[U-Boot] [linux-sunxi] Re: [PATCH] video: add cfb console driver for sunxi

[Michal Suchanek <hramrach@gmail.com>]

On 4 August 2014 17:31, Henrik Nordstr?m <henrik@henriknordstrom.net> wrote:
> m?n 2014-08-04 klockan 17:05 +0200 skrev Luc Verhaegen:
>> But... What do we do when u-boot sets up cfb, without setting up a
>> simplefb node in the dt. Or what do we do when a simplefb node is set
>> up, but no simplefb code is included in the kernel? Well, we then either
>> need to claim the clocks, and make sure that nothing else touches the
>> memory, or we need to cleanly disable the display engine. But which do
>> we choose, do we keep the u-boot output forever, or do we