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1/*
2 * Created: Tue Feb 2 08:37:54 1999 by faith@valinux.com
3 *
4 * Copyright 1999 Precision Insight, Inc., Cedar Park, Texas.
5 * Copyright 2000 VA Linux Systems, Inc., Sunnyvale, California.
6 * All Rights Reserved.
7 *
8 * Author Rickard E. (Rik) Faith <faith@valinux.com>
9 * Author Gareth Hughes <gareth@valinux.com>
10 *
11 * Permission is hereby granted, free of charge, to any person obtaining a
12 * copy of this software and associated documentation files (the "Software"),
13 * to deal in the Software without restriction, including without limitation
14 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
15 * and/or sell copies of the Software, and to permit persons to whom the
16 * Software is furnished to do so, subject to the following conditions:
17 *
18 * The above copyright notice and this permission notice (including the next
19 * paragraph) shall be included in all copies or substantial portions of the
20 * Software.
21 *
22 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
23 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
24 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
25 * VA LINUX SYSTEMS AND/OR ITS SUPPLIERS BE LIABLE FOR ANY CLAIM, DAMAGES OR
26 * OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
27 * ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
28 * OTHER DEALINGS IN THE SOFTWARE.
29 */
30
31#include <linux/slab.h>
32
33#include <drm/drm_auth.h>
34#include <drm/drm_drv.h>
35#include <drm/drm_file.h>
36#include <drm/drm_lease.h>
37#include <drm/drm_print.h>
38
39#include "drm_internal.h"
40#include "drm_legacy.h"
41
42/**
43 * DOC: master and authentication
44 *
45 * &struct drm_master is used to track groups of clients with open
46 * primary/legacy device nodes. For every &struct drm_file which has had at
47 * least once successfully became the device master (either through the
48 * SET_MASTER IOCTL, or implicitly through opening the primary device node when
49 * no one else is the current master that time) there exists one &drm_master.
50 * This is noted in &drm_file.is_master. All other clients have just a pointer
51 * to the &drm_master they are associated with.
52 *
53 * In addition only one &drm_master can be the current master for a &drm_device.
54 * It can be switched through the DROP_MASTER and SET_MASTER IOCTL, or
55 * implicitly through closing/openeing the primary device node. See also
56 * drm_is_current_master().
57 *
58 * Clients can authenticate against the current master (if it matches their own)
59 * using the GETMAGIC and AUTHMAGIC IOCTLs. Together with exchanging masters,
60 * this allows controlled access to the device for an entire group of mutually
61 * trusted clients.
62 */
63
64int drm_getmagic(struct drm_device *dev, void *data, struct drm_file *file_priv)
65{
66 struct drm_auth *auth = data;
67 int ret = 0;
68
69 mutex_lock(&dev->master_mutex);
70 if (!file_priv->magic) {
71 ret = idr_alloc(&file_priv->master->magic_map, file_priv,
72 1, 0, GFP_KERNEL);
73 if (ret >= 0)
74 file_priv->magic = ret;
75 }
76 auth->magic = file_priv->magic;
77 mutex_unlock(&dev->master_mutex);
78
79 DRM_DEBUG("%u\n", auth->magic);
80
81 return ret < 0 ? ret : 0;
82}
83
84int drm_authmagic(struct drm_device *dev, void *data,
85 struct drm_file *file_priv)
86{
87 struct drm_auth *auth = data;
88 struct drm_file *file;
89
90 DRM_DEBUG("%u\n", auth->magic);
91
92 mutex_lock(&dev->master_mutex);
93 file = idr_find(&file_priv->master->magic_map, auth->magic);
94 if (file) {
95 file->authenticated = 1;
96 idr_replace(&file_priv->master->magic_map, NULL, auth->magic);
97 }
98 mutex_unlock(&dev->master_mutex);
99
100 return file ? 0 : -EINVAL;
