1================================
2Devres - Managed Device Resource
3================================
4
5Tejun Heo	<teheo@suse.de>
6
7First draft	10 January 2007
8
9.. contents
10
11   1. Intro			: Huh? Devres?
12   2. Devres			: Devres in a nutshell
13   3. Devres Group		: Group devres'es and release them together
14   4. Details			: Life time rules, calling context, ...
15   5. Overhead			: How much do we have to pay for this?
16   6. List of managed interfaces: Currently implemented managed interfaces
17
18
191. Intro
20--------
21
22devres came up while trying to convert libata to use iomap.  Each
23iomapped address should be kept and unmapped on driver detach.  For
24example, a plain SFF ATA controller (that is, good old PCI IDE) in
25native mode makes use of 5 PCI BARs and all of them should be
26maintained.
27
28As with many other device drivers, libata low level drivers have
29sufficient bugs in ->remove and ->probe failure path.  Well, yes,
30that's probably because libata low level driver developers are lazy
31bunch, but aren't all low level driver developers?  After spending a
32day fiddling with braindamaged hardware with no document or
33braindamaged document, if it's finally working, well, it's working.
34
35For one reason or another, low level drivers don't receive as much
36attention or testing as core code, and bugs on driver detach or
37initialization failure don't happen often enough to be noticeable.
38Init failure path is worse because it's much less travelled while
39needs to handle multiple entry points.
40
41So, many low level drivers end up leaking resources on driver detach
42and having half broken failure path implementation in ->probe() which
43would leak resources or even cause oops when failure occurs.  iomap
44adds more to this mix.  So do msi and msix.
45
46
472. Devres
48---------
49
50devres is basically linked list of arbitrarily sized memory areas
51associated with a struct device.  Each devres entry is associated with
52a release function.  A devres can be released in several ways.  No
53matter what, all devres entries are released on driver detach.  On
54release, the associated release function is invoked and then the
55devres entry is freed.
56
57Managed interface is created for resources commonly used by device
58drivers using devres.  For example, coherent DMA memory is acquired
59using dma_alloc_coherent().  The managed version is called
60dmam_alloc_coherent().  It is identical to dma_alloc_coherent() except
61for the DMA memory allocated using it is managed and will be
62automatically released on driver detach.  Implementation looks like
63the following::
64
65  struct dma_devres {
66	size_t		size;
67	void		*vaddr;
68	dma_addr_t	dma_handle;
69  };
70
71  static void dmam_coherent_release(struct device *dev, void *res)
72  {
73	struct dma_devres *this = res;
74
75	dma_free_coherent(dev, this->size, this->vaddr, this->dma_handle);
76  }
77
78  dmam_alloc_coherent(dev, size, dma_handle, gfp)
79  {
80	struct dma_devres *dr;
81	void *vaddr;
82
83	dr = devres_alloc(dmam_coherent_release, sizeof(*dr), gfp);
84	...
85
86	/* alloc DMA memory as usual */
87	vaddr = dma_alloc_coherent(...);
88	...
89
90	/* record size, vaddr, dma_handle in dr */
91	dr->vaddr = vaddr;
92	...
93
94	devres_add(dev, dr);
95
96	return vaddr;
97  }
98
99If a driver uses dmam_alloc_coherent(), the area is guaranteed to be
100freed whether initialization fails half-way or the device gets
101detached.  If most resources are acquired using managed interface, a
102driver can have much simpler init and exit code.  Init path basically
103looks like the following::
104
105  my_init_one()
106  {
107	struct mydev *d;
108
109	d = devm_kzalloc(dev, sizeof(*d), GFP_KERNEL);
110	if (!d)
111		return -ENOMEM;
112
113	d->ring = dmam_alloc_coherent(...);
114	if (!d->ring)
115		return -ENOMEM;
116
117	if (check something)
118		return -EINVAL;
119	...
120
121	return register_to_upper_layer(d);
122  }
123
124And exit path::
125
126  my_remove_one()
127  {
128	unregister_from_upper_layer(d);
129	shutdown_my_hardware();
130  }
131
132As shown above, low level drivers can be simplified a lot by using
133devres.  Complexity is shifted from less maintained low level drivers
134to better maintained higher layer.  Also, as init failure path is
135shared with exit path, both can get more testing.
136
137Note though that when converting current calls or assignments to
138managed devm_* versions it is up to you to check if internal operations
139like allocating memory, have failed. Managed resources pertains to the
140freeing of these resources *only* - all other checks needed are still
141on you. In some cases this may mean introducing checks that were not
142necessary before moving to the managed devm_* calls.
