1 .. _device_link: 2 3 ============ 4 Device links 5 ============ 6 7 By default, the driver core only enforces dependencies between devices 8 that are borne out of a parent/child relationship within the device 9 hierarchy: When suspending, resuming or shutting down the system, devices 10 are ordered based on this relationship, i.e. children are always suspended 11 before their parent, and the parent is always resumed before its children. 12 13 Sometimes there is a need to represent device dependencies beyond the 14 mere parent/child relationship, e.g. between siblings, and have the 15 driver core automatically take care of them. 16 17 Secondly, the driver core by default does not enforce any driver presence 18 dependencies, i.e. that one device must be bound to a driver before 19 another one can probe or function correctly. 20 21 Often these two dependency types come together, so a device depends on 22 another one both with regards to driver presence *and* with regards to 23 suspend/resume and shutdown ordering. 24 25 Device links allow representation of such dependencies in the driver core. 26 27 In its standard or *managed* form, a device link combines *both* dependency 28 types: It guarantees correct suspend/resume and shutdown ordering between a 29 "supplier" device and its "consumer" devices, and it guarantees driver 30 presence on the supplier. The consumer devices are not probed before the 31 supplier is bound to a driver, and they're unbound before the supplier 32 is unbound. 33 34 When driver presence on the supplier is irrelevant and only correct 35 suspend/resume and shutdown ordering is needed, the device link may 36 simply be set up with the ``DL_FLAG_STATELESS`` flag. In other words, 37 enforcing driver presence on the supplier is optional. 38 39 Another optional feature is runtime PM integration: By setting the 40 ``DL_FLAG_PM_RUNTIME`` flag on addition of the device link, the PM core 41 is instructed to runtime resume the supplier and keep it active 42 whenever and for as long as the consumer is runtime resumed. 43 44 Usage 45 ===== 46 47 The earliest point in time when device links can be added is after 48 :c:func:`device_add()` has been called for the supplier and 49 :c:func:`device_initialize()` has been called for the consumer. 50 51 It is legal to add them later, but care must be taken that the system 52 remains in a consistent state: E.g. a device link cannot be added in 53 the midst of a suspend/resume transition, so either commencement of 54 such a transition needs to be prevented with :c:func:`lock_system_sleep()`, 55 or the device link needs to be added from a function which is guaranteed 56 not to run in parallel to a suspend/resume transition, such as from a 57 device ``->probe`` callback or a boot-time PCI quirk. 58 59 Another example for an inconsistent state would be a device link that 60 represents a driver presence dependency, yet is added from the consumer's 61 ``->probe`` callback while the supplier hasn't started to probe yet: Had the 62 driver core known about the device link earlier, it wouldn't have probed the 63 consumer in the first place. The onus is thus on the consumer to check 64 presence of the supplier after adding the link, and defer probing on 65 non-presence. [Note that it is valid to create a link from the consumer's 66 ``->probe`` callback while the supplier is still probing, but the consumer must 67 know that the supplier is functional already at the link creation time (that is 68 the case, for instance, if the consumer has just acquired some resources that 69 would not have been available had the supplier not been functional then).] 70 71 If a device link with ``DL_FLAG_STATELESS`` set (i.e. a stateless device link) 72 is added in the ``->probe`` callback of the supplier or consumer driver, it is 73 typically deleted in its ``->remove`` callback for symmetry. That way, if the 74 driver is compiled as a module, the device link is added on module load and 75 orderly deleted on unload. The same restrictions that apply to device link 76 addition (e.g. exclusion of a parallel suspend/resume transition) apply equally 77 to deletion. Device links managed by the driver core are deleted automatically 78 by it. 79 80 Several flags may be specified on device link addition, two of which 81 have already been mentioned above: ``DL_FLAG_STATELESS`` to express that no 82 driver presence dependency is needed (but only correct suspend/resume and 83 shutdown ordering) and ``DL_FLAG_PM_RUNTIME`` to express that runtime PM 84 integration is desired. 