Documentation/driver-model/devres.txt

   1 Devres - Managed Device Resource
   2 ================================
   3
   4 Tejun Heo       <teheo@suse.de>
   5
   6 First draft     10 January 2007
   7
   8
   9 1. Intro                        : Huh? Devres?
  10 2. Devres                       : Devres in a nutshell
  11 3. Devres Group                 : Group devres'es and release them together
  12 4. Details                      : Life time rules, calling context, ...
  13 5. Overhead                     : How much do we have to pay for this?
  14 6. List of managed interfaces   : Currently implemented managed interfaces
  15
  16
  17   1. Intro
  18   --------
  19
  20 devres came up while trying to convert libata to use iomap.  Each
  21 iomapped address should be kept and unmapped on driver detach.  For
  22 example, a plain SFF ATA controller (that is, good old PCI IDE) in
  23 native mode makes use of 5 PCI BARs and all of them should be
  24 maintained.
  25
  26 As with many other device drivers, libata low level drivers have
  27 sufficient bugs in ->remove and ->probe failure path.  Well, yes,
  28 that's probably because libata low level driver developers are lazy
  29 bunch, but aren't all low level driver developers?  After spending a
  30 day fiddling with braindamaged hardware with no document or
  31 braindamaged document, if it's finally working, well, it's working.
  32
  33 For one reason or another, low level drivers don't receive as much
  34 attention or testing as core code, and bugs on driver detach or
  35 initialization failure don't happen often enough to be noticeable.
  36 Init failure path is worse because it's much less travelled while
  37 needs to handle multiple entry points.
  38
  39 So, many low level drivers end up leaking resources on driver detach
  40 and having half broken failure path implementation in ->probe() which
  41 would leak resources or even cause oops when failure occurs.  iomap
  42 adds more to this mix.  So do msi and msix.
  43
  44
  45   2. Devres
  46   ---------
  47
  48 devres is basically linked list of arbitrarily sized memory areas
  49 associated with a struct device.  Each devres entry is associated with
  50 a release function.  A devres can be released in several ways.  No
  51 matter what, all devres entries are released on driver detach.  On
  52 release, the associated release function is invoked and then the
  53 devres entry is freed.
  54
  55 Managed interface is created for resources commonly used by device
  56 drivers using devres.  For example, coherent DMA memory is acquired
  57 using dma_alloc_coherent().  The managed version is called
  58 dmam_alloc_coherent().  It is identical to dma_alloc_coherent() except
  59 for the DMA memory allocated using it is managed and will be
  60 automatically released on driver detach.  Implementation looks like
  61 the following.
  62
  63   struct dma_devres {
  64         size_t          size;
  65         void            *vaddr;
  66         dma_addr_t      dma_handle;
  67   };
  68
  69   static void dmam_coherent_release(struct device *dev, void *res)
  70   {
  71         struct dma_devres *this = res;
  72
  73         dma_free_coherent(dev, this->size, this->vaddr, this->dma_handle);
  74   }
  75
  76   dmam_alloc_coherent(dev, size, dma_handle, gfp)
  77   {
  78         struct dma_devres *dr;
  79         void *vaddr;
  80
  81         dr = devres_alloc(dmam_coherent_release, sizeof(*dr), gfp);
  82         ...
  83
  84         /* alloc DMA memory as usual */
  85         vaddr = dma_alloc_coherent(...);
  86         ...
  87
  88         /* record size, vaddr, dma_handle in dr */
  89         dr->vaddr = vaddr;
  90         ...
  91
  92         devres_add(dev, dr);
  93
  94         return vaddr;
  95   }
  96
  97 If a driver uses dmam_alloc_coherent(), the area is guaranteed to be
  98 freed whether initialization fails half-way or the device gets
  99 detached.  If most resources are acquired using managed interface, a
 100 driver can have much simpler init and exit code.  Init path basically
 101 looks like the following.
 102
 103   my_init_one()
 104   {
 105         struct mydev *d;
 106
 107         d = devm_kzalloc(dev, sizeof(*d), GFP_KERNEL);
 108         if (!d)
 109                 return -ENOMEM;
 110
 111         d->ring = dmam_alloc_coherent(...);
 112         if (!d->ring)
 113                 return -ENOMEM;
 114
 115         if (check something)
 116                 return -EINVAL;
 117         ...
