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

Frank Rowand
2019-08-01T19:28:17Z

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
>