85 86 Two other flags are specifically targeted at use cases where the device 87 link is added from the consumer's ``->probe`` callback: ``DL_FLAG_RPM_ACTIVE`` 88 can be specified to runtime resume the supplier and prevent it from suspending 89 before the consumer is runtime suspended. ``DL_FLAG_AUTOREMOVE_CONSUMER`` 90 causes the device link to be automatically purged when the consumer fails to 91 probe or later unbinds. 92 93 Similarly, when the device link is added from supplier's ``->probe`` callback, 94 ``DL_FLAG_AUTOREMOVE_SUPPLIER`` causes the device link to be automatically 95 purged when the supplier fails to probe or later unbinds. 96 97 If neither ``DL_FLAG_AUTOREMOVE_CONSUMER`` nor ``DL_FLAG_AUTOREMOVE_SUPPLIER`` 98 is set, ``DL_FLAG_AUTOPROBE_CONSUMER`` can be used to request the driver core 99 to probe for a driver for the consumer driver on the link automatically after 100 a driver has been bound to the supplier device. 101 102 Note, however, that any combinations of ``DL_FLAG_AUTOREMOVE_CONSUMER``, 103 ``DL_FLAG_AUTOREMOVE_SUPPLIER`` or ``DL_FLAG_AUTOPROBE_CONSUMER`` with 104 ``DL_FLAG_STATELESS`` are invalid and cannot be used. 105 106 Limitations 107 =========== 108 109 Driver authors should be aware that a driver presence dependency for managed 110 device links (i.e. when ``DL_FLAG_STATELESS`` is not specified on link addition) 111 may cause probing of the consumer to be deferred indefinitely. This can become 112 a problem if the consumer is required to probe before a certain initcall level 113 is reached. Worse, if the supplier driver is blacklisted or missing, the 114 consumer will never be probed. 115 116 Moreover, managed device links cannot be deleted directly. They are deleted 117 by the driver core when they are not necessary any more in accordance with the 118 ``DL_FLAG_AUTOREMOVE_CONSUMER`` and ``DL_FLAG_AUTOREMOVE_SUPPLIER`` flags. 119 However, stateless device links (i.e. device links with ``DL_FLAG_STATELESS`` 120 set) are expected to be removed by whoever called :c:func:`device_link_add()` 121 to add them with the help of either :c:func:`device_link_del()` or 122 :c:func:`device_link_remove()`. 123 124 Passing ``DL_FLAG_RPM_ACTIVE`` along with ``DL_FLAG_STATELESS`` to 125 :c:func:`device_link_add()` may cause the PM-runtime usage counter of the 126 supplier device to remain nonzero after a subsequent invocation of either 127 :c:func:`device_link_del()` or :c:func:`device_link_remove()` to remove the 128 device link returned by it. This happens if :c:func:`device_link_add()` is 129 called twice in a row for the same consumer-supplier pair without removing the 130 link between these calls, in which case allowing the PM-runtime usage counter 131 of the supplier to drop on an attempt to remove the link may cause it to be 132 suspended while the consumer is still PM-runtime-active and that has to be 133 avoided. [To work around this limitation it is sufficient to let the consumer 134 runtime suspend at least once, or call :c:func:`pm_runtime_set_suspended()` for 135 it with PM-runtime disabled, between the :c:func:`device_link_add()` and 136 :c:func:`device_link_del()` or :c:func:`device_link_remove()` calls.] 137 138 Sometimes drivers depend on optional resources. They are able to operate 139 in a degraded mode (reduced feature set or performance) when those resources 140 are not present. An example is an SPI controller that can use a DMA engine 141 or work in PIO mode. The controller can determine presence of the optional 142 resources at probe time but on non-presence there is no way to know whether 143 they will become available in the near future (due to a supplier driver 144 probing) or never. Consequently it cannot be determined whether to defer 145 probing or not. It would be possible to notify drivers when optional 146 resources become available after probing, but it would come at a high cost 147 for drivers as switching between modes of operation at runtime based on the 148 availability of such resources would be much more complex than a mechanism 149 based on probe deferral. In any case optional resources are beyond the 150 scope of device links. 