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Subject: Re: [PATCH v9 0/7] Solve postboot supplier cleanup and optimize probe
 ordering
To:     Saravana Kannan <saravanak@google.com>
Cc:     Rob Herring <robh+dt@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        David Collins <collinsd@codeaurora.org>,
        Android Kernel Team <kernel-team@android.com>
References: <20190731221721.187713-1-saravanak@google.com>
 <919b66e9-9708-de34-41cd-e448838b130c@gmail.com>
 <CAGETcx8LqeOXD5zPsLuxoG5pR9VZ_v=PQfRf-aFwCSaW4kwoxA@mail.gmail.com>
From:   Frank Rowand <frowand.list@gmail.com>
Message-ID: <7a0ee940-f81f-36b9-93e7-2b4c242360c9@gmail.com>
Date:   Fri, 9 Aug 2019 22:20:52 -0700
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On 8/9/19 10:00 PM, Saravana Kannan wrote:
> On Fri, Aug 9, 2019 at 7:57 PM Frank Rowand <frowand.list@gmail.com> wrote:
>>
>> Hi Saravana,
>>
>> On 7/31/19 3:17 PM, 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()
>>>
>>> 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)
>>
>> Some maintainers have strong opinions about whether change logs should be:
>>
>>   (1) only in patch 0
>>   (2) only in the specific patches that are changed
>>   (3) both in patch 0 and in the specific patches that are changed.
>>
>> I can adapt to any of the three styles.  But for style "(1)" please
>> list which specific patch has changed for each item in the change list.
>>
> 
> Thanks for the context Frank. I'm okay with (1) or (2) but I'll stick
> with (1) for this series. Didn't realize there were options (2) and
> (3). Since you started reviewing from v7, I'll do that in the future
> updates? Also, I haven't forgotten your emails. Just tied up with
> something else for a few days. I'll get to your emails next week.

Yes, starting with future updates is fine, no need to redo the v9
change logs.

No problem on the timing.  I figured you were busy or away from the
internet.

-Frank

> 
> Thanks,
> Saravana
>