101}
102
103struct drm_master *drm_master_create(struct drm_device *dev)
104{
105 struct drm_master *master;
106
107 master = kzalloc(sizeof(*master), GFP_KERNEL);
108 if (!master)
109 return NULL;
110
111 kref_init(&master->refcount);
112 drm_master_legacy_init(master);
113 idr_init(&master->magic_map);
114 master->dev = dev;
115
116 /* initialize the tree of output resource lessees */
117 INIT_LIST_HEAD(&master->lessees);
118 INIT_LIST_HEAD(&master->lessee_list);
119 idr_init(&master->leases);
120 idr_init(&master->lessee_idr);
121
122 return master;
123}
124
125static int drm_set_master(struct drm_device *dev, struct drm_file *fpriv,
126 bool new_master)
127{
128 int ret = 0;
129
130 dev->master = drm_master_get(fpriv->master);
131 if (dev->driver->master_set) {
132 ret = dev->driver->master_set(dev, fpriv, new_master);
133 if (unlikely(ret != 0)) {
134 drm_master_put(&dev->master);
135 }
136 }
137
138 return ret;
139}
140
141static int drm_new_set_master(struct drm_device *dev, struct drm_file *fpriv)
142{
143 struct drm_master *old_master;
144 int ret;
145
146 lockdep_assert_held_once(&dev->master_mutex);
147
148 WARN_ON(fpriv->is_master);
149 old_master = fpriv->master;
150 fpriv->master = drm_master_create(dev);
151 if (!fpriv->master) {
152 fpriv->master = old_master;
153 return -ENOMEM;
154 }
155
156 if (dev->driver->master_create) {
157 ret = dev->driver->master_create(dev, fpriv->master);
158 if (ret)
159 goto out_err;
160 }
161 fpriv->is_master = 1;
162 fpriv->authenticated = 1;
163
164 ret = drm_set_master(dev, fpriv, true);
165 if (ret)
166 goto out_err;
167
168 if (old_master)
169 drm_master_put(&old_master);
170
171 return 0;
172
173out_err:
174 /* drop references and restore old master on failure */
175 drm_master_put(&fpriv->master);
176 fpriv->master = old_master;
177 fpriv->is_master = 0;
178
179 return ret;
180}
181
182int drm_setmaster_ioctl(struct drm_device *dev, void *data,
183 struct drm_file *file_priv)
184{
185 int ret = 0;
186
187 mutex_lock(&dev->master_mutex);
188 if (drm_is_current_master(file_priv))
189 goto out_unlock;
190
191 if (dev->master) {
192 ret = -EINVAL;
193 goto out_unlock;
194 }
195
196 if (!file_priv->master) {
197 ret = -EINVAL;
198 goto out_unlock;
199 }
200
201 if (!file_priv->is_master) {
202 ret = drm_new_set_master(dev, file_priv);
203 goto out_unlock;
204 }
205
206 if (file_priv->master->lessor != NULL) {
207 DRM_DEBUG_LEASE("Attempt to set lessee %d as master\n", file_priv->master->lessee_id);
208 ret = -EINVAL;
209 goto out_unlock;
210 }
211
212 ret = drm_set_master(dev, file_priv, false);
213out_unlock:
214 mutex_unlock(&dev->master_mutex);
215 return ret;
216}
217
218static void drm_drop_master(struct drm_device *dev,
219 struct drm_file *fpriv)
220{
221 if (dev->driver->master_drop)
222 dev->driver->master_drop(dev, fpriv);
223 drm_master_put(&dev->master);
224}
225
226int drm_dropmaster_ioctl(struct drm_device *dev, void *data,
227 struct drm_file *file_priv)
228{
229 int ret = -EINVAL;
230
231 mutex_lock(&dev->master_mutex);
232 if (!drm_is_current_master(file_priv))
233 goto out_unlock;
234
235 if (!dev->master)
236 goto out_unlock;
237
238 if (file_priv->master->lessor != NULL) {
239 DRM_DEBUG_LEASE("Attempt to drop lessee %d as master\n", file_priv->master->lessee_id);
240 ret = -EINVAL;
241 goto out_unlock;
242 }
243
244 ret = 0;
245 drm_drop_master(dev, file_priv);
246out_unlock:
247 mutex_unlock(&dev->master_mutex);
248 return ret;
249}
250
251int drm_master_open(struct drm_file *file_priv)
252{
253 struct drm_device *dev = file_priv->minor->dev;
254 int ret = 0;
255
256 /* if there is no current master make this fd it, but do not create
257 * any master object for render clients */