151 152 Examples 153 ======== 154 155 * An MMU device exists alongside a busmaster device, both are in the same 156 power domain. The MMU implements DMA address translation for the busmaster 157 device and shall be runtime resumed and kept active whenever and as long 158 as the busmaster device is active. The busmaster device's driver shall 159 not bind before the MMU is bound. To achieve this, a device link with 160 runtime PM integration is added from the busmaster device (consumer) 161 to the MMU device (supplier). The effect with regards to runtime PM 162 is the same as if the MMU was the parent of the master device. 163 164 The fact that both devices share the same power domain would normally 165 suggest usage of a struct dev_pm_domain or struct generic_pm_domain, 166 however these are not independent devices that happen to share a power 167 switch, but rather the MMU device serves the busmaster device and is 168 useless without it. A device link creates a synthetic hierarchical 169 relationship between the devices and is thus more apt. 170 171 * A Thunderbolt host controller comprises a number of PCIe hotplug ports 172 and an NHI device to manage the PCIe switch. On resume from system sleep, 173 the NHI device needs to re-establish PCI tunnels to attached devices 174 before the hotplug ports can resume. If the hotplug ports were children 175 of the NHI, this resume order would automatically be enforced by the 176 PM core, but unfortunately they're aunts. The solution is to add 177 device links from the hotplug ports (consumers) to the NHI device 178 (supplier). A driver presence dependency is not necessary for this 179 use case. 180 181 * Discrete GPUs in hybrid graphics laptops often feature an HDA controller 182 for HDMI/DP audio. In the device hierarchy the HDA controller is a sibling 183 of the VGA device, yet both share the same power domain and the HDA 184 controller is only ever needed when an HDMI/DP display is attached to the 185 VGA device. A device link from the HDA controller (consumer) to the 186 VGA device (supplier) aptly represents this relationship. 187 188 * ACPI allows definition of a device start order by way of _DEP objects. 189 A classical example is when ACPI power management methods on one device 190 are implemented in terms of I\ :sup:`2`\ C accesses and require a specific 191 I\ :sup:`2`\ C controller to be present and functional for the power 192 management of the device in question to work. 193 194 * In some SoCs a functional dependency exists from display, video codec and 195 video processing IP cores on transparent memory access IP cores that handle 196 burst access and compression/decompression. 197 198 Alternatives 199 ============ 200 201 * A struct dev_pm_domain can be used to override the bus, 202 class or device type callbacks. It is intended for devices sharing 203 a single on/off switch, however it does not guarantee a specific 204 suspend/resume ordering, this needs to be implemented separately. 205 It also does not by itself track the runtime PM status of the involved 206 devices and turn off the power switch only when all of them are runtime 207 suspended. Furthermore it cannot be used to enforce a specific shutdown 208 ordering or a driver presence dependency. 209 210 * A struct generic_pm_domain is a lot more heavyweight than a 211 device link and does not allow for shutdown ordering or driver presence 212 dependencies. It also cannot be used on ACPI systems. 213 214 Implementation 215 ============== 216 217 The device hierarchy, which -- as the name implies -- is a tree, 218 becomes a directed acyclic graph once device links are added. 219 220 Ordering of these devices during suspend/resume is determined by the 221 dpm_list. During shutdown it is determined by the devices_kset. With 222 no device links present, the two lists are a flattened, one-dimensional 223 representations of the device tree such that a device is placed behind 224 all its ancestors. That is achieved by traversing the ACPI namespace 225 or OpenFirmware device tree top-down and appending devices to the lists 226 as they are discovered. 227 228 Once device links are added, the lists need to satisfy the additional 229 constraint that a device is placed behind all its suppliers, recursively. 