Date:   Thu, 25 Jul 2019 14:04:23 -0700
From:   Frank Rowand <frowand.list@...il.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Saravana Kannan <saravanak@...gle.com>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        David Collins <collinsd@...eaurora.org>,
        kernel-team@...roid.com
Subject: Re: [PATCH v7 0/7] Solve postboot supplier cleanup and optimize probe
 ordering

On 7/25/19 6:42 AM, Greg Kroah-Hartman wrote:
> On Tue, Jul 23, 2019 at 05:10:53PM -0700, Saravana Kannan 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()
>>
>> 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/
> 
> 
> This looks sane to me.  Anyone have any objections for me queueing this
> up for my tree to get into linux-next now?

I would like for the series to get into linux-next sooner than later,
and spend some time there.  

I am _slightly_ more optimistic than Rob that sitting in linux-next for
an extended period might reveal any latent issues, so I would like for
the series to be in linux-next for an extended period of time.  (Yes,
my understanding is that Linus does not like patches to be in linux-next
if they are not targeted for the next merge window, but I prefer that
this patch series spend as much time in linux-next as possible).

I have been waiting for the changes to settle down before bringing up
the issue of devicetree overlays.  Now that the code seems to be
settling down, I need to look at how these changes impact overlays.
So I do not think the patches will be ready for a Linus pull request
until overlays are considered.

-Frank

> 
> thanks,
> 
> greg k-h
> 

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