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Subject: Re: [RESEND PATCH v1 1/5] of/platform: Speed up
 of_find_device_by_node()
To:     Rob Herring <robh+dt@kernel.org>,
        Sandeep Patil <sspatil@android.com>,
        Saravana Kannan <saravanak@google.com>
Cc:     Mark Rutland <mark.rutland@arm.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        David Collins <collinsd@codeaurora.org>,
        devicetree@vger.kernel.org,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Android Kernel Team <kernel-team@android.com>
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From:   Frank Rowand <frowand.list@gmail.com>
Message-ID: <75be9e83-4d56-6080-7011-0c79b70c8cb9@gmail.com>
Date:   Wed, 12 Jun 2019 09:07:10 -0700
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On 6/12/19 6:53 AM, Rob Herring wrote:
> On Tue, Jun 11, 2019 at 3:52 PM Sandeep Patil <sspatil@android.com> wrote:
>>
>> On Tue, Jun 11, 2019 at 01:56:25PM -0700, 'Saravana Kannan' via kernel-team wrote:
>>> On Tue, Jun 11, 2019 at 8:18 AM Frank Rowand <frowand.list@gmail.com> wrote:
>>>>
>>>> Hi Saravana,
>>>>
>>>> On 6/10/19 10:36 AM, Rob Herring wrote:
>>>>> Why are you resending this rather than replying to Frank's last
>>>>> comments on the original?
>>>>
>>>> Adding on a different aspect...  The independent replies from three different
>>>> maintainers (Rob, Mark, myself) pointed out architectural issues with the
>>>> patch series.  There were also some implementation issues brought out.
>>>> (Although I refrained from bringing up most of my implementation issues
>>>> as they are not relevant until architecture issues are resolved.)
>>>
>>> Right, I'm not too worried about the implementation issues before we
>>> settle on the architectural issues. Those are easy to fix.
>>>
>>> Honestly, the main points that the maintainers raised are:
>>> 1) This is a configuration property and not describing the device.
>>> Just use the implicit dependencies coming from existing bindings.
>>>
>>> I gave a bunch of reasons for why I think it isn't an OS configuration
>>> property. But even if that's not something the maintainers can agree
>>> to, I gave a concrete example (cyclic dependencies between clock
>>> provider hardware) where the implicit dependencies would prevent one
>>> of the devices from probing till the end of time. So even if the
>>> maintainers don't agree we should always look at "depends-on" to
>>> decide the dependencies, we still need some means to override the
>>> implicit dependencies where they don't match the real dependency. Can
>>> we use depends-on as an override when the implicit dependencies aren't
>>> correct?
>>>
>>> 2) This doesn't need to be solved because this is just optimizing
>>> probing or saving power ("we should get rid of this auto disabling"):
>>>
>>> I explained why this patch series is not just about optimizing probe
>>> ordering or saving power. And why we can't ignore auto disabling
>>> (because it's more than just auto disabling). The kernel is currently
>>> broken when trying to use modules in ARM SoCs (probably in other
>>> systems/archs too, but I can't speak for those).
>>>
>>> 3) Concerns about backwards compatibility
>>>
>>> I pointed out why the current scheme (depends-on being the only source
>>> of dependency) doesn't break compatibility. And if we go with
>>> "depends-on" as an override what we could do to keep backwards
>>> compatibility. Happy to hear more thoughts or discuss options.
>>>
>>> 4) How the "sync_state" would work for a device that supplies multiple
>>> functionalities but a limited driver.
>>
>> <snip>
>> To be clear, all of above are _real_ problems that stops us from efficiently
>> load device drivers as modules for Android.
>>
>> So, if 'depends-on' doesn't seem like the right approach and "going back to
>> the drawing board" is the ask, could you please point us in the right
>> direction?
> 
> Use the dependencies which are already there in DT. That's clocks,
> pinctrl, regulators, interrupts, gpio at a minimum. I'm simply not
> going to accept duplicating all those dependencies in DT. The downside
> for the kernel is you have to address these one by one and can't have
> a generic property the driver core code can parse. After that's in
> place, then maybe we can consider handling any additional dependencies
> not already captured in DT. Once all that is in place, we can probably
> sort device and/or driver lists to optimize the probe order (maybe the
> driver core already does that now?).
> 
> Get rid of the auto disabling of clocks and regulators in
> late_initcall. It's simply not a valid marker that boot is done when
> modules are involved. We probably can't get rid of it as lot's of
> platforms rely on that, so it will have to be opt out. Make it the
> platform's responsibility for ensuring a consistent state.

Setting aside modules for one moment, why is there any auto disabling
of clocks and regulators in late initcall????  Deferred probe processing
does not begin until deferred_probe_initcall() sets
driver_deferred_probe_enable to true.  No late_initcall function
should ever depend on ordering with respect to any other late_initcall.
(And yes, I know that among various initcall levels, there have been
games played to get a certain amount of ordering, but that is at
best fragile.)

In addition to modules, devicetree overlays need to be considered.

Just as modules can result in a driver appearing after boot finishes,
overlays can result in new devicetree nodes (and thus dependencies)
appearing after boot finishes.

-Frank

> 
> Perhaps we need a 'boot done' or 'stop deferring probe' trigger from
> userspace in order to make progress if dependencies are missing. Or
> maybe just some timeout would be sufficient. I think this is probably
> more useful for development than in a shipping product. Even if you
> could fallback to polling mode instead of interrupts for example, I
> doubt you would want to in a product.
> 
> You should also keep in mind that everything needed for a console has
> to be built in. Maybe Android can say the console isn't needed, but in
> general we can't.
> 
> Rob
> .
>