 118
 119         return register_to_upper_layer(d);
 120   }
 121
 122 And exit path,
 123
 124   my_remove_one()
 125   {
 126         unregister_from_upper_layer(d);
 127         shutdown_my_hardware();
 128   }
 129
 130 As shown above, low level drivers can be simplified a lot by using
 131 devres.  Complexity is shifted from less maintained low level drivers
 132 to better maintained higher layer.  Also, as init failure path is
 133 shared with exit path, both can get more testing.
 134
 135
 136   3. Devres group
 137   ---------------
 138
 139 Devres entries can be grouped using devres group.  When a group is
 140 released, all contained normal devres entries and properly nested
 141 groups are released.  One usage is to rollback series of acquired
 142 resources on failure.  For example,
 143
 144   if (!devres_open_group(dev, NULL, GFP_KERNEL))
 145         return -ENOMEM;
 146
 147   acquire A;
 148   if (failed)
 149         goto err;
 150
 151   acquire B;
 152   if (failed)
 153         goto err;
 154   ...
 155
 156   devres_remove_group(dev, NULL);
 157   return 0;
 158
 159  err:
 160   devres_release_group(dev, NULL);
 161   return err_code;
 162
 163 As resource acquisition failure usually means probe failure, constructs
 164 like above are usually useful in midlayer driver (e.g. libata core
 165 layer) where interface function shouldn't have side effect on failure.
 166 For LLDs, just returning error code suffices in most cases.
 167
 168 Each group is identified by void *id.  It can either be explicitly
 169 specified by @id argument to devres_open_group() or automatically
 170 created by passing NULL as @id as in the above example.  In both
 171 cases, devres_open_group() returns the group's id.  The returned id
 172 can be passed to other devres functions to select the target group.
 173 If NULL is given to those functions, the latest open group is
 174 selected.
 175
 176 For example, you can do something like the following.
 177
 178   int my_midlayer_create_something()
 179   {
 180         if (!devres_open_group(dev, my_midlayer_create_something, GFP_KERNEL))
 181                 return -ENOMEM;
 182
 183         ...
 184
 185         devres_close_group(dev, my_midlayer_create_something);
 186         return 0;
 187   }
 188
 189   void my_midlayer_destroy_something()
 190   {
 191         devres_release_group(dev, my_midlayer_create_something);
 192   }
 193
 194
 195   4. Details
 196   ----------
 197
 198 Lifetime of a devres entry begins on devres allocation and finishes
 199 when it is released or destroyed (removed and freed) - no reference
 200 counting.
 201
 202 devres core guarantees atomicity to all basic devres operations and
 203 has support for single-instance devres types (atomic
 204 lookup-and-add-if-not-found).  Other than that, synchronizing
 205 concurrent accesses to allocated devres data is caller's
 206 responsibility.  This is usually non-issue because bus ops and
 207 resource allocations already do the job.
 208
 209 For an example of single-instance devres type, read pcim_iomap_table()
 210 in lib/devres.c.
 211
 212 All devres interface functions can be called without context if the
 213 right gfp mask is given.
 214
 215
 216   5. Overhead
 217   -----------
 218
 219 Each devres bookkeeping info is allocated together with requested data
 220 area.  With debug option turned off, bookkeeping info occupies 16
 221 bytes on 32bit machines and 24 bytes on 64bit (three pointers rounded
 222 up to ull alignment).  If singly linked list is used, it can be
 223 reduced to two pointers (8 bytes on 32bit, 16 bytes on 64bit).
 224
 225 Each devres group occupies 8 pointers.  It can be reduced to 6 if
 226 singly linked list is used.
 227
 228 Memory space overhead on ahci controller with two ports is between 300
 229 and 400 bytes on 32bit machine after naive conversion (we can
 230 certainly invest a bit more effort into libata core layer).
