) {
	lockdep_assert_held(&ctx->ww_ctx);

	if (!ww_mutex_trylock(&lock->mutex))
	return -EBUSY;
	else
	return 0;
	} else if (interruptible && slow) {
	ret = ww_mutex_lock_slow_interruptible(&lock->mutex, &ctx->ww_ctx);
	} else if (interruptible) {
	ret = ww_mutex_lock_interruptible(&lock->mutex, &ctx->ww_ctx);
	} else if (slow) {
	ww_mutex_lock_slow(&lock->mutex, &ctx->ww_ctx);
	ret = 0;
	} else {
	ret = ww_mutex_lock(&lock->mutex, &ctx->ww_ctx);
	}
	if (!ret) {
	WARN_ON(!list_empty(&lock->head));
	list_add(&lock->head, &ctx->locked);
	} else if (ret == -EALREADY) {
	/* we already hold the lock.. this is fine. For atomic
	* we will need to be able to drm_modeset_lock() things
	* without having to keep track of what is already locked
	* or not.
	*/
	ret = 0;
	} else if (ret == -EDEADLK) {
	ctx->contended = lock;
	}

	return ret;
	}

	static int modeset_backoff(struct drm_modeset_acquire_ctx *ctx,
	bool interruptible)
	{
	struct drm_modeset_lock *contended = ctx->contended;

	ctx->contended = NULL;

	if (WARN_ON(!contended))
	return 0;

	drm_modeset_drop_locks(ctx);

	return modeset_lock(contended, ctx, interruptible, true);
	}

	/**
	* drm_modeset_backoff - deadlock avoidance backoff
	* @ctx: the acquire context
	*
	* If deadlock is detected (ie. drm_modeset_lock() returns -EDEADLK),
	* you must call this function to drop all currently held locks and
	* block until the contended lock becomes available.
	*/
	void drm_modeset_backoff(struct drm_modeset_acquire_ctx *ctx)
	{
	modeset_backoff(ctx, false);
	}
	EXPORT_SYMBOL(drm_modeset_backoff);

	/**
	* drm_modeset_backoff_interruptible - deadlock avoidance backoff
	* @ctx: the acquire context
	*
	* Interruptible version of drm_modeset_backoff()
	*/
	int drm_modeset_backoff_interruptible(struct drm_modeset_acquire_ctx *ctx)
	{
	return modeset_backoff(ctx, true);
	}
	EXPORT_SYMBOL(drm_modeset_backoff_interruptible);

	/**
	* drm_modeset_lock_init - initialize lock
	* @lock: lock to init
	*/
	void drm_modeset_lock_init(struct drm_modeset_lock *lock)
	{
	ww_mutex_init(&lock->mutex, &crtc_ww_class);
	INIT_LIST_HEAD(&lock->head);
	}
	EXPORT_SYMBOL(drm_modeset_lock_init);

	/**
	* drm_modeset_lock - take modeset lock
	* @lock: lock to take
	* @ctx: acquire ctx
	*
	* If ctx is not NULL, then its ww acquire context is used and the
	* lock will be tracked by the context and can be released by calling
	* drm_modeset_drop_locks(). If -EDEADLK is returned, this means a
	* deadlock scenario has been detected and it is an error to attempt
	* to take any more locks without first calling drm_modeset_backoff().
	*/
	int drm_modeset_lock(struct drm_modeset_lock *lock,
	struct drm_modeset_acquire_ctx *ctx)
	{
	if (ctx)
	return modeset_lock(lock, ctx, false, false);

	ww_mutex_lock(&lock->mutex, NULL);
	return 0;
	}
	EXPORT_SYMBOL(drm_modeset_lock);

	/**
	* drm_modeset_lock_interruptible - take modeset lock
	* @lock: lock to take
	* @ctx: acquire ctx
	*
	* Interruptible version of drm_modeset_lock()
	*/
	int drm_modeset_lock_interruptible(struct drm_modeset_lock *lock,
	struct drm_modeset_acquire_ctx *ctx)
	{
	if (ctx)
	return modeset_lock(lock, ctx, true, false);

	return ww_mutex_lock_interruptible(&lock->mutex, NULL);
	}
	EXPORT_SYMBOL(drm_modeset_lock_interruptible);

	/**
	* drm_modeset_unlock - drop modeset lock
	* @lock: lock to release
	*/
	void drm_modeset_unlock(struct drm_modeset_lock *lock)
	{
	list_del_init(&lock->head);
	ww_mutex_unlock(&lock->mutex);
	}
	EXPORT_SYMBOL(drm_modeset_unlock);

	/**
	* drm_modeset_lock_all_ctx - take all modeset locks
	* @dev: DRM device
	* @ctx: lock acquisition context
	*
	* This function takes all modeset locks, suitable where a more fine-grained
	* scheme isn't (yet) implemented.
	*
	* Unlike drm_modeset_lock_all(), it doesn't take the &drm_mode_config.mutex
	* since that lock isn't required for modeset state changes. Callers which
	* need to grab that lock too need to do so outside of the acquire context
	* @ctx.
	*
	* Locks acquired with this function should be released by calling the
	* drm_modeset_drop_locks() function on @ctx.
	*
	* Returns: 0 on success or a negative error-code on failure.
	*/
	int drm_modeset_lock_all_ctx(struct drm_device *dev,
	struct drm_modeset_acquire_ctx *ctx)
	{
	struct drm_crtc *crtc;
	struct drm_plane *plane;
	int ret;

	ret = drm_modeset_lock(&dev->mode_config.connection_mutex, ctx);
	if (ret)
	return ret;

	drm_for_each_crtc(crtc, dev) {
	ret = drm_modeset_lock(&crtc->mutex, ctx);
	if (ret)
	return ret;
	}

	drm_for_each_plane(plane, dev) {
	ret = drm_modeset_lock(&plane->mutex, ctx);
	if (ret)
	return ret;
	}

	return 0;
	}
	EXPORT_SYMBOL(drm_modeset_lock_all_ctx);