summaryrefslogtreecommitdiffstats
path: root/Documentation/driver-api/driver-model/devres.rst
blob: 4249eb4239e07c80bb30574495ad5bcbd0534569 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
================================
Devres - Managed Device Resource
================================

Tejun Heo	<teheo@suse.de>

First draft	10 January 2007

.. contents

   1. Intro			: Huh? Devres?
   2. Devres			: Devres in a nutshell
   3. Devres Group		: Group devres'es and release them together
   4. Details			: Life time rules, calling context, ...
   5. Overhead			: How much do we have to pay for this?
   6. List of managed interfaces: Currently implemented managed interfaces


1. Intro
--------

devres came up while trying to convert libata to use iomap.  Each
iomapped address should be kept and unmapped on driver detach.  For
example, a plain SFF ATA controller (that is, good old PCI IDE) in
native mode makes use of 5 PCI BARs and all of them should be
maintained.

As with many other device drivers, libata low level drivers have
sufficient bugs in ->remove and ->probe failure path.  Well, yes,
that's probably because libata low level driver developers are lazy
bunch, but aren't all low level driver developers?  After spending a
day fiddling with braindamaged hardware with no document or
braindamaged document, if it's finally working, well, it's working.

For one reason or another, low level drivers don't receive as much
attention or testing as core code, and bugs on driver detach or
initialization failure don't happen often enough to be noticeable.
Init failure path is worse because it's much less travelled while
needs to handle multiple entry points.

So, many low level drivers end up leaking resources on driver detach
and having half broken failure path implementation in ->probe() which
would leak resources or even cause oops when failure occurs.  iomap
adds more to this mix.  So do msi and msix.


2. Devres
---------

devres is basically linked list of arbitrarily sized memory areas
associated with a struct device.  Each devres entry is associated with
a release function.  A devres can be released in several ways.  No
matter what, all devres entries are released on driver detach.  On
release, the associated release function is invoked and then the
devres entry is freed.

Managed interface is created for resources commonly used by device
drivers using devres.  For example, coherent DMA memory is acquired
using dma_alloc_coherent().  The managed version is called
dmam_alloc_coherent().  It is identical to dma_alloc_coherent() except
for the DMA memory allocated using it is managed and will be
automatically released on driver detach.  Implementation looks like
the following::

  struct dma_devres {
	size_t		size;
	void		*vaddr;
	dma_addr_t	dma_handle;
  };

  static void dmam_coherent_release(struct device *dev, void *res)
  {
	struct dma_devres *this = res;

	dma_free_coherent(dev, this->size, this->vaddr, this->dma_handle);
  }

  dmam_alloc_coherent(dev, size, dma_handle, gfp)
  {
	struct dma_devres *dr;
	void *vaddr;

	dr = devres_alloc(dmam_coherent_release, sizeof(*dr), gfp);
	...

	/* alloc DMA memory as usual */
	vaddr = dma_alloc_coherent(...);
	...

	/* record size, vaddr, dma_handle in dr */
	dr->vaddr = vaddr;
	...

	devres_add(dev, dr);

	return vaddr;
  }

If a driver uses dmam_alloc_coherent(), the area is guaranteed to be
freed whether initialization fails half-way or the device gets
detached.  If most resources are acquired using managed interface, a
driver can have much simpler init and exit code.  Init path basically
looks like the following::

  my_init_one()
  {
	struct mydev *d;

	d = devm_kzalloc(dev, sizeof(*d), GFP_KERNEL);
	if (!d)
		return -ENOMEM;

	d->ring = dmam_alloc_coherent(...);
	if (!d->ring)
		return -ENOMEM;

	if (check something)
		return -EINVAL;
	...

	return register_to_upper_layer(d);
  }

And exit path::

  my_remove_one()
  {
	unregister_from_upper_layer(d);
	shutdown_my_hardware();
  }

As shown above, low level drivers can be simplified a lot by using
devres.  Complexity is shifted from less maintained low level drivers
to better maintained higher layer.  Also, as init failure path is
shared with exit path, both can get more testing.

Note though that when converting current calls or assignments to
managed devm_* versions it is up to you to check if internal operations
like allocating memory, have failed. Managed resources pertains to the
freeing of these resources *only* - all other checks needed are still
on you. In some cases this may mean introducing checks that were not
necessary before moving to the managed devm_* calls.


