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Subject: Re: [PATCH v9 0/7] Solve postboot supplier cleanup and optimize probe
 ordering
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Saravana Kannan <saravanak@google.com>
Cc:     Rob Herring <robh+dt@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
        David Collins <collinsd@codeaurora.org>,
        kernel-team@android.com
References: <20190731221721.187713-1-saravanak@google.com>
 <20190801061209.GA3570@kroah.com>
From:   Frank Rowand <frowand.list@gmail.com>
Message-ID: <5a1e785d-075e-19a0-7d3d-949e1b65d726@gmail.com>
Date:   Thu, 1 Aug 2019 12:28:13 -0700
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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.

Thanks,

Frank

> 
> thanks for sticking with this!
> 
> greg k-h
>