On Fri, Jul 19, 2019 at 11:16 PM 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. > > I've also not updated this patch series to handle the new patch [1] from > Rafael. Will do that once this patch series is close to being Acked. > > [1] - https://lore.kernel.org/lkml/3121545.4lOhFoIcdQ@kreacher/ > > -Saravana > > > Saravana Kannan (7): > driver core: Add support for linking devices during device addition > driver core: Add edit_links() callback for drivers > of/platform: Add functional dependency link from DT bindings > driver core: Add sync_state driver/bus callback > of/platform: Pause/resume sync state during init and > of_platform_populate() > of/platform: Create device links for all child-supplier depencencies > of/platform: Don't create device links for default busses > > .../admin-guide/kernel-parameters.txt | 5 + > drivers/base/core.c | 168 ++++++++++++++++ > drivers/base/dd.c | 29 +++ > drivers/of/platform.c | 182 ++++++++++++++++++ > include/linux/device.h | 47 +++++ > 5 files changed, 431 insertions(+) Update: Tested this refactor on hardware by backporting to a 4.14 kernel. Works just as it did before the refactor. Also, nudge to make sure this series isn't lost over the weekend email snooze. -Saravana
