On Thu, Aug 29, 2019 at 2:46 AM Saravana Kannan <saravanak@xxxxxxxxxx> wrote: > > Add device-links to track functional dependencies between devices > after they are created (but before they are probed) by looking at > their common DT bindings like clocks, interconnects, etc. > > Having functional dependencies automatically added before the devices > are probed, provides the following benefits: > > - Optimizes device probe order and avoids the useless work of > attempting probes of devices that will not probe successfully > (because their suppliers aren't present or haven't probed yet). > > For example, in a commonly available mobile SoC, registering just > one consumer device's driver at an initcall level earlier than the > supplier device's driver causes 11 failed probe attempts before the > consumer device probes successfully. This was with a kernel with all > the drivers statically compiled in. This problem gets a lot worse if > all the drivers are loaded as modules without direct symbol > dependencies. > > - Supplier devices like clock providers, interconnect providers, etc > need to keep the resources they provide active and at a particular > state(s) during boot up even if their current set of consumers don't > request the resource to be active. This is because the rest of the > consumers might not have probed yet and turning off the resource > before all the consumers have probed could lead to a hang or > undesired user experience. > > Some frameworks (Eg: regulator) handle this today by turning off > "unused" resources at late_initcall_sync and hoping all the devices > have probed by then. This is not a valid assumption for systems with > loadable modules. Other frameworks (Eg: clock) just don't handle > this due to the lack of a clear signal for when they can turn off > resources. This leads to downstream hacks to handle cases like this > that can easily be solved in the upstream kernel. > > By linking devices before they are probed, we give suppliers a clear > count of the number of dependent consumers. Once all of the > consumers are active, the suppliers can turn off the unused > resources without making assumptions about the number of consumers. > > By default we just add device-links to track "driver presence" (probe > succeeded) of the supplier device. If any other functionality provided > by device-links are needed, it is left to the consumer/supplier > devices to change the link when they probe. > > v1 -> v2: > - Drop patch to speed up of_find_device_by_node() > - Drop depends-on property and use existing bindings > > v2 -> v3: > - Refactor the code to have driver core initiate the linking of devs > - Have driver core link consumers to supplier before it's probed > - Add support for drivers to edit the device links before probing > > v3 -> v4: > - Tested edit_links() on system with cyclic dependency. Works. > - Added some checks to make sure device link isn't attempted from > parent device node to child device node. > - Added way to pause/resume sync_state callbacks across > of_platform_populate(). > - Recursively parse DT node to create device links from parent to > suppliers of parent and all child nodes. > > v4 -> v5: > - Fixed copy-pasta bugs with linked list handling > - Walk up the phandle reference till I find an actual device (needed > for regulators to work) > - Added support for linking devices from regulator DT bindings > - Tested the whole series again to make sure cyclic dependencies are > broken with edit_links() and regulator links are created properly. > > v5 -> v6: > - Split, squashed and reordered some of the patches. > - Refactored the device linking code to follow the same code pattern for > any property. > > v6 -> v7: > - No functional changes. > - Renamed i to index > - Added comment to clarify not having to check property name for every > index > - Added "matched" variable to clarify code. No functional change. > - Added comments to include/linux/device.h for add_links() > > v7 -> v8: > - Rebased on top of linux-next to handle device link changes in [1] > > v8 -> v9: > - Fixed kbuild test bot reported errors (docs and const) > > v9->v10: > - Changes made based on reviews on LKML [2] and discussions at ELC [3] > - Dropped the edit_links() patch > - Dropped the patch that skips linking for default bus nodes > - 1/7: Changed from bus.add_links() to fwnode.ops.add_links() > - 1/7: Update device link doc > - 1/7: Lots of comments/fn doc updates > - 1/7: Renamed device_link_check_waiting_consumers() to > device_link_add_missing_supplier_links() > - 2/7: Moved DT parsing/linking code from of/platform.c to of/property.c Why? You'll notice that of/property.c doesn't know anything about platform_device (and struct device): $ git grep platform_device -- drivers/of/property.c $ Everything related to platform_device goes in of/platform.c. Everything related to struct device only goes in of/device.c. I'd be okay with a new file for this too. Rob