258 mutex_lock(&dev->master_mutex);
259 if (!dev->master)
260 ret = drm_new_set_master(dev, file_priv);
261 else
262 file_priv->master = drm_master_get(dev->master);
263 mutex_unlock(&dev->master_mutex);
264
265 return ret;
266}
267
268void drm_master_release(struct drm_file *file_priv)
269{
270 struct drm_device *dev = file_priv->minor->dev;
271 struct drm_master *master = file_priv->master;
272
273 mutex_lock(&dev->master_mutex);
274 if (file_priv->magic)
275 idr_remove(&file_priv->master->magic_map, file_priv->magic);
276
277 if (!drm_is_current_master(file_priv))
278 goto out;
279
280 drm_legacy_lock_master_cleanup(dev, master);
281
282 if (dev->master == file_priv->master)
283 drm_drop_master(dev, file_priv);
284out:
285 if (drm_core_check_feature(dev, DRIVER_MODESET) && file_priv->is_master) {
286 /* Revoke any leases held by this or lessees, but only if
287 * this is the "real" master
288 */
289 drm_lease_revoke(master);
290 }
291
292 /* drop the master reference held by the file priv */
293 if (file_priv->master)
294 drm_master_put(&file_priv->master);
295 mutex_unlock(&dev->master_mutex);
296}
297
298/**
299 * drm_is_current_master - checks whether @priv is the current master
300 * @fpriv: DRM file private
301 *
302 * Checks whether @fpriv is current master on its device. This decides whether a
303 * client is allowed to run DRM_MASTER IOCTLs.
304 *
305 * Most of the modern IOCTL which require DRM_MASTER are for kernel modesetting
306 * - the current master is assumed to own the non-shareable display hardware.
307 */
308bool drm_is_current_master(struct drm_file *fpriv)
309{
310 return fpriv->is_master && drm_lease_owner(fpriv->master) == fpriv->minor->dev->master;
311}
312EXPORT_SYMBOL(drm_is_current_master);
313
314/**
315 * drm_master_get - reference a master pointer
316 * @master: &struct drm_master
317 *
318 * Increments the reference count of @master and returns a pointer to @master.
319 */
320struct drm_master *drm_master_get(struct drm_master *master)
321{
322 kref_get(&master->refcount);
323 return master;
324}
325EXPORT_SYMBOL(drm_master_get);
326
327static void drm_master_destroy(struct kref *kref)
328{
329 struct drm_master *master = container_of(kref, struct drm_master, refcount);
330 struct drm_device *dev = master->dev;
331
332 if (drm_core_check_feature(dev, DRIVER_MODESET))
333 drm_lease_destroy(master);
334
335 if (dev->driver->master_destroy)
336 dev->driver->master_destroy(dev, master);
337
338 drm_legacy_master_rmmaps(dev, master);
339
340 idr_destroy(&master->magic_map);
341 idr_destroy(&master->leases);
342 idr_destroy(&master->lessee_idr);
343
344 kfree(master->unique);
345 kfree(master);
346}
347
348/**
349 * drm_master_put - unreference and clear a master pointer
350 * @master: pointer to a pointer of &struct drm_master
351 *
352 * This decrements the &drm_master behind @master and sets it to NULL.
353 */
354void drm_master_put(struct drm_master **master)
355{
356 kref_put(&(*master)->refcount, drm_master_destroy);
357 *master = NULL;
358}
359EXPORT_SYMBOL(drm_master_put);
360
361/* Used by drm_client and drm_fb_helper */
362bool drm_master_internal_acquire(struct drm_device *dev)
363{
364 mutex_lock(&dev->master_mutex);
365 if (dev->master) {
366 mutex_unlock(&dev->master_mutex);
367 return false;
368 }
369
370 return true;
371}
372EXPORT_SYMBOL(drm_master_internal_acquire);
373
374/* Used by drm_client and drm_fb_helper */
375void drm_master_internal_release(struct drm_device *dev)
376{
377 mutex_unlock(&dev->master_mutex);
378}
379EXPORT_SYMBOL(drm_master_internal_release);
1/*
2 * Created: Tue Feb 2 08:37:54 1999 by faith@valinux.com
3 *
4 * Copyright 1999 Precision Insight, Inc., Cedar Park, Texas.
