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 MEM
 237   devm_kzalloc()
 238   devm_kfree()
 239
 240 IO region
 241   devm_request_region()
 242   devm_request_mem_region()
 243   devm_release_region()
 244   devm_release_mem_region()
 245
 246 IRQ
 247   devm_request_irq()
 248   devm_free_irq()
 249
 250 DMA
 251   dmam_alloc_coherent()
 252   dmam_free_coherent()
 253   dmam_alloc_noncoherent()
 254   dmam_free_noncoherent()
 255   dmam_declare_coherent_memory()
 256   dmam_pool_create()
 257   dmam_pool_destroy()
 258
 259 PCI
 260   pcim_enable_device()  : after success, all PCI ops become managed
 261   pcim_pin_device()     : keep PCI device enabled after release
 262
 263 IOMAP
 264   devm_ioport_map()
 265   devm_ioport_unmap()
 266   devm_ioremap()
 267   devm_ioremap_nocache()
 268   devm_iounmap()
 269   devm_ioremap_resource() : checks resource, requests memory region, ioremaps
 270   devm_request_and_ioremap() : obsoleted by devm_ioremap_resource()
 271   pcim_iomap()
 272   pcim_iounmap()
 273   pcim_iomap_table()    : array of mapped addresses indexed by BAR
 274   pcim_iomap_regions()  : do request_region() and iomap() on multiple BARs
 275
 276 REGULATOR
 277   devm_regulator_get()
 278   devm_regulator_put()
 279   devm_regulator_bulk_get()
 280
 281 CLOCK
 282   devm_clk_get()
 283   devm_clk_put()
 284
 285 PINCTRL
 286   devm_pinctrl_get()
 287   devm_pinctrl_put()
 288
 289 PWM
 290   devm_pwm_get()
 291   devm_pwm_put()
 292
 293 PHY
 294   devm_usb_get_phy()
 295   devm_usb_put_phy()