143
144
1453. Devres group
146---------------
147
148Devres entries can be grouped using devres group.  When a group is
149released, all contained normal devres entries and properly nested
150groups are released.  One usage is to rollback series of acquired
151resources on failure.  For example::
152
153  if (!devres_open_group(dev, NULL, GFP_KERNEL))
154	return -ENOMEM;
155
156  acquire A;
157  if (failed)
158	goto err;
159
160  acquire B;
161  if (failed)
162	goto err;
163  ...
164
165  devres_remove_group(dev, NULL);
166  return 0;
167
168 err:
169  devres_release_group(dev, NULL);
170  return err_code;
171
172As resource acquisition failure usually means probe failure, constructs
173like above are usually useful in midlayer driver (e.g. libata core
174layer) where interface function shouldn't have side effect on failure.
175For LLDs, just returning error code suffices in most cases.
176
177Each group is identified by `void *id`.  It can either be explicitly
178specified by @id argument to devres_open_group() or automatically
179created by passing NULL as @id as in the above example.  In both
180cases, devres_open_group() returns the group's id.  The returned id
181can be passed to other devres functions to select the target group.
182If NULL is given to those functions, the latest open group is
183selected.
184
185For example, you can do something like the following::
186
187  int my_midlayer_create_something()
188  {
189	if (!devres_open_group(dev, my_midlayer_create_something, GFP_KERNEL))
190		return -ENOMEM;
191
192	...
193
194	devres_close_group(dev, my_midlayer_create_something);
195	return 0;
196  }
197
198  void my_midlayer_destroy_something()
199  {
200	devres_release_group(dev, my_midlayer_create_something);
201  }
202
203
2044. Details
205----------
206
207Lifetime of a devres entry begins on devres allocation and finishes
208when it is released or destroyed (removed and freed) - no reference
209counting.
210
211devres core guarantees atomicity to all basic devres operations and
212has support for single-instance devres types (atomic
213lookup-and-add-if-not-found).  Other than that, synchronizing
214concurrent accesses to allocated devres data is caller's
215responsibility.  This is usually non-issue because bus ops and
216resource allocations already do the job.
217
218For an example of single-instance devres type, read pcim_iomap_table()
219in lib/devres.c.
220
221All devres interface functions can be called without context if the
222right gfp mask is given.
223
224
2255. Overhead
226-----------
227
228Each devres bookkeeping info is allocated together with requested data
229area.  With debug option turned off, bookkeeping info occupies 16
230bytes on 32bit machines and 24 bytes on 64bit (three pointers rounded
231up to ull alignment).  If singly linked list is used, it can be
232reduced to two pointers (8 bytes on 32bit, 16 bytes on 64bit).
233
234Each devres group occupies 8 pointers.  It can be reduced to 6 if
235singly linked list is used.
236
237Memory space overhead on ahci controller with two ports is between 300
238and 400 bytes on 32bit machine after naive conversion (we can
239certainly invest a bit more effort into libata core layer).
240
241
2426. List of managed interfaces
243-----------------------------
244
245CLOCK
246  devm_clk_get()
247  devm_clk_get_optional()
248  devm_clk_put()
249  devm_clk_bulk_get()
250  devm_clk_bulk_get_all()
251  devm_clk_bulk_get_optional()
252  devm_get_clk_from_child()
253  devm_clk_hw_register()
254  devm_of_clk_add_hw_provider()
255  devm_clk_hw_register_clkdev()
256
257DMA
258  dmaenginem_async_device_register()
259  dmam_alloc_coherent()
260  dmam_alloc_attrs()
261  dmam_free_coherent()
262  dmam_pool_create()
263  dmam_pool_destroy()
264
265DRM
266  devm_drm_dev_alloc()
267
268GPIO
269  devm_gpiod_get()
270  devm_gpiod_get_array()
271  devm_gpiod_get_array_optional()
272  devm_gpiod_get_index()