3. Devres group
---------------

Devres entries can be grouped using devres group.  When a group is
released, all contained normal devres entries and properly nested
groups are released.  One usage is to rollback series of acquired
resources on failure.  For example::

  if (!devres_open_group(dev, NULL, GFP_KERNEL))
	return -ENOMEM;

  acquire A;
  if (failed)
	goto err;

  acquire B;
  if (failed)
	goto err;
  ...

  devres_remove_group(dev, NULL);
  return 0;

 err:
  devres_release_group(dev, NULL);
  return err_code;

As resource acquisition failure usually means probe failure, constructs
like above are usually useful in midlayer driver (e.g. libata core
layer) where interface function shouldn't have side effect on failure.
For LLDs, just returning error code suffices in most cases.

Each group is identified by `void *id`.  It can either be explicitly
specified by @id argument to devres_open_group() or automatically
created by passing NULL as @id as in the above example.  In both
cases, devres_open_group() returns the group's id.  The returned id
can be passed to other devres functions to select the target group.
If NULL is given to those functions, the latest open group is
selected.

For example, you can do something like the following::

  int my_midlayer_create_something()
  {
	if (!devres_open_group(dev, my_midlayer_create_something, GFP_KERNEL))
		return -ENOMEM;

	...

	devres_close_group(dev, my_midlayer_create_something);
	return 0;
  }

  void my_midlayer_destroy_something()
  {
	devres_release_group(dev, my_midlayer_create_something);
  }


4. Details
----------

Lifetime of a devres entry begins on devres allocation and finishes
when it is released or destroyed (removed and freed) - no reference
counting.

devres core guarantees atomicity to all basic devres operations and
has support for single-instance devres types (atomic
lookup-and-add-if-not-found).  Other than that, synchronizing
concurrent accesses to allocated devres data is caller's
responsibility.  This is usually non-issue because bus ops and
resource allocations already do the job.

For an example of single-instance devres type, read pcim_iomap_table()
in lib/devres.c.

All devres interface functions can be called without context if the
right gfp mask is given.


5. Overhead
-----------

Each devres bookkeeping info is allocated together with requested data
area.  With debug option turned off, bookkeeping info occupies 16
bytes on 32bit machines and 24 bytes on 64bit (three pointers rounded
up to ull alignment).  If singly linked list is used, it can be
reduced to two pointers (8 bytes on 32bit, 16 bytes on 64bit).

Each devres group occupies 8 pointers.  It can be reduced to 6 if
singly linked list is used.

Memory space overhead on ahci controller with two ports is between 300
and 400 bytes on 32bit machine after naive conversion (we can
certainly invest a bit more effort into libata core layer).


6. List of managed interfaces
-----------------------------

CLOCK
  devm_clk_get()
  devm_clk_get_optional()
  devm_clk_put()
  devm_clk_bulk_get()
  devm_clk_bulk_get_all()
  devm_clk_bulk_get_optional()
  devm_get_clk_from_child()
  devm_clk_hw_register()
  devm_of_clk_add_hw_provider()
  devm_clk_hw_register_clkdev()

DMA
  dmaenginem_async_device_register()
  dmam_alloc_coherent()
  dmam_alloc_attrs()
  dmam_free_coherent()
  dmam_pool_create()
  dmam_pool_destroy()

DRM
  devm_drm_dev_alloc()

GPIO
  devm_gpiod_get()
  devm_gpiod_get_array()
  devm_gpiod_get_array_optional()
  devm_gpiod_get_index()
  devm_gpiod_get_index_optional()
  devm_gpiod_get_optional()
  devm_gpiod_put()
  devm_gpiod_unhinge()
  devm_gpiochip_add_data()
  devm_gpio_request()
  devm_gpio_request_one()

I2C
  devm_i2c_add_adapter()
  devm_i2c_new_dummy_device()

IIO
  devm_iio_device_alloc()
  devm_iio_device_register()
  devm_iio_dmaengine_buffer_setup()
  devm_iio_kfifo_buffer_setup()
  devm_iio_kfifo_buffer_setup_ext()
  devm_iio_map_array_register()
  devm_iio_triggered_buffer_setup()
  devm_iio_triggered_buffer_setup_ext()
  devm_iio_trigger_alloc()
  devm_iio_trigger_register()
  devm_iio_channel_get()
  devm_iio_channel_get_all()
  devm_iio_hw_consumer_alloc()
  devm_fwnode_iio_channel_get_by_name()

INPUT
  devm_input_allocate_device()

IO region
  devm_release_mem_region()
  devm_release_region()
  devm_release_resource()
  devm_request_mem_region()
  devm_request_free_mem_region()
  devm_request_region()
  devm_request_resource()

