[linux-sunxi] Re: [PATCH 4/4] simplefb: add clock handling code

[Julian Calaby]

Hi All,

On Wed, Aug 27, 2014 at 6:37 PM, Geert Uytterhoeven
<geert at linux-m68k.org> wrote:
> Hi Maxime,
>
> On Wed, Aug 27, 2014 at 10:22 AM, Maxime Ripard
> <maxime.ripard at free-electrons.com> wrote:
>> On Wed, Aug 27, 2014 at 08:40:57AM +0100, Mark Brown wrote:
>>> On Tue, Aug 26, 2014 at 08:40:09PM +0200, Henrik Nordström wrote:
>>>
>>> > It is not clear to me where the hardware resources should be listed in
>>> > DT, being it a simplefb node or part of the actual hardware device node
>>> > properly marked as dynamic boot defaults or something else? It's
>>> > somewhere inbetween hardware and virtual device, and somewhat volatile.
>>> > As far as simplefb is concerned it is a hardware desription of the
>>> > framebuffer, but for a kms driver it's no more than firmware handover of
>>> > boottime settings and ceases to exists once the kms driver have
>>> > reconfigured the hardware.
>>>
>>> Is simplefb something that should be in the device tree distinctly in
>>> the first place - shouldn't it be a subset of the functionality of the
>>> video nodes?  It's the same hardware being driven differently.
>>
>> Therorically, yes, but that would mean knowing beforehand what the
>> final binding will look like, even before submitting the driver. Since
>> the bindings are always reviewed, and most of the time changed
>> slightly, that wouldn't work very well with the DT as a stable ABI
>> policy I guess.
>
> If you don't know how the bindings for a device will look like at the time of
> writing your DTS, you're always screwed, whether you add a simpefb
> node or not.
>
> If you know how the bindings look like, just add the device, with an extra
> "linux,simplefb" compatibility value.
> If you don't know how the bindings look like, do your utter best in
> guessing. Your DTS must be amended later anyway, either because
> you guessed wrong[*] (in case you added a node to have simplefb
> working), or because you have to add a real device node (in case you
> didn't add one for simplefb).
>
> [*] Actually you may have guessed right, in which case it'll continue
>     working as-is, and everybody will be happy.
>     Whether you want to keep backwards-compatibility in your future driver
>     with the "guessed wrong" node is up to you.

I apologise if I'm stepping on anyone's toes or horribly
misrepresenting device tree's capabilities, but couldn't we start out
with something like:

disp at whatever {
    compatible = "sunxi,sun4i-disp";
    clocks = <&stuff 1>, <&stuff 2>, <&stuff 3>;
}

as our binding?

u-boot could then set up a framebuffer and mangle this to:

disp at whatever {
    compatible = "sunxi,sun4i-disp", "linux,simplefb";
    clocks = <&stuff 1>, <&stuff 2>, <&stuff 3>;
    // simplefb stuff
}

Hell, if we have a reserved memory driver, couldn't we then mangle
this to something like:

disp at whatever {
    compatible = "sunxi,sun4i-disp", "linux,simplefb";
    clocks = <&stuff 1>, <&stuff 2>, <&stuff 3>;
    // simplefb stuff
    // reserved mem stuff
}

simplefb is modified to be smart enough to grab the clocks (that's a
small amount of code that almost every DT-enabled driver has, right?)
and if we're going to use the reserved memory driver to ensure nobody
messes with it's in-memory framebuffer, it'll have to deal with that
too.

Eventually sunxi-kms gets submitted, and it takes the
"sunxi,sun4i-disp" compatible, which we've already defined, however we
need some more stuff so we just add more stuff to the binding and end
up with something like:

disp at whatever {
    compatible = "sunxi,sun4i-disp";
    clocks = <&stuff 1>, <&stuff 2>, <&stuff 3>;
    // sunxi-kms stuff
}

I believe there'd be no backwards compatibility issues as we're only
adding, not subtracting or modifying. We already know which clocks are
required for the display IP, right? And if not, we can add them. I
believe this minimises the mangling u-boot has to do (at least
initially) and is forward and backward compatible.

That'd work, right?

Thanks,

-- 
Julian Calaby

Email: julian.calaby at gmail.com
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