230 To ensure this, upon addition of the device link the consumer and the 231 entire sub-graph below it (all children and consumers of the consumer) 232 are moved to the end of the list. (Call to :c:func:`device_reorder_to_tail()` 233 from :c:func:`device_link_add()`.) 234 235 To prevent introduction of dependency loops into the graph, it is 236 verified upon device link addition that the supplier is not dependent 237 on the consumer or any children or consumers of the consumer. 238 (Call to :c:func:`device_is_dependent()` from :c:func:`device_link_add()`.) 239 If that constraint is violated, :c:func:`device_link_add()` will return 240 ``NULL`` and a ``WARNING`` will be logged. 241 242 Notably this also prevents the addition of a device link from a parent 243 device to a child. However the converse is allowed, i.e. a device link 244 from a child to a parent. Since the driver core already guarantees 245 correct suspend/resume and shutdown ordering between parent and child, 246 such a device link only makes sense if a driver presence dependency is 247 needed on top of that. In this case driver authors should weigh 248 carefully if a device link is at all the right tool for the purpose. 249 A more suitable approach might be to simply use deferred probing or 250 add a device flag causing the parent driver to be probed before the 251 child one. 252 253 State machine 254 ============= 255 256 .. kernel-doc:: include/linux/device.h 257 :functions: device_link_state 258 259 :: 260 261 .=============================. 262 | | 263 v | 264 DORMANT <=> AVAILABLE <=> CONSUMER_PROBE => ACTIVE 265 ^ | 266 | | 267 '============ SUPPLIER_UNBIND <============' 268 269 * The initial state of a device link is automatically determined by 270 :c:func:`device_link_add()` based on the driver presence on the supplier 271 and consumer. If the link is created before any devices are probed, it 272 is set to ``DL_STATE_DORMANT``. 273 274 * When a supplier device is bound to a driver, links to its consumers 275 progress to ``DL_STATE_AVAILABLE``. 276 (Call to :c:func:`device_links_driver_bound()` from 277 :c:func:`driver_bound()`.) 278 279 * Before a consumer device is probed, presence of supplier drivers is 280 verified by checking the consumer device is not in the wait_for_suppliers 281 list and by checking that links to suppliers are in ``DL_STATE_AVAILABLE`` 282 state. The state of the links is updated to ``DL_STATE_CONSUMER_PROBE``. 283 (Call to :c:func:`device_links_check_suppliers()` from 284 :c:func:`really_probe()`.) 285 This prevents the supplier from unbinding. 286 (Call to :c:func:`wait_for_device_probe()` from 287 :c:func:`device_links_unbind_consumers()`.) 288 289 * If the probe fails, links to suppliers revert back to ``DL_STATE_AVAILABLE``. 290 (Call to :c:func:`device_links_no_driver()` from :c:func:`really_probe()`.) 291 292 * If the probe succeeds, links to suppliers progress to ``DL_STATE_ACTIVE``. 293 (Call to :c:func:`device_links_driver_bound()` from :c:func:`driver_bound()`.) 294 295 * When the consumer's driver is later on removed, links to suppliers revert 296 back to ``DL_STATE_AVAILABLE``. 297 (Call to :c:func:`__device_links_no_driver()` from 298 :c:func:`device_links_driver_cleanup()`, which in turn is called from 299 :c:func:`__device_release_driver()`.) 300 301 * Before a supplier's driver is removed, links to consumers that are not 302 bound to a driver are updated to ``DL_STATE_SUPPLIER_UNBIND``. 303 (Call to :c:func:`device_links_busy()` from 304 :c:func:`__device_release_driver()`.) 305 This prevents the consumers from binding. 306 (Call to :c:func:`device_links_check_suppliers()` from 307 :c:func:`really_probe()`.) 308 Consumers that are bound are freed from their driver; consumers that are 309 probing are waited for until they are done. 310 (Call to :c:func:`device_links_unbind_consumers()` from 311 :c:func:`__device_release_driver()`.) 312 Once all links to consumers are in ``DL_STATE_SUPPLIER_UNBIND`` state, 313 the supplier driver is released and the links revert to ``DL_STATE_DORMANT``. 314 (Call to :c:func:`device_links_driver_cleanup()` from 315 :c:func:`__device_release_driver()`.) 316 317 API 318 === 319 320 See device_link_add(), device_link_del() and device_link_remove().
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