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From:   Rob Herring <robh+dt@kernel.org>
Date:   Wed, 12 Jun 2019 07:53:39 -0600
Message-ID: <CAL_Jsq+V9QUBpzmPyYjWe93-06-mpU=5JmUqvf-QsnuLxPnmUA@mail.gmail.com>
Subject: Re: [RESEND PATCH v1 1/5] of/platform: Speed up of_find_device_by_node()
To:     Sandeep Patil <sspatil@android.com>,
        Saravana Kannan <saravanak@google.com>
Cc:     Frank Rowand <frowand.list@gmail.com>,
        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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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.

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