5 * Copyright 2000 VA Linux Systems, Inc., Sunnyvale, California.
6 * All Rights Reserved.
7 *
8 * Author Rickard E. (Rik) Faith <faith@valinux.com>
9 * Author Gareth Hughes <gareth@valinux.com>
10 *
11 * Permission is hereby granted, free of charge, to any person obtaining a
12 * copy of this software and associated documentation files (the "Software"),
13 * to deal in the Software without restriction, including without limitation
14 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
15 * and/or sell copies of the Software, and to permit persons to whom the
16 * Software is furnished to do so, subject to the following conditions:
17 *
18 * The above copyright notice and this permission notice (including the next
19 * paragraph) shall be included in all copies or substantial portions of the
20 * Software.
21 *
22 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
23 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
24 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
25 * VA LINUX SYSTEMS AND/OR ITS SUPPLIERS BE LIABLE FOR ANY CLAIM, DAMAGES OR
26 * OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
27 * ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
28 * OTHER DEALINGS IN THE SOFTWARE.
29 */
30
31#include <drm/drmP.h>
32#include "drm_internal.h"
33#include "drm_legacy.h"
34#include <drm/drm_lease.h>
35
36/**
37 * DOC: master and authentication
38 *
39 * &struct drm_master is used to track groups of clients with open
40 * primary/legacy device nodes. For every &struct drm_file which has had at
41 * least once successfully became the device master (either through the
42 * SET_MASTER IOCTL, or implicitly through opening the primary device node when
43 * no one else is the current master that time) there exists one &drm_master.
44 * This is noted in &drm_file.is_master. All other clients have just a pointer
45 * to the &drm_master they are associated with.
46 *
47 * In addition only one &drm_master can be the current master for a &drm_device.
48 * It can be switched through the DROP_MASTER and SET_MASTER IOCTL, or
49 * implicitly through closing/openeing the primary device node. See also
50 * drm_is_current_master().
51 *
52 * Clients can authenticate against the current master (if it matches their own)
53 * using the GETMAGIC and AUTHMAGIC IOCTLs. Together with exchanging masters,
54 * this allows controlled access to the device for an entire group of mutually
55 * trusted clients.
56 */
57
58int drm_getmagic(struct drm_device *dev, void *data, struct drm_file *file_priv)
59{
60 struct drm_auth *auth = data;
61 int ret = 0;
62
63 mutex_lock(&dev->master_mutex);
64 if (!file_priv->magic) {
65 ret = idr_alloc(&file_priv->master->magic_map, file_priv,
66 1, 0, GFP_KERNEL);
67 if (ret >= 0)
68 file_priv->magic = ret;
69 }
70 auth->magic = file_priv->magic;
71 mutex_unlock(&dev->master_mutex);
72
73 DRM_DEBUG("%u\n", auth->magic);
74
75 return ret < 0 ? ret : 0;
76}
77
78int drm_authmagic(struct drm_device *dev, void *data,
79 struct drm_file *file_priv)
80{
81 struct drm_auth *auth = data;
82 struct drm_file *file;
83
84 DRM_DEBUG("%u\n", auth->magic);
85
86 mutex_lock(&dev->master_mutex);
87 file = idr_find(&file_priv->master->magic_map, auth->magic);
88 if (file) {
89 file->authenticated = 1;
90 idr_replace(&file_priv->master->magic_map, NULL, auth->magic);
91 }
92 mutex_unlock(&dev->master_mutex);
93
94 return file ? 0 : -EINVAL;
95}
96
97struct drm_master *drm_master_create(struct drm_device *dev)
98{
99 struct drm_master *master;
100
101 master = kzalloc(sizeof(*master), GFP_KERNEL);
102 if (!master)
103 return NULL;
104
105 kref_init(&master->refcount);
106 spin_lock_init(&master->lock.spinlock);
107 init_waitqueue_head(&master->lock.lock_queue);
108 idr_init(&master->magic_map);
109 master->dev = dev;
110
111 /* initialize the tree of output resource lessees */
112 master->lessor = NULL;
113 master->lessee_id = 0;
114 INIT_LIST_HEAD(&master->lessees);
