On Wed, Aug 27, 2014 at 02:56:14PM +0200, Thierry Reding wrote:

quoted

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So second of all, Thierry, what exactly is the technical argument against
adding support for dealing with clocks to simplefb ?

I've already stated technical arguments against it. You could just go
back and read the thread again, or would you rather want me to repeat
them for your convenience?

It is a long thread, IIRC your main arguments against this are:

1) You don't want this kind of platform-specific knowledge in simplefb
2) That simplefb is supposed to be simple, and this is not simple

As for 1. several people have already argued that clocks as such are
an abstract concept, found one many platforms and as such are not
platform specific.

That alone doesn't justify this patch. SoCs often have a requirement to
enable clocks in a specific order, for example. Other systems may need
to coordinate clocks with resets in a specific order or enable power
domains and such. All that is very SoC specific and if we don't make it
very clear what this driver assumes about the state of the system, then
we can't object to anybody adding support for those later on either. The
result of that is going to be a driver that needs to handle all kinds of
combinations of resources.

Which is not an issue in our case, since the firmware enabled them
already.

So while clocks themselves are fairly generic the way in which they need
to be used is highly platform-specific.

Let me also reiterate what I've said a couple of times already. The
issue here isn't that simplefb needs to manage these clocks in any way.
Firmware has already set them up so that display works. The reason why
you need this patch is so that the clock framework doesn't automatically
turn them off. With the proposed patch the driver enables these clocks,
but that's not what it needs to do, they are already on. It's a work
around to prevent the clock framework into not disabling them.

So as far as I'm concerned this points to a fundamental issue with how
the clock framework handles unused clocks. Therefore that is the problem
that should be solved, not worked around.

Well, this is again a philosophical difference them. I find like the
right thing to do for the clock framework to disable the clocks that
are enabled if no one reports them as used. That also comes down to
all the drivers that require a clock to stay enabled should report to
the clock framework to tell that it uses this clock. Fortunately, we
have everything in place to do so.

quoted

Any generic hw driver dealing with embedded platforms, be it ahci, ohci,
ehci, etc. has ended up adding abstract support for things like clocks
(and other resources). I say abstract support here, since to all these
drivers the support was added in a way that no platform specific knowledge
is necessary, they just deal with clocks not caring about the specific
underlying clock implementation, clock names, etc.

Yes, I know that. You and I both had a very similar discussion not so
long ago. But the above aren't quite the same as simplefb. The devices
follow at least a well-known standard (EHCI, OHCI, AHCI), so there's
bound to be more commonalities between them than between the various
display devices that simplefb potentially can support.

Interestingly though, if you look at what hardware really is supported
by the generic drivers that deal with embedded platforms, there isn't
all that much diversity (I've manually stripped out non-DTS occurrences
since they aren't relevant for this discussion):

	$ git grep -n generic-ohci
	arch/arm/boot/dts/sun4i-a10.dtsi:451:           compatible = "allwinner,sun4i-a10-ohci", "generic-ohci";
	arch/arm/boot/dts/sun4i-a10.dtsi:492:           compatible = "allwinner,sun4i-a10-ohci", "generic-ohci";
	arch/arm/boot/dts/sun5i-a10s.dtsi:403:          compatible = "allwinner,sun5i-a10s-ohci", "generic-ohci";
	arch/arm/boot/dts/sun5i-a13.dtsi:376:           compatible = "allwinner,sun5i-a13-ohci", "generic-ohci";
	arch/arm/boot/dts/sun6i-a31.dtsi:410:           compatible = "allwinner,sun6i-a31-ohci", "generic-ohci";
	arch/arm/boot/dts/sun6i-a31.dtsi:432:           compatible = "allwinner,sun6i-a31-ohci", "generic-ohci";
	arch/arm/boot/dts/sun6i-a31.dtsi:443:           compatible = "allwinner,sun6i-a31-ohci", "generic-ohci";
	arch/arm/boot/dts/sun7i-a20.dtsi:535:           compatible = "allwinner,sun7i-a20-ohci", "generic-ohci";
	arch/arm/boot/dts/sun7i-a20.dtsi:576:           compatible = "allwinner,sun7i-a20-ohci", "generic-ohci";
	arch/powerpc/boot/dts/akebono.dts:144:          compatible = "ibm,476gtr-ohci", "generic-ohci";
	arch/powerpc/boot/dts/akebono.dts:151:          compatible = "ibm,476gtr-ohci", "generic-ohci";

	$ git grep -n generic-ehci
	arch/arm/boot/dts/rk3288.dtsi:199:              compatible = "generic-ehci";
	arch/arm/boot/dts/rk3288.dtsi:210:              compatible = "generic-ehci";
	arch/arm/boot/dts/sun4i-a10.dtsi:441:           compatible = "allwinner,sun4i-a10-ehci", "generic-ehci";
	arch/arm/boot/dts/sun4i-a10.dtsi:482:           compatible = "allwinner,sun4i-a10-ehci", "generic-ehci";
	arch/arm/boot/dts/sun5i-a10s.dtsi:393:          compatible = "allwinner,sun5i-a10s-ehci", "generic-ehci";
	arch/arm/boot/dts/sun5i-a13.dtsi:366:           compatible = "allwinner,sun5i-a13-ehci", "generic-ehci";
	arch/arm/boot/dts/sun6i-a31.dtsi:399:           compatible = "allwinner,sun6i-a31-ehci", "generic-ehci";
	arch/arm/boot/dts/sun6i-a31.dtsi:421:           compatible = "allwinner,sun6i-a31-ehci", "generic-ehci";
	arch/arm/boot/dts/sun7i-a20.dtsi:525:           compatible = "allwinner,sun7i-a20-ehci", "generic-ehci";
	arch/arm/boot/dts/sun7i-a20.dtsi:566:           compatible = "allwinner,sun7i-a20-ehci", "generic-ehci";
	arch/powerpc/boot/dts/akebono.dts:130:          compatible = "ibm,476gtr-ehci", "generic-ehci";

For generic-ahci there aren't any matches, but here are a few that are
marked compatible with it using different compatible strings:

	$ git grep -n snps,spear-ahci
	arch/arm/boot/dts/spear1310.dtsi:60:            compatible = "snps,spear-ahci";
	arch/arm/boot/dts/spear1310.dtsi:69:            compatible = "snps,spear-ahci";
	arch/arm/boot/dts/spear1310.dtsi:78:            compatible = "snps,spear-ahci";
	arch/arm/boot/dts/spear1340.dtsi:43:            compatible = "snps,spear-ahci";

	$ git grep -n snps,exynos5440-ahci
	arch/arm/boot/dts/exynos5440.dtsi:241:          compatible = "snps,exynos5440-ahci";

	$ git grep -n 'ibm,476gtr-ahci'
	arch/powerpc/boot/dts/akebono.dts:123:          compatible = "ibm,476gtr-ahci";

	$ git grep -n 'snps,dwc-ahci'
	arch/arm/boot/dts/dra7.dtsi:1049:               compatible = "snps,dwc-ahci";
	arch/arm/boot/dts/exynos5250.dtsi:265:          compatible = "snps,dwc-ahci";
	arch/arm/boot/dts/omap5.dtsi:919:               compatible = "snps,dwc-ahci";

That looks fairly homogenous to me. There are also things like this
(from the ahci_platform.c driver):

	if (of_device_is_compatible(dev->of_node, "hisilicon,hisi-ahci"))
		hpriv->flags |= AHCI_HFLAG_NO_FBS | AHCI_HFLAG_NO_NCQ;

With more supported hardware there's bound to be more quirks like that.

So there's a couple of SoCs and boards that actually are generic enough
to work with a generic driver. And then there's a whole bunch of other
drivers for hardware that's compliant with the same standard yet needs
different drivers. To me that's a clear indication that there isn't
enough genericity to warrant a generic driver in the first place.

The commonality is in the functionality and defined by the standard
registers. But there's little to no commonality in how that interface is
glued into the SoC. Luckily the above subsystems implement the standard
hardware programming in a library, so that non-generic drivers can still
make use of most of the generic code.

To be fair, these additions are a couple of release old, and are quite
recent. You can't really make any judgement based on barely 2 releases
of history.

quoted

If you look at how almost all dt bindings work, then the dt node for
a device specifies the resources needed, I don't see why simplefb would
be so special that it should be different here. I agree that it is
important to get the abstractions right here, but to me it seems that
the right abstraction is to be consistent with how all other devices
are abstracted and to add needed resources to the dt node for the
simplefb.

But simplefb is fundamentally different from other devices. It isn't a
physical device at all. It's a virtual device that reuses resources as
set up by some other piece of code (firmware). It implies that there's
nothing that needs to be managed. It should only create a framebuffer
with the given parameters and allow the kernel (and userspace) to render
into it.

Nothing should be managed, but everything should stay as is. Again,
this is something that we all seem to agree on, yet differ completely
on how to implement that.

The only way you can deal with such virtual, completely generic devices
is by being very explicit about the requirements. For simplefb the
assumptions are that firmware set everything up and passes information
about what it set up to the kernel so that it can be reused. None of the
resources need to be explicitly managed because they have all been
properly configured. For that reason, again, I think the right way is
for the kernel not to switch off any of the used resources.

So, you're saying that the firmware should inform the kernel about
what clocks it has set up?

I do agree on that too.

And it looks like we reached an agreement then, since it's exactly
what this patch is relying on.

If you want to equate simplefb to other drivers then it is fundamentally
broken anyway. Given only what's in the DTB the simplefb driver won't be
able to do anything useful. Consider what would happen if the firmware
didn't set up all the resources. Then the DT is missing resets, power
supplies and all that to make it work.

That would mean that the framebuffer wasn't working in the first
place, which breaks the initial assumption, doesn't it?

quoted

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so I think we should keep the discussion going for a bit and
see if we really can't come up with something that will fix this for a
more general case rather than just your particular use-case. It's
evidently a recurring problem that others suffer from, too.

Yes needing resources despite the device being behind some generic
interface where the main driver code for that interface has no knowledge
about such resources sofar, because e.g. on PC-s this was not necessary
is a recurring problem, which has been solved already 3 times at least,
see the ohci-, ehci- and ahci-platform drivers, and for all 3 the conclusion
was that the best solution was to simple add the resources to the dt-node,
and add code to claim and enable them.

And as I pointed out above in all three cases the generic platform
driver is only marginally useful because many SoCs have specific
requirements that make them incompatible with the generic driver.

s/marginally useful/marginally used/

-- 
Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com
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