linux-kernel - Re: [PATCH v9 0/7] Solve postboot supplier cleanup and optimize probe ordering

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Date:   Thu, 1 Aug 2019 12:59:25 -0700
From:   Frank Rowand <frowand.list@...il.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Saravana Kannan <saravanak@...gle.com>,
        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 v9 0/7] Solve postboot supplier cleanup and optimize probe
 ordering

On 8/1/19 12:32 PM, Greg Kroah-Hartman wrote:
> On Thu, Aug 01, 2019 at 12:28:13PM -0700, Frank Rowand wrote:
>> Hi Greg,
>>
>> On 7/31/19 11:12 PM, Greg Kroah-Hartman wrote:
>>> On Wed, Jul 31, 2019 at 03:17:13PM -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.
>>>
>>> All now queued up in my driver-core-testing branch, and if 0-day is
>>> happy with this, will move it to my "real" driver-core-next branch in a
>>> day or so to get included in linux-next.
>>
>> I have been slow in getting my review out.
>>
>> This patch series is not yet ready for sending to Linus, so if putting
>> this in linux-next implies that it will be in your next pull request
>> to Linus, please do not put it in linux-next.
> 
> It means that it will be in my pull request for 5.4-rc1, many many
> waeeks away from now.

If you are willing to revert the series before the pull request _if_ I
have significant review issues in the next couple of days, then I am happy
to see the patches get exposure in linux-next.

-Frank

> 
> thanks,
> 
> greg k-h
>