On Thu, Aug 29, 2019 at 9:43 AM Rob Herring <robh+dt@xxxxxxxxxx> wrote: > > 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. The only platform_device related code in what got moved to of/property.c is the call to of_find_device_by_node(). And that's because I forgot that function returns a platform_device --- it should really have been called of_find_plat_device_by_node() or something similar. Outside of that, of/property.c makes sense because that's where the fwnode ops are implemented. As you mentioned in the other email, just searching platform_bus is not sufficient. So I'll have to figure something out for that. Once I do, I think the code will be fine in of/property.c as it shouldn't have any reference to platform_device. Thanks for catching what I missed. -Saravana