273  devm_gpiod_get_index_optional()
274  devm_gpiod_get_optional()
275  devm_gpiod_put()
276  devm_gpiod_unhinge()
277  devm_gpiochip_add_data()
278  devm_gpio_request()
279  devm_gpio_request_one()
280  devm_gpio_free()
281
282I2C
283  devm_i2c_new_dummy_device()
284
285IIO
286  devm_iio_device_alloc()
287  devm_iio_device_register()
288  devm_iio_dmaengine_buffer_setup()
289  devm_iio_kfifo_buffer_setup()
290  devm_iio_map_array_register()
291  devm_iio_triggered_buffer_setup()
292  devm_iio_trigger_alloc()
293  devm_iio_trigger_register()
294  devm_iio_channel_get()
295  devm_iio_channel_get_all()
296
297INPUT
298  devm_input_allocate_device()
299
300IO region
301  devm_release_mem_region()
302  devm_release_region()
303  devm_release_resource()
304  devm_request_mem_region()
305  devm_request_region()
306  devm_request_resource()
307
308IOMAP
309  devm_ioport_map()
310  devm_ioport_unmap()
311  devm_ioremap()
312  devm_ioremap_uc()
313  devm_ioremap_wc()
314  devm_ioremap_np()
315  devm_ioremap_resource() : checks resource, requests memory region, ioremaps
316  devm_ioremap_resource_wc()
317  devm_platform_ioremap_resource() : calls devm_ioremap_resource() for platform device
318  devm_platform_ioremap_resource_byname()
319  devm_platform_get_and_ioremap_resource()
320  devm_iounmap()
321  pcim_iomap()
322  pcim_iomap_regions()	: do request_region() and iomap() on multiple BARs
323  pcim_iomap_table()	: array of mapped addresses indexed by BAR
324  pcim_iounmap()
325
326IRQ
327  devm_free_irq()
328  devm_request_any_context_irq()
329  devm_request_irq()
330  devm_request_threaded_irq()
331  devm_irq_alloc_descs()
332  devm_irq_alloc_desc()
333  devm_irq_alloc_desc_at()
334  devm_irq_alloc_desc_from()
335  devm_irq_alloc_descs_from()
336  devm_irq_alloc_generic_chip()
337  devm_irq_setup_generic_chip()
338  devm_irq_sim_init()
339
340LED
341  devm_led_classdev_register()
342  devm_led_classdev_unregister()
343
344MDIO
345  devm_mdiobus_alloc()
346  devm_mdiobus_alloc_size()
347  devm_mdiobus_register()
348  devm_of_mdiobus_register()
349
350MEM
351  devm_free_pages()
352  devm_get_free_pages()
353  devm_kasprintf()
354  devm_kcalloc()
355  devm_kfree()
356  devm_kmalloc()
357  devm_kmalloc_array()
358  devm_kmemdup()
359  devm_krealloc()
360  devm_kstrdup()
361  devm_kvasprintf()
362  devm_kzalloc()
363
364MFD
365  devm_mfd_add_devices()
366
367MUX
368  devm_mux_chip_alloc()
369  devm_mux_chip_register()
370  devm_mux_control_get()
371  devm_mux_state_get()
372
373NET
374  devm_alloc_etherdev()
375  devm_alloc_etherdev_mqs()
376  devm_register_netdev()
377
378PER-CPU MEM
379  devm_alloc_percpu()
380  devm_free_percpu()
381
382PCI
383  devm_pci_alloc_host_bridge()  : managed PCI host bridge allocation
384  devm_pci_remap_cfgspace()	: ioremap PCI configuration space
385  devm_pci_remap_cfg_resource()	: ioremap PCI configuration space resource
386  pcim_enable_device()		: after success, all PCI ops become managed
387  pcim_pin_device()		: keep PCI device enabled after release
388
389PHY
390  devm_usb_get_phy()
391  devm_usb_put_phy()
392
393PINCTRL
394  devm_pinctrl_get()
395  devm_pinctrl_put()
396  devm_pinctrl_register()
397  devm_pinctrl_unregister()
398
399POWER
400  devm_reboot_mode_register()
401  devm_reboot_mode_unregister()
402
403PWM
404  devm_pwm_get()
405  devm_of_pwm_get()
406  devm_fwnode_pwm_get()
407
408REGULATOR
409  devm_regulator_bulk_get()
410  devm_regulator_get()
411  devm_regulator_put()
412  devm_regulator_register()
413
414RESET
415  devm_reset_control_get()
416  devm_reset_controller_register()
417
418RTC
419  devm_rtc_device_register()
420  devm_rtc_allocate_device()
421  devm_rtc_register_device()
422  devm_rtc_nvmem_register()
423
424SERDEV
425  devm_serdev_device_open()
426
427SLAVE DMA ENGINE
428  devm_acpi_dma_controller_register()
429
430SPI
431  devm_spi_register_master()
432
433WATCHDOG
434  devm_watchdog_register_device()
435