 231
 232
 233   6. List of managed interfaces
 234   -----------------------------
 235
 236 CLOCK
 237   devm_clk_get()
 238   devm_clk_put()
 239   devm_clk_hw_register()
 240
 241 DMA
 242   dmam_alloc_coherent()
 243   dmam_alloc_attrs()
 244   dmam_declare_coherent_memory()
 245   dmam_free_coherent()
 246   dmam_pool_create()
 247   dmam_pool_destroy()
 248
 249 GPIO
 250   devm_gpiod_get()
 251   devm_gpiod_get_index()
 252   devm_gpiod_get_index_optional()
 253   devm_gpiod_get_optional()
 254   devm_gpiod_put()
 255   devm_gpiochip_add_data()
 256   devm_gpiochip_remove()
 257   devm_gpio_request()
 258   devm_gpio_request_one()
 259   devm_gpio_free()
 260
 261 IIO
 262   devm_iio_device_alloc()
 263   devm_iio_device_free()
 264   devm_iio_device_register()
 265   devm_iio_device_unregister()
 266   devm_iio_kfifo_allocate()
 267   devm_iio_kfifo_free()
 268   devm_iio_triggered_buffer_setup()
 269   devm_iio_triggered_buffer_cleanup()
 270   devm_iio_trigger_alloc()
 271   devm_iio_trigger_free()
 272   devm_iio_trigger_register()
 273   devm_iio_trigger_unregister()
 274   devm_iio_channel_get()
 275   devm_iio_channel_release()
 276   devm_iio_channel_get_all()
 277   devm_iio_channel_release_all()
 278
 279 INPUT
 280   devm_input_allocate_device()
 281
 282 IO region
 283   devm_release_mem_region()
 284   devm_release_region()
 285   devm_release_resource()
 286   devm_request_mem_region()
 287   devm_request_region()
 288   devm_request_resource()
 289
 290 IOMAP
 291   devm_ioport_map()
 292   devm_ioport_unmap()
 293   devm_ioremap()
 294   devm_ioremap_nocache()
 295   devm_ioremap_wc()
 296   devm_ioremap_resource() : checks resource, requests memory region, ioremaps
 297   devm_iounmap()
 298   pcim_iomap()
 299   pcim_iomap_regions()  : do request_region() and iomap() on multiple BARs
 300   pcim_iomap_table()    : array of mapped addresses indexed by BAR
 301   pcim_iounmap()
 302
 303 IRQ
 304   devm_free_irq()
 305   devm_request_any_context_irq()
 306   devm_request_irq()
 307   devm_request_threaded_irq()
 308   devm_irq_alloc_descs()
 309   devm_irq_alloc_desc()
 310   devm_irq_alloc_desc_at()
 311   devm_irq_alloc_desc_from()
 312   devm_irq_alloc_descs_from()
 313   devm_irq_alloc_generic_chip()
 314   devm_irq_setup_generic_chip()
 315
 316 LED
 317   devm_led_classdev_register()
 318   devm_led_classdev_unregister()
 319
 320 MDIO
 321   devm_mdiobus_alloc()
 322   devm_mdiobus_alloc_size()
 323   devm_mdiobus_free()
 324
 325 MEM
 326   devm_free_pages()
 327   devm_get_free_pages()
 328   devm_kasprintf()
 329   devm_kcalloc()
 330   devm_kfree()
 331   devm_kmalloc()
 332   devm_kmalloc_array()
 333   devm_kmemdup()
 334   devm_kstrdup()
 335   devm_kvasprintf()
 336   devm_kzalloc()
 337
 338 MFD
 339   devm_mfd_add_devices()
 340
 341 MUX
 342   devm_mux_chip_alloc()
 343   devm_mux_chip_register()
 344   devm_mux_control_get()
 345
 346 PER-CPU MEM
 347   devm_alloc_percpu()
 348   devm_free_percpu()
 349
 350 PCI
 351   devm_pci_alloc_host_bridge()  : managed PCI host bridge allocation
 352   devm_pci_remap_cfgspace()     : ioremap PCI configuration space
 353   devm_pci_remap_cfg_resource() : ioremap PCI configuration space resource
 354   pcim_enable_device()          : after success, all PCI ops become managed
 355   pcim_pin_device()             : keep PCI device enabled after release
 356
 357 PHY
 358   devm_usb_get_phy()
 359   devm_usb_put_phy()
 360
 361 PINCTRL
 362   devm_pinctrl_get()
 363   devm_pinctrl_put()
 364   devm_pinctrl_register()
 365   devm_pinctrl_unregister()
 366
 367 POWER
 368   devm_reboot_mode_register()
 369   devm_reboot_mode_unregister()
 370
 371 PWM
 372   devm_pwm_get()
 373   devm_pwm_put()
 374
 375 REGULATOR
 376   devm_regulator_bulk_get()
 377   devm_regulator_get()
 378   devm_regulator_put()
 379   devm_regulator_register()
 380
 381 RESET
 382   devm_reset_control_get()
 383   devm_reset_controller_register()
 384
 385 SLAVE DMA ENGINE
 386   devm_acpi_dma_controller_register()
 387
 388 SPI
 389   devm_spi_register_master()
 390
 391 WATCHDOG
 392   devm_watchdog_register_device()