1 /* $NetBSD: drm_modeset_lock.c,v 1.5 2021/12/18 23:44:57 riastradh Exp $ */
2
3 /*
4 * Copyright (C) 2014 Red Hat
5 * Author: Rob Clark <robdclark@gmail.com>
6 *
7 * Permission is hereby granted, free of charge, to any person obtaining a
8 * copy of this software and associated documentation files (the "Software"),
9 * to deal in the Software without restriction, including without limitation
10 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
11 * and/or sell copies of the Software, and to permit persons to whom the
12 * Software is furnished to do so, subject to the following conditions:
13 *
14 * The above copyright notice and this permission notice shall be included in
15 * all copies or substantial portions of the Software.
16 *
17 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
18 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
19 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
20 * THE COPYRIGHT HOLDER(S) OR AUTHOR(S) BE LIABLE FOR ANY CLAIM, DAMAGES OR
21 * OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
22 * ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
23 * OTHER DEALINGS IN THE SOFTWARE.
24 */
25
26 #include <sys/cdefs.h>
27 __KERNEL_RCSID(0, "$NetBSD: drm_modeset_lock.c,v 1.5 2021/12/18 23:44:57 riastradh Exp $");
28
29 #include <drm/drm_atomic.h>
30 #include <drm/drm_crtc.h>
31 #include <drm/drm_device.h>
32 #include <drm/drm_modeset_lock.h>
33
34 /**
35 * DOC: kms locking
36 *
37 * As KMS moves toward more fine grained locking, and atomic ioctl where
38 * userspace can indirectly control locking order, it becomes necessary
39 * to use &ww_mutex and acquire-contexts to avoid deadlocks. But because
40 * the locking is more distributed around the driver code, we want a bit
41 * of extra utility/tracking out of our acquire-ctx. This is provided
42 * by &struct drm_modeset_lock and &struct drm_modeset_acquire_ctx.
43 *
44 * For basic principles of &ww_mutex, see: Documentation/locking/ww-mutex-design.rst
45 *
46 * The basic usage pattern is to::
47 *
48 * drm_modeset_acquire_init(ctx, DRM_MODESET_ACQUIRE_INTERRUPTIBLE)
49 * retry:
50 * foreach (lock in random_ordered_set_of_locks) {
51 * ret = drm_modeset_lock(lock, ctx)
52 * if (ret == -EDEADLK) {
53 * ret = drm_modeset_backoff(ctx);
54 * if (!ret)
55 * goto retry;
56 * }
57 * if (ret)
58 * goto out;
59 * }
60 * ... do stuff ...
61 * out:
62 * drm_modeset_drop_locks(ctx);
63 * drm_modeset_acquire_fini(ctx);
64 *
65 * For convenience this control flow is implemented in
66 * DRM_MODESET_LOCK_ALL_BEGIN() and DRM_MODESET_LOCK_ALL_END() for the case
67 * where all modeset locks need to be taken through drm_modeset_lock_all_ctx().
68 *
69 * If all that is needed is a single modeset lock, then the &struct
70 * drm_modeset_acquire_ctx is not needed and the locking can be simplified
71 * by passing a NULL instead of ctx in the drm_modeset_lock() call or
72 * calling drm_modeset_lock_single_interruptible(). To unlock afterwards
73 * call drm_modeset_unlock().
74 *
75 * On top of these per-object locks using &ww_mutex there's also an overall
76 * &drm_mode_config.mutex, for protecting everything else. Mostly this means
77 * probe state of connectors, and preventing hotplug add/removal of connectors.
78 *
79 * Finally there's a bunch of dedicated locks to protect drm core internal
80 * lists and lookup data structures.
81 */
82
83 static DEFINE_WW_CLASS(crtc_ww_class);
84
85 /**
86 * drm_modeset_lock_all - take all modeset locks
87 * @dev: DRM device
88 *
89 * This function takes all modeset locks, suitable where a more fine-grained
90 * scheme isn't (yet) implemented. Locks must be dropped by calling the
91 * drm_modeset_unlock_all() function.
92 *
93 * This function is deprecated. It allocates a lock acquisition context and
94 * stores it in &drm_device.mode_config. This facilitate conversion of
95 * existing code because it removes the need to manually deal with the
96 * acquisition context, but it is also brittle because the context is global
97 * and care must be taken not to nest calls. New code should use the
98 * drm_modeset_lock_all_ctx() function and pass in the context explicitly.
