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    SubjectRe: [PATCH v10 0/7] Solve postboot supplier cleanup and optimize probe ordering
    On Thu, Aug 29, 2019 at 9:43 AM Rob Herring <robh+dt@kernel.org> wrote:
    >
    > On Thu, Aug 29, 2019 at 2:46 AM Saravana Kannan <saravanak@google.com> 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

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