IOMAP
  devm_ioport_map()
  devm_ioport_unmap()
  devm_ioremap()
  devm_ioremap_uc()
  devm_ioremap_wc()
  devm_ioremap_resource() : checks resource, requests memory region, ioremaps
  devm_ioremap_resource_wc()
  devm_platform_ioremap_resource() : calls devm_ioremap_resource() for platform device
  devm_platform_ioremap_resource_byname()
  devm_platform_get_and_ioremap_resource()
  devm_iounmap()
  pcim_iomap()
  pcim_iomap_regions()	: do request_region() and iomap() on multiple BARs
  pcim_iomap_table()	: array of mapped addresses indexed by BAR
  pcim_iounmap()

IRQ
  devm_free_irq()
  devm_request_any_context_irq()
  devm_request_irq()
  devm_request_threaded_irq()
  devm_irq_alloc_descs()
  devm_irq_alloc_desc()
  devm_irq_alloc_desc_at()
  devm_irq_alloc_desc_from()
  devm_irq_alloc_descs_from()
  devm_irq_alloc_generic_chip()
  devm_irq_setup_generic_chip()
  devm_irq_domain_create_sim()

LED
  devm_led_classdev_register()
  devm_led_classdev_register_ext()
  devm_led_classdev_unregister()
  devm_led_trigger_register()
  devm_of_led_get()

MDIO
  devm_mdiobus_alloc()
  devm_mdiobus_alloc_size()
  devm_mdiobus_register()
  devm_of_mdiobus_register()

MEM
  devm_free_pages()
  devm_get_free_pages()
  devm_kasprintf()
  devm_kcalloc()
  devm_kfree()
  devm_kmalloc()
  devm_kmalloc_array()
  devm_kmemdup()
  devm_krealloc()
  devm_kstrdup()
  devm_kstrdup_const()
  devm_kvasprintf()
  devm_kzalloc()

MFD
  devm_mfd_add_devices()

MUX
  devm_mux_chip_alloc()
  devm_mux_chip_register()
  devm_mux_control_get()
  devm_mux_state_get()

NET
  devm_alloc_etherdev()
  devm_alloc_etherdev_mqs()
  devm_register_netdev()

PER-CPU MEM
  devm_alloc_percpu()
  devm_free_percpu()

PCI
  devm_pci_alloc_host_bridge()  : managed PCI host bridge allocation
  devm_pci_remap_cfgspace()	: ioremap PCI configuration space
  devm_pci_remap_cfg_resource()	: ioremap PCI configuration space resource
  pcim_enable_device()		: after success, all PCI ops become managed
  pcim_pin_device()		: keep PCI device enabled after release

PHY
  devm_usb_get_phy()
  devm_usb_get_phy_by_node()
  devm_usb_get_phy_by_phandle()
  devm_usb_put_phy()

PINCTRL
  devm_pinctrl_get()
  devm_pinctrl_put()
  devm_pinctrl_get_select()
  devm_pinctrl_register()
  devm_pinctrl_register_and_init()
  devm_pinctrl_unregister()

POWER
  devm_reboot_mode_register()
  devm_reboot_mode_unregister()

PWM
  devm_pwmchip_add()
  devm_pwm_get()
  devm_fwnode_pwm_get()

REGULATOR
  devm_regulator_bulk_register_supply_alias()
  devm_regulator_bulk_get()
  devm_regulator_bulk_get_const()
  devm_regulator_bulk_get_enable()
  devm_regulator_bulk_put()
  devm_regulator_get()
  devm_regulator_get_enable()
  devm_regulator_get_enable_optional()
  devm_regulator_get_exclusive()
  devm_regulator_get_optional()
  devm_regulator_irq_helper()
  devm_regulator_put()
  devm_regulator_register()
  devm_regulator_register_notifier()
  devm_regulator_register_supply_alias()
  devm_regulator_unregister_notifier()

RESET
  devm_reset_control_get()
  devm_reset_controller_register()

RTC
  devm_rtc_device_register()
  devm_rtc_allocate_device()
  devm_rtc_register_device()
  devm_rtc_nvmem_register()

SERDEV
  devm_serdev_device_open()

SLAVE DMA ENGINE
  devm_acpi_dma_controller_register()
  devm_acpi_dma_controller_free()

SPI
  devm_spi_alloc_master()
  devm_spi_alloc_slave()
  devm_spi_register_master()

WATCHDOG
  devm_watchdog_register_device()