99 */
drm_modeset_lock_all(struct drm_device * dev)100 void drm_modeset_lock_all(struct drm_device *dev)
101 {
102 struct drm_mode_config *config = &dev->mode_config;
103 struct drm_modeset_acquire_ctx *ctx;
104 int ret;
105
106 ctx = kzalloc(sizeof(*ctx), GFP_KERNEL | __GFP_NOFAIL);
107 if (WARN_ON(!ctx))
108 return;
109
110 mutex_lock(&config->mutex);
111
112 drm_modeset_acquire_init(ctx, 0);
113
114 retry:
115 ret = drm_modeset_lock_all_ctx(dev, ctx);
116 if (ret < 0) {
117 if (ret == -EDEADLK) {
118 drm_modeset_backoff(ctx);
119 goto retry;
120 }
121
122 drm_modeset_acquire_fini(ctx);
123 kfree(ctx);
124 return;
125 }
126 ww_acquire_done(&ctx->ww_ctx);
127
128 WARN_ON(config->acquire_ctx);
129
130 /*
131 * We hold the locks now, so it is safe to stash the acquisition
132 * context for drm_modeset_unlock_all().
133 */
134 config->acquire_ctx = ctx;
135
136 drm_warn_on_modeset_not_all_locked(dev);
137 }
138 EXPORT_SYMBOL(drm_modeset_lock_all);
139
140 /**
141 * drm_modeset_unlock_all - drop all modeset locks
142 * @dev: DRM device
143 *
144 * This function drops all modeset locks taken by a previous call to the
145 * drm_modeset_lock_all() function.
146 *
147 * This function is deprecated. It uses the lock acquisition context stored
148 * in &drm_device.mode_config. This facilitates conversion of existing
149 * code because it removes the need to manually deal with the acquisition
150 * context, but it is also brittle because the context is global and care must
151 * be taken not to nest calls. New code should pass the acquisition context
152 * directly to the drm_modeset_drop_locks() function.
153 */
drm_modeset_unlock_all(struct drm_device * dev)154 void drm_modeset_unlock_all(struct drm_device *dev)
155 {
156 struct drm_mode_config *config = &dev->mode_config;
157 struct drm_modeset_acquire_ctx *ctx = config->acquire_ctx;
158
159 if (WARN_ON(!ctx))
160 return;
161
162 config->acquire_ctx = NULL;
163 drm_modeset_drop_locks(ctx);
164 drm_modeset_acquire_fini(ctx);
165
166 kfree(ctx);
167
168 mutex_unlock(&dev->mode_config.mutex);
169 }
170 EXPORT_SYMBOL(drm_modeset_unlock_all);
171
172 /**
173 * drm_warn_on_modeset_not_all_locked - check that all modeset locks are locked
174 * @dev: device
175 *
176 * Useful as a debug assert.
177 */
drm_warn_on_modeset_not_all_locked(struct drm_device * dev)178 void drm_warn_on_modeset_not_all_locked(struct drm_device *dev)
179 {
180 struct drm_crtc *crtc;
181
182 /* Locking is currently fubar in the panic handler. */
183 if (oops_in_progress)
184 return;
185
186 drm_for_each_crtc(crtc, dev)
187 WARN_ON(!drm_modeset_is_locked(&crtc->mutex));
188
189 WARN_ON(!drm_modeset_is_locked(&dev->mode_config.connection_mutex));
190 WARN_ON(!mutex_is_locked(&dev->mode_config.mutex));
191 }
192 EXPORT_SYMBOL(drm_warn_on_modeset_not_all_locked);
193
194 /**
195 * drm_modeset_acquire_init - initialize acquire context
196 * @ctx: the acquire context
197 * @flags: 0 or %DRM_MODESET_ACQUIRE_INTERRUPTIBLE
198 *
199 * When passing %DRM_MODESET_ACQUIRE_INTERRUPTIBLE to @flags,
200 * all calls to drm_modeset_lock() will perform an interruptible
201 * wait.
