Hi!

On Wed, Aug 27, 2014 at 10:37:36AM +0200, Geert Uytterhoeven wrote:

Hi Maxime,

On Wed, Aug 27, 2014 at 10:22 AM, Maxime Ripard
[off-list ref] wrote:

quoted

On Wed, Aug 27, 2014 at 08:40:57AM +0100, Mark Brown wrote:

quoted

On Tue, Aug 26, 2014 at 08:40:09PM +0200, Henrik Nordstr?m wrote:

quoted

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).

Let's be conservative and consider the case where we would guess
wrong.

If we just rely on a simplefb node, when reviewing and integrating the
"new" bindings to describe accureately the various IPs involved in the
display path, we would obviously create new compatibles for
them. Since it's new compatibles, we can come up with any binding we'd
like, without have to consider the backward compatibility, since it's
a new binding.

Then, we just remove the simplefb, all is good.

If we were to try to create our bindings for all the IPs involved, and
were not pleased with the binding anymore when merging the driver,
then we would have to break the bindings, since we don't introduce a
new compatible anymore, but modifying an existing one.

Maxime

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