115 INIT_LIST_HEAD(&master->lessee_list);
116 idr_init(&master->leases);
117 idr_init(&master->lessee_idr);
118
119 return master;
120}
121
122static int drm_set_master(struct drm_device *dev, struct drm_file *fpriv,
123 bool new_master)
124{
125 int ret = 0;
126
127 dev->master = drm_master_get(fpriv->master);
128 if (dev->driver->master_set) {
129 ret = dev->driver->master_set(dev, fpriv, new_master);
130 if (unlikely(ret != 0)) {
131 drm_master_put(&dev->master);
132 }
133 }
134
135 return ret;
136}
137
138static int drm_new_set_master(struct drm_device *dev, struct drm_file *fpriv)
139{
140 struct drm_master *old_master;
141 int ret;
142
143 lockdep_assert_held_once(&dev->master_mutex);
144
145 old_master = fpriv->master;
146 fpriv->master = drm_master_create(dev);
147 if (!fpriv->master) {
148 fpriv->master = old_master;
149 return -ENOMEM;
150 }
151
152 if (dev->driver->master_create) {
153 ret = dev->driver->master_create(dev, fpriv->master);
154 if (ret)
155 goto out_err;
156 }
157 fpriv->is_master = 1;
158 fpriv->authenticated = 1;
159
160 ret = drm_set_master(dev, fpriv, true);
161 if (ret)
162 goto out_err;
163
164 if (old_master)
165 drm_master_put(&old_master);
166
167 return 0;
168
169out_err:
170 /* drop references and restore old master on failure */
171 drm_master_put(&fpriv->master);
172 fpriv->master = old_master;
173
174 return ret;
175}
176
177int drm_setmaster_ioctl(struct drm_device *dev, void *data,
178 struct drm_file *file_priv)
179{
180 int ret = 0;
181
182 mutex_lock(&dev->master_mutex);
183 if (drm_is_current_master(file_priv))
184 goto out_unlock;
185
186 if (dev->master) {
187 ret = -EINVAL;
188 goto out_unlock;
189 }
190
191 if (!file_priv->master) {
192 ret = -EINVAL;
193 goto out_unlock;
194 }
195
196 if (!file_priv->is_master) {
197 ret = drm_new_set_master(dev, file_priv);
198 goto out_unlock;
199 }
200
201 if (file_priv->master->lessor != NULL) {
202 DRM_DEBUG_LEASE("Attempt to set lessee %d as master\n", file_priv->master->lessee_id);
203 ret = -EINVAL;
204 goto out_unlock;
205 }
206
207 ret = drm_set_master(dev, file_priv, false);
208out_unlock:
209 mutex_unlock(&dev->master_mutex);
210 return ret;
211}
212
213static void drm_drop_master(struct drm_device *dev,
214 struct drm_file *fpriv)
215{
216 if (dev->driver->master_drop)
217 dev->driver->master_drop(dev, fpriv);
218 drm_master_put(&dev->master);
219}
220
221int drm_dropmaster_ioctl(struct drm_device *dev, void *data,
222 struct drm_file *file_priv)
223{
224 int ret = -EINVAL;
225
226 mutex_lock(&dev->master_mutex);
227 if (!drm_is_current_master(file_priv))
228 goto out_unlock;
229
230 if (!dev->master)
231 goto out_unlock;
232
233 if (file_priv->master->lessor != NULL) {
234 DRM_DEBUG_LEASE("Attempt to drop lessee %d as master\n", file_priv->master->lessee_id);
235 ret = -EINVAL;
236 goto out_unlock;
237 }
238
239 ret = 0;
240 drm_drop_master(dev, file_priv);
241out_unlock:
242 mutex_unlock(&dev->master_mutex);
243 return ret;
244}
245
246int drm_master_open(struct drm_file *file_priv)
247{
248 struct drm_device *dev = file_priv->minor->dev;
249 int ret = 0;
250
251 /* if there is no current master make this fd it, but do not create
252 * any master object for render clients */
253 mutex_lock(&dev->master_mutex);
254 if (!dev->master)
255 ret = drm_new_set_master(dev, file_priv);
256 else
257 file_priv->master = drm_master_get(dev->master);
258 mutex_unlock(&dev->master_mutex);
259
260 return ret;
261}
262
263void drm_master_release(struct drm_file *file_priv)
264{
265 struct drm_device *dev = file_priv->minor->dev;
266 struct drm_master *master = file_priv->master;
267
268 mutex_lock(&dev->master_mutex);
269 if (file_priv->magic)
270 idr_remove(&file_priv->master->magic_map, file_priv->magic);
271
272 if (!drm_is_current_master(file_priv))
273 goto out;
274
275 if (drm_core_check_feature(dev, DRIVER_LEGACY)) {
276 /*
277 * Since the master is disappearing, so is the
278 * possibility to lock.
279 */
280 mutex_lock(&dev->struct_mutex);
281 if (master->lock.hw_lock) {
282 if (dev->sigdata.lock == master->lock.hw_lock)
283 dev->sigdata.lock = NULL;
284 master->lock.hw_lock = NULL;
285 master->lock.file_priv = NULL;
286 wake_up_interruptible_all(&master->lock.lock_queue);
287 }
288 mutex_unlock(&dev->struct_mutex);
289 }
290
291 if (dev->master == file_priv->master)
292 drm_drop_master(dev, file_priv);
293out:
294 if (drm_core_check_feature(dev, DRIVER_MODESET) && file_priv->is_master) {
295 /* Revoke any leases held by this or lessees, but only if
296 * this is the "real" master
297 */
298 drm_lease_revoke(master);
299 }
300
301 /* drop the master reference held by the file priv */
302 if (file_priv->master)
303 drm_master_put(&file_priv->master);
304 mutex_unlock(&dev->master_mutex);
305}
306
307/**
308 * drm_is_current_master - checks whether @priv is the current master
309 * @fpriv: DRM file private
310 *
311 * Checks whether @fpriv is current master on its device. This decides whether a
312 * client is allowed to run DRM_MASTER IOCTLs.
313 *
314 * Most of the modern IOCTL which require DRM_MASTER are for kernel modesetting
315 * - the current master is assumed to own the non-shareable display hardware.
316 */
317bool drm_is_current_master(struct drm_file *fpriv)
318{
319 return fpriv->is_master && drm_lease_owner(fpriv->master) == fpriv->minor->dev->master;
320}
321EXPORT_SYMBOL(drm_is_current_master);
322
323/**
324 * drm_master_get - reference a master pointer
325 * @master: &struct drm_master
326 *
327 * Increments the reference count of @master and returns a pointer to @master.
328 */
329struct drm_master *drm_master_get(struct drm_master *master)
330{
331 kref_get(&master->refcount);
332 return master;
333}
334EXPORT_SYMBOL(drm_master_get);
335
336static void drm_master_destroy(struct kref *kref)
337{
338 struct drm_master *master = container_of(kref, struct drm_master, refcount);
339 struct drm_device *dev = master->dev;
340
341 if (drm_core_check_feature(dev, DRIVER_MODESET))
342 drm_lease_destroy(master);
343
344 if (dev->driver->master_destroy)
345 dev->driver->master_destroy(dev, master);
346
347 drm_legacy_master_rmmaps(dev, master);
348
349 idr_destroy(&master->magic_map);
350 idr_destroy(&master->leases);
351 idr_destroy(&master->lessee_idr);
352
353 kfree(master->unique);
354 kfree(master);
355}
356
357/**
358 * drm_master_put - unreference and clear a master pointer
359 * @master: pointer to a pointer of &struct drm_master
360 *
361 * This decrements the &drm_master behind @master and sets it to NULL.
362 */
363void drm_master_put(struct drm_master **master)
364{
365 kref_put(&(*master)->refcount, drm_master_destroy);
366 *master = NULL;
367}
368EXPORT_SYMBOL(drm_master_put);