Message-ID: <20150429145114.GW6325@pengutronix.de>
Date:	Wed, 29 Apr 2015 16:51:14 +0200
From:	Sascha Hauer <s.hauer@...gutronix.de>
To:	Lee Jones <lee.jones@...aro.org>
Cc:	Maxime Ripard <maxime.ripard@...e-electrons.com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	kernel@...inux.com, mturquette@...aro.org, sboyd@...eaurora.org,
	devicetree@...r.kernel.org, geert@...ux-m68k.org
Subject: Re: [PATCH v6 4/4] clk: dt: Introduce binding for always-on clock
 support

On Wed, Apr 29, 2015 at 03:17:51PM +0100, Lee Jones wrote:
> On Wed, 22 Apr 2015, Maxime Ripard wrote:
> 
> > On Wed, Apr 08, 2015 at 06:23:44PM +0100, Lee Jones wrote:
> > > On Wed, 08 Apr 2015, Maxime Ripard wrote:
> > > 
> > > > On Wed, Apr 08, 2015 at 11:38:32AM +0100, Lee Jones wrote:
> > > > > On Wed, 08 Apr 2015, Maxime Ripard wrote:
> > > > > 
> > > > > > On Wed, Apr 08, 2015 at 09:14:50AM +0100, Lee Jones wrote:
> > > > > > > > > +
> > > > > > > > > +		    This property is not to be abused.  It is only to be used to
> > > > > > > > > +		    protect platforms from being crippled by gated clocks, not
> > > > > > > > > +		    as a convenience function to avoid using the framework
> > > > > > > > > +		    correctly inside device drivers.
> > > > > > > > 
> > > > > > > > Disregarding what's stated here, I'm pretty sure that this will
> > > > > > > > actually happen. Where do you place the cursor?
> > > > > > > 
> > > > > > > That's up to Mike.
> > > > > > 
> > > > > > Except that Mike won't review any of the DT changes, so he won't be
> > > > > > able to refrain users from using it. Let alone out-of-tree DTs using a
> > > > > > mainline kernel.
> > > > > 
> > > > > Ideally Mike should be Cc'ed on patches using clock bindings, but if
> > > > > he isn't the DT guys are smart enough to either make the right
> > > > > decisions themselves (Rob has Acked these bindings already, so will be
> > > > > on the lookout for misuse, I'm sure), or ask for Mike's help.
> > > > 
> > > > Yeah, right, as if this strategy really worked in the past....
> > > > 
> > > > Do we really want to look at even the DT bindings that have actually
> > > > been reviewed by maintainers that got merged?
> > > > 
> > > > They don't have time for that, which is totally fine, but we really
> > > > should bury our head in the sand by actually thinking they will review
> > > > every single DT-related patch.
> > > > 
> > > > Using that as an argument is just plain denial of what really happened
> > > > for the past 4 years.
> > > 
> > > I agree that it's a problem, but this is a process problem and has
> > > nothing to do with this set.  If you have a problem with the current
> > > process and have a better alternative, submit your thoughts to the DT
> > > list.  Rejecting all new bindings because you are frightened that they
> > > will be used in a manner that they were not intended is not the way to
> > > go though.
> > 
> > I'm not saying that this binding should not go in because of a process
> > issue.
> > 
> > I'm saying that discarding arguments against your binding by adding
> > restrictions that cannot be enforced is not reasonable.
> 
> I'm open to constructive suggestions/alternatives.
> 
> Hand rolling this stuff in C per vendor is not of of them.
> 
> > > > > > > > Should we create a new driver for our RAM controller, or do we want to
> > > > > > > > use clock-always-on?
> > > > > > > 
> > > > > > > I would say that if all the driver did was to enable clocks, then you
> > > > > > > should use this instead.  This binding was designed specifically for
> > > > > > > that purpose.
> > > > > > > 
> > > > > > > However, if the aforementioned driver clock can be safely gated, then
> > > > > > > it should not be an always-on clock.
> > > > > > 
> > > > > > Yeah, of course, I understand the original intent of it, but that
> > > > > > argument, which might very well be true at one point in time, might
> > > > > > not be true anymore two or three releases later.
> > > > > 
> > > > > Why?  The H/W isn't going to change in two or three releases.  The
> > > > > clocks designated as 'always-on' will have to be on forever, or
> > > > > synonymously, 'always'.
> > > > >
> > > > > > And that driver might actually rely on the fact that the clock is shut
> > > > > > down, which won't be the case.
> > > > > 
> > > > > I think you are missing the point of this binding.  The driver can
> > > > > never rely on that in this use-case.  If the clock is off, there is no
> > > > > device driver, period. 
> > > > 
> > > > Ok. So CPU hotplug or cpuidle is not a thing then? I'm pretty sure the
> > > > PM guys will be happy to hear that.
> > > > 
> > > > And they are not device drivers, are not mandatory in the system, and
> > > > it's usually a good thing to keep the CPU running whenever you don't
> > > > have such drivers.
> > > > 
> > > > > > Introducing a DT interface solely by refering to the current state of
> > > > > > a driver is a bit odd.
> > > > > 
> > > > > I'm not sure I get your point.  This binding has nothing to do with
> > > > > drivers.
> > > > 
> > > > It's all about drivers. Or rather all about missing drivers.
> > > 
> > > I think you are going to have to be more forthcoming with your issues
> > > with this binding, because I'm struggling to understand what your
> > > problem with it is.  You have already pointed me to vendors which have
> > > a genuine/valid need for it.  But instead you'd prefer they hand-roll
> > > their own implementations over multiple lines of C code (each).
> > 
> > I told you already.
> > 
> > If you have that property, there's absolutely no way to do any kind of
> > clock management in the future.
> > 
> > It might be fine for your use case, but see my point about the
> > unreasonable restriction. People are going to use it for clocks that
> > just don't have a driver *yet*, and when that driver will be merged we
> > will end up with a driver that (for example) makes the assumption that
> > the clock has been shut down to reset the IP, that might or might not
> > be the case, depending on how old the DT is exactly.
> 
> There is a need for this binding, but as you say, it must not be
> abused.  So how to we get people not to use it willy-nilly?
> 
> IMO, if people choose to ignore the stark warning in the documentation
> then that's they're lookout.  I guess you'd like to wrap them in more
> cotton wool than I would.  That's fine too, but how.
> 
> > This will be even a bigger madness if you ask me.
> > 
> > > > > > > > Do we really want to enforce this if we ever gain a driver that would
> > > > > > > > actually be able to manage its clock (like do we want the CPU clock to
> > > > > > > > never *ever* be gated just because we don't have a cpuidle/hotplug
> > > > > > > > driver yet?)
> > > > > > > 
> > > > > > > As I've just mentioned, if a clock 'can' be turned off, this binding
> > > > > > > should never be used. Situations where using always-on as a stop-gap
> > > > > > > due to a lack of current functionality is what the paragraph above is
> > > > > > > trying to mitigate.
> > > > > > 
> > > > > > But it's not really what this property is about. What this property
> > > > > > describes is that these clocks should never be gated. Any point in
> > > > > > time during the life of the system AND with in any kernel version.
> > > > > 
> > > > > You got it, that's correct -- these clocks should never be gated.
> > > > > 
> > > > > So why would that ever change?  If that is likely (or even possible)
> > > > > to change in the future then this binding should not be used.
> > > > >
> > > > > To reiterate; this binding should be used on ungatable clocks only.
> > > > > Non-negotiable, non-changeable either by the introduction of new
> > > > > functionality/support or kernel version.
> > > > 
> > > > I'm pretty sure that if that patch gets merged, by the end of the
> > > > year, there will be "incorrect" users by your standards.
> > > 
> > > It's possible to abuse any binding.  I don't see why you are so
> > > offended of this one in particular.
> > 
> > I'm not offended, I just tried to push the same kind of patches two
> > years ago, with Mike pushing back, and actually came to see that he
> > was right a few monthes down the road.
> 
> Well this was suggested by Mike.  I even have his Ack already.  So I
> guess he too has changed the error of his ways. :)
> 
> > And yeah, your point that any binding can be abused is true. This one
> > is only so easy to abuse it's not even funny.
> > 
> > > > If you introduce a feature, you should expect people to use
> > > > it.  If not, what's the point?
> > > 
> > > By your own admission, there are genuine users for this binding and I
> > > expect people to use it.
> > 
> > The only thing that we really disagree upon is that whether that
> > restriction will really be followed. You expect people to, I
> > don't. It's the fundamental disagreement we have, that really prevent
> > any purely technical discussion.
> > 
> > Maybe we can try to address that before moving forward?
> 
> I'd like that.
> 
> If a firm warning isn't good enough, then what will be?
> 
> > > > > > > > Have you seen the numerous NAK on such approach Mike did?
> > > > > > > 
> > > > > > > I haven't, but the folks reviewing previous versions have.  Do you
> > > > > > > have something specific in mind that you'd like to bring to my
> > > > > > > attention?
> > > > > > 
> > > > > > Unfortunately, I haven't been able to dig out such mails. But it's why
> > > > > > we ended up with clock protection code in various clock drivers
> > > > > > including:
> > > > > > 
> > > > > > AT91: http://lxr.free-electrons.com/source/drivers/clk/at91/clk-slow.c#L484
> > > > > > iMX28: http://lxr.free-electrons.com/source/drivers/clk/mxs/clk-imx28.c#L154
> > > > > > Rockchip: http://lxr.free-electrons.com/source/drivers/clk/rockchip/clk.c#L320
> > > > > > sunXi: http://lxr.free-electrons.com/source/drivers/clk/sunxi/clk-sunxi.c#L1183
> > > > > > Zynq: http://lxr.free-electrons.com/source/drivers/clk/zynq/clkc.c#L504
> > > > > > 
> > > > > > Which is much more flexible, since you won't have to modify the DT to
> > > > > > change which clocks are to be left enabled, as well as way easier to
> > > > > > debug if you ever have to remove that property from the DT.
> > > > > 
> > > > > You're right, you don't have the change the DT in these cases.  You
> > > > > have to write new C code, which is _less_ flexible.
> > > > 
> > > > I'm sorry to learn that you never heard of that stable-DT thing.
> > > > 
> > > > And a bit sorry to see that a maintainer is really seeing C as not
> > > > flexible.
> > > 
> > > You're putting words in my mouth.  I didn't say C was not flexible.
> > > 
> > > I'm referencing the original DT pros i.e. it is possible to supply a
> > > different configuration without the need to compile the kernel.
> > > That's certainly true in this case.  We can provide a clk-provider and
> > > tag it as always-on, all without re-compile.
> > > 
> > > > > So all these platforms are adding their own hand-rolled version of
> > > > > this binding, adding more duplication and cruft to the kernel.
> > > > > Instead they can use this 'always-on' and we can consolidate and strip
> > > > > it all out.
> > > > 
> > > > Except that all these platforms are actually not implementing a
> > > > binding, ie not an interface with the DT they are bound to. Each and
> > > > every of these platforms can change that list whenever they wish, just
> > > > by sending a single one-liner patch (just like the DT, really.).
> > > > 
> > > > Which is not something that you can achieve with a DT binding.
> > > 
> > > Once again, can you give me more information about why you have such a
> > > problem with this binding.  I wish for it to be stable/ABI, I wish for
> > > it never to be removed, I envisage it will always be needed, so what's
> > > the problem? 
> > > 
> > > Do you have a vested interest that I am missing?
> > > 
> > > Perhaps an example of possible calamity will help convince me that
> > > you're not completely wrong and blowing everything out of proportion
> > > for no good reason.
> > 
> > Let's say you've introduced such a clock in kernel 4.0 for the memory
> > clock.
> > 
> > At some point down the road, you create a ddrfreq driver (if that ever
> > exists). You have a new driver, which will manage the clock.
> > 
> > In that driver, for some reason, you have to shutdown the clock to
> > reset the DDR controller. Of course that also means that you will be
> > removing the clk-always-on property from your DT.
> > 
> > You will have in your driver something like:
> > 
> > /* Reset our controller */
> > clk_disable(clk);
> > clk_enable(clk):
> > 
> > And then, you expect your controller to be in its out-of-reset
> > state. Which will be the case with a new DT, and not with the old one,
> > probably creating all kind of very entertaining issues to debug.
> > 
> > All of this wouldn't be the case if you had this inside the kernel,
> > since (hopefully) the kernel is consistent with itself.
> 
> Surely you must have realised already that DTBs are more tightly
> coupled to kernel versions than we would have initially liked?  It's
> naive to assume that old DTBs will 'just-work' with newer kernels.
> 
> Wrong decisions related to DT are being made daily.  Adding mistakenly
> or foolhardily adding 'clk-always-on' to a DTS is not going to be the
> sole cause of breakage somewhere down the line.

It may not be the sole cause of breakage, but one we can avoid.

> 
> Pushing back on the acceptance of this binding based on idealistic,
> possibly already out-of-date premise is just frustrating.
> 
> This useful binding should be accepted and people should not abuse
> it.  If they do and the vendor Maintainer's review and accept then
> they have no foundation for recourse.

Maxime gave a good example for a clock which looks like a candidate for
clk-always-on but needs to be turned off in a later kernel revision.
Could you elaborate why you still want to have the clk-always-on in the
device tree instead of in the kernel where it can be removed when
necessary? What's your problem with enabling the critical clocks using
clk_prepare_enable like other SoCs already do?

Sascha

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