Date:   Mon, 22 Jul 2019 16:47:15 -0700
From:   Saravana Kannan <saravanak@...gle.com>
To:     Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Frank Rowand <frowand.list@...il.com>
Cc:     "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        David Collins <collinsd@...eaurora.org>,
        Android Kernel Team <kernel-team@...roid.com>
Subject: Re: [PATCH v6 0/7] Solve postboot supplier cleanup and optimize probe ordering

On Fri, Jul 19, 2019 at 11:16 PM Saravana Kannan <saravanak@...gle.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.
>
> 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

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