202 */
drm_modeset_acquire_init(struct drm_modeset_acquire_ctx * ctx,uint32_t flags)203 void drm_modeset_acquire_init(struct drm_modeset_acquire_ctx *ctx,
204 uint32_t flags)
205 {
206 memset(ctx, 0, sizeof(*ctx));
207 ww_acquire_init(&ctx->ww_ctx, &crtc_ww_class);
208 INIT_LIST_HEAD(&ctx->locked);
209
210 if (flags & DRM_MODESET_ACQUIRE_INTERRUPTIBLE)
211 ctx->interruptible = true;
212 }
213 EXPORT_SYMBOL(drm_modeset_acquire_init);
214
215 /**
216 * drm_modeset_acquire_fini - cleanup acquire context
217 * @ctx: the acquire context
218 */
drm_modeset_acquire_fini(struct drm_modeset_acquire_ctx * ctx)219 void drm_modeset_acquire_fini(struct drm_modeset_acquire_ctx *ctx)
220 {
221 ww_acquire_fini(&ctx->ww_ctx);
222 }
223 EXPORT_SYMBOL(drm_modeset_acquire_fini);
224
225 /**
226 * drm_modeset_drop_locks - drop all locks
227 * @ctx: the acquire context
228 *
229 * Drop all locks currently held against this acquire context.
230 */
drm_modeset_drop_locks(struct drm_modeset_acquire_ctx * ctx)231 void drm_modeset_drop_locks(struct drm_modeset_acquire_ctx *ctx)
232 {
233 WARN_ON(ctx->contended);
234 while (!list_empty(&ctx->locked)) {
235 struct drm_modeset_lock *lock;
236
237 lock = list_first_entry(&ctx->locked,
238 struct drm_modeset_lock, head);
239
240 drm_modeset_unlock(lock);
241 }
242 }
243 EXPORT_SYMBOL(drm_modeset_drop_locks);
244
modeset_lock(struct drm_modeset_lock * lock,struct drm_modeset_acquire_ctx * ctx,bool interruptible,bool slow)245 static inline int modeset_lock(struct drm_modeset_lock *lock,
246 struct drm_modeset_acquire_ctx *ctx,
247 bool interruptible, bool slow)
248 {
249 int ret;
250
251 WARN_ON(ctx->contended);
252
253 if (ctx->trylock_only) {
254 lockdep_assert_held(&ctx->ww_ctx);
255
256 if (!ww_mutex_trylock(&lock->mutex))
257 return -EBUSY;
258 else
259 return 0;
260 } else if (interruptible && slow) {
261 ret = ww_mutex_lock_slow_interruptible(&lock->mutex, &ctx->ww_ctx);
262 } else if (interruptible) {
263 ret = ww_mutex_lock_interruptible(&lock->mutex, &ctx->ww_ctx);
264 } else if (slow) {
265 ww_mutex_lock_slow(&lock->mutex, &ctx->ww_ctx);
266 ret = 0;
267 } else {
268 ret = ww_mutex_lock(&lock->mutex, &ctx->ww_ctx);
269 }
270 if (!ret) {
271 WARN_ON(!list_empty(&lock->head));
272 list_add(&lock->head, &ctx->locked);
273 } else if (ret == -EALREADY) {
274 /* we already hold the lock.. this is fine. For atomic
275 * we will need to be able to drm_modeset_lock() things
276 * without having to keep track of what is already locked
277 * or not.
278 */
279 ret = 0;
280 } else if (ret == -EDEADLK) {
281 ctx->contended = lock;
282 }
283
284 return ret;
285 }
286
287 /**
288 * drm_modeset_backoff - deadlock avoidance backoff
289 * @ctx: the acquire context
290 *
291 * If deadlock is detected (ie. drm_modeset_lock() returns -EDEADLK),
292 * you must call this function to drop all currently held locks and
293 * block until the contended lock becomes available.
294 *
295 * This function returns 0 on success, or -ERESTARTSYS if this context
296 * is initialized with %DRM_MODESET_ACQUIRE_INTERRUPTIBLE and the
297 * wait has been interrupted.
298 */
drm_modeset_backoff(struct drm_modeset_acquire_ctx * ctx)299 int drm_modeset_backoff(struct drm_modeset_acquire_ctx *ctx)
300 {
301 struct drm_modeset_lock *contended = ctx->contended;
302
303 ctx->contended = NULL;
304
305 if (WARN_ON(!contended))
306 return 0;
307
308 drm_modeset_drop_locks(ctx);
309
310 return modeset_lock(contended, ctx, ctx->interruptible, true);
311 }
312 EXPORT_SYMBOL(drm_modeset_backoff);
313
314 /**
315 * drm_modeset_lock_init - initialize lock
316 * @lock: lock to init
317 */
drm_modeset_lock_init(struct drm_modeset_lock * lock)318 void drm_modeset_lock_init(struct drm_modeset_lock *lock)
319 {
320 ww_mutex_init(&lock->mutex, &crtc_ww_class);
321 INIT_LIST_HEAD(&lock->head);
322 }
323 EXPORT_SYMBOL(drm_modeset_lock_init);
324
325 /**
326 * drm_modeset_lock - take modeset lock
327 * @lock: lock to take
328 * @ctx: acquire ctx
329 *
330 * If @ctx is not NULL, then its ww acquire context is used and the
331 * lock will be tracked by the context and can be released by calling
332 * drm_modeset_drop_locks(). If -EDEADLK is returned, this means a
333 * deadlock scenario has been detected and it is an error to attempt
334 * to take any more locks without first calling drm_modeset_backoff().
335 *
336 * If the @ctx is not NULL and initialized with
337 * %DRM_MODESET_ACQUIRE_INTERRUPTIBLE, this function will fail with
338 * -ERESTARTSYS when interrupted.
339 *
340 * If @ctx is NULL then the function call behaves like a normal,
341 * uninterruptible non-nesting mutex_lock() call.
342 */
drm_modeset_lock(struct drm_modeset_lock * lock,struct drm_modeset_acquire_ctx * ctx)343 int drm_modeset_lock(struct drm_modeset_lock *lock,
344 struct drm_modeset_acquire_ctx *ctx)
345 {
346 if (ctx)
347 return modeset_lock(lock, ctx, ctx->interruptible, false);
348
349 ww_mutex_lock(&lock->mutex, NULL);
350 return 0;
351 }
352 EXPORT_SYMBOL(drm_modeset_lock);
353
354 /**
355 * drm_modeset_lock_single_interruptible - take a single modeset lock
356 * @lock: lock to take
357 *
358 * This function behaves as drm_modeset_lock() with a NULL context,
359 * but performs interruptible waits.
360 *
361 * This function returns 0 on success, or -ERESTARTSYS when interrupted.
362 */
drm_modeset_lock_single_interruptible(struct drm_modeset_lock * lock)363 int drm_modeset_lock_single_interruptible(struct drm_modeset_lock *lock)
364 {
365 return ww_mutex_lock_interruptible(&lock->mutex, NULL);
366 }
367 EXPORT_SYMBOL(drm_modeset_lock_single_interruptible);
368
369 /**
370 * drm_modeset_unlock - drop modeset lock
371 * @lock: lock to release
372 */
drm_modeset_unlock(struct drm_modeset_lock * lock)373 void drm_modeset_unlock(struct drm_modeset_lock *lock)
374 {
375 list_del_init(&lock->head);
376 ww_mutex_unlock(&lock->mutex);
377 }
378 EXPORT_SYMBOL(drm_modeset_unlock);
379
380 /**
381 * drm_modeset_lock_all_ctx - take all modeset locks
382 * @dev: DRM device
383 * @ctx: lock acquisition context
384 *
385 * This function takes all modeset locks, suitable where a more fine-grained
386 * scheme isn't (yet) implemented.
387 *
388 * Unlike drm_modeset_lock_all(), it doesn't take the &drm_mode_config.mutex
389 * since that lock isn't required for modeset state changes. Callers which
390 * need to grab that lock too need to do so outside of the acquire context
391 * @ctx.
392 *
393 * Locks acquired with this function should be released by calling the
394 * drm_modeset_drop_locks() function on @ctx.
395 *
396 * See also: DRM_MODESET_LOCK_ALL_BEGIN() and DRM_MODESET_LOCK_ALL_END()
397 *
398 * Returns: 0 on success or a negative error-code on failure.
399 */
drm_modeset_lock_all_ctx(struct drm_device * dev,struct drm_modeset_acquire_ctx * ctx)400 int drm_modeset_lock_all_ctx(struct drm_device *dev,
401 struct drm_modeset_acquire_ctx *ctx)
402 {
403 struct drm_private_obj *privobj;
404 struct drm_crtc *crtc;
405 struct drm_plane *plane;
406 int ret;
407
408 ret = drm_modeset_lock(&dev->mode_config.connection_mutex, ctx);
409 if (ret)
410 return ret;
411
412 drm_for_each_crtc(crtc, dev) {
413 ret = drm_modeset_lock(&crtc->mutex, ctx);
414 if (ret)
415 return ret;
416 }
417
418 drm_for_each_plane(plane, dev) {
419 ret = drm_modeset_lock(&plane->mutex, ctx);
420 if (ret)
421 return ret;
422 }
423
424 drm_for_each_privobj(privobj, dev) {
425 ret = drm_modeset_lock(&privobj->lock, ctx);
426 if (ret)
427 return ret;
428 }
429
430 return 0;
431 }
432 EXPORT_SYMBOL(drm_modeset_lock_all_ctx);
433