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Date:   Fri, 24 May 2019 07:52:17 +0200
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     Saravana Kannan <saravanak@google.com>
Cc:     Rob Herring <robh+dt@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Frank Rowand <frowand.list@gmail.com>,
        devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@android.com
Subject: Re: [PATCH v1 0/5] Solve postboot supplier cleanup and optimize
 probe ordering
Message-ID: <20190524055217.GC31664@kroah.com>
References: <20190524010117.225219-1-saravanak@google.com>
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On Thu, May 23, 2019 at 06:01:11PM -0700, Saravana Kannan wrote:
> Add a generic "depends-on" property that allows specifying mandatory
> functional dependencies between devices. Add device-links after the
> devices are created (but before they are probed) by looking at this
> "depends-on" property.
> 
> This property is used instead of existing DT properties that specify
> phandles of other devices (Eg: clocks, pinctrl, regulators, etc). This
> is because not all resources referred to by existing DT properties are
> mandatory functional dependencies. Some devices/drivers might be able
> to operate with reduced functionality when some of the resources
> aren't available. For example, a device could operate in polling mode
> if no IRQ is available, a device could skip doing power management if
> clock or voltage control isn't available and they are left on, etc.
> 
> So, adding mandatory functional dependency links between devices by
> looking at referred phandles in DT properties won't work as it would
> prevent probing devices that could be probed. By having an explicit
> depends-on property, we can handle these cases correctly.
> 
> Having functional dependencies explicitly called out in DT and
> 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, regulators 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.

Somewhere in this wall of text you need to say:
	MAKES DEVICES BOOT FASTER!
right?  :)

So in short, this solves the issue of deferred probing with systems with
loads of modules for platform devices and device tree, in that now you
have a chance to probe devices in the correct order saving loads of busy
loops.

A good thing, I like this, very nice work, all of these are:
	Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
but odds are I'll take this through my tree, so I'll add my s-o-b then.
But only after the DT people agree on the new entry.

thanks,

greg k-h