Hi,

On 08/29/2014 09:01 AM, Thierry Reding wrote:

On Thu, Aug 28, 2014 at 02:15:40PM +0200, Hans de Goede wrote:

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On 08/27/2014 04:16 PM, Thierry Reding wrote:

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Hmm I see, in my mind the problem is not that the clk framework disables
unused clocks, but that no one is marking the clocks in question as used.
Someone should mark these clocks as used so that they do not get disabled.

We could add a clk_mark_in_use function and have the simplefb patch call
that instead of clk_prepare_and_enable, or maybe even better just only
claim the clks and teach the clk framework to not disable claimed clk
in its cleanup run. That would make it clear that simplefb is not enabling
anything, just claiming resource its need to avoid them getting removed
from underneath simplefb, would that work for you ?

That would be more accurate, but it boils down to the same problem where
we still need a driver to claim the resources in some way whereas the
problem is fundamentally one where the bootloader should be telling the
kernel not to disable it. It's in fact the bootloader that's claiming
the resources.

Yes, but those resources do not belong to the bootloader in a sense
that traditional bootloader / firmware claimed resources (e.g. acpi
reserved resources) do. These traditional resources are claimed for ever.

I thought that at least on x86 there was a way for the kernel to reclaim
memory set apart for an early framebuffer.

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Where as these resources are claimed by the bootloader to keep the simplefb
it provides working, as soon as the simplefb is no longer used, they become
unused.

Right. And when simplefb goes away it is because a real driver is taking
over, in which case it will claim the resources explicitly.

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The copy and paste is for code that's platform specific. The clocks have
different names, resets are different, supplies are different. The fact
that many can currently use the same driver is likely just coincidence
rather than design and it's entirely possible that at some point they'll
add support for a more advanced feature that makes them incompatible
with the rest of the generic drivers. And then you have a big mess
because you need to add quirks all over the place.

And this isn't all that far off-topic, since simplefb also needs to deal
with this kind of situation. And what I've been arguing is that in order
to really be generic it has to make assumptions, one of which is that it
uses only resources that it doesn't need to explicitly handle.

I can understand that you're worried about generic ?hci drivers dealing with
clocks / resets / etc. As there may be strict ordering requirements there,
but for simplefb that is not the case.

All we're asking for is for a way to communicate 2 things to the kernel:

1) These resources are in use (we are not asking the kernel to do anything
with them, rather the opposite, we're asking to leave them alone so no
ordering issues)

Right. That's the issue that needs to be solved. We still only disagree
on how it should be solved.

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2) Tie these resources to simplefb so that the kernel can know when they
are no longer in use, and it may e.g. re-use the memory

For the memory there shouldn't be a problem because it's already in the
DT node anyway. It has to be there so that simplefb knows where to write
to.

The memory information currently in the dt node is not complete enough to
reclaim memory, e.g. the sunxi driver always reserves 8M, but the simplefb
reg entry only describes the part actually used for the framebuffer.

For all the other resources, if 1) is solved properly then 2) becomes a
non-issue.

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To me the most logical and also most "correct" way of modelling this is to
put the resources inside the simplefb dt node.

Except that simplefb isn't a real device, so there's a hard time
justifying its existence in DT as it is. Claiming resources from a
virtual device doesn't sound correct to me at all.

Yet you want it to claim memory that does not seem consistent to me.

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The key word here is "the used resources" to me this is not about simlefb
managing resources, but marking them as used as long as it needs them, like
it will need to do for the reserved mem chunk.

The difference between the clocks and the memory resource is that the
driver needs to directly access the memory (it needs to map it and
provide a userspace mapping for it) whereas it doesn't need to touch the
clocks (except to workaround a Linux-specific implementation detail).

Erm, no, the need to map the memory and the memory being a resource
which may be released are an orthogonal problem. E.g. a system with
dedicated framebuffer memory won't need to use a reserved main-memory
chunk, nor need to worry about returning that mem when simplefb is no
longer in use.

I would think the memory should still be reserved anyway to make sure
nothing else is writing over it. And it's in the device tree anyway
because the driver needs to know where to put framebuffer content.

No, the address were to write framebuffer contents currently is in the
simplefb node, not the actually backing memory info. I can fully see
some crazy hardware where a chunk of main memory is reserved for some
funky video engine, and then that chunk of memory gets accessed through
io-mem addresses for framebuffer access (e.g. the hardware may need this
to know when the fb is dirtied).

So
the point I was trying to make is that we can't treat the memory in the
same way as clocks because it needs to be explicitly managed. Whereas
clocks don't. The driver is simply too generic to know what to do with
the clocks. It doesn't know what frequency they should be running at or
what they're used for, so by any definition of what DT should describe
they're useless for this virtual device.

I don't see why you keep insisting that the memory is so special, it is
just another resource which we need to claim, and tie to the simplefb node
so that it can be re-used (or disabled) when simplefb is no longer used
(it could e.g. be unbound explicitly on headless systems).

Furthermore it's fairly likely that as your kernel support progresses
you'll find that the driver all of a sudden needs to manage some other
type of resource that you just haven't needed until now because it may
default to being always on. Then you'll have a hard time keeping
backwards-compatibility and will have to resort to the kinds of hacks
that you don't want to see in the kernel.

kernel support? This is about u-boot's video code and the simplefb bindings.

So it really boils down to the DT needing to describe a complete device
with all the dependencies. And just clocks aren't the complete
description. But if you want the complete description then you're going
to need a complete driver as well and simplefb is out of the picture
anyway.

Yes we eventually need a full kernel driver, with its own bindings, that
is not what this is about.

Once again, the current, basic form of the binding allows for a
completely generic implementation of the driver because it makes
assumptions about firmware setting things up the right way so that the
driver doesn't have to.

Except that it does not work because it does not claim the necessary
resources.

We seem to be at a point where this discussion is just going in circles,
and since there seems to be no reasonable alternative to the proposed
solution of adding the clocks to the simplefb node, can you please just
stop blocking progress on this?

Your objection to this has been duly noted, but no one is forcing you or
other people to actually use the clocks property in your implementation
of simplefb. So if your 101% certain that this is a bad idea, can you
please just let us (the sunxi people) make our own mistakes and learn
from those ? As we actually plan to use the clocks property, we will be
the ones which will have to deal with any issues. For the record, I
don't think this is a bad idea myself, but that just brings us full
circle again.

I'm willing to look into just marking the clocks as used, instead of
actually enabling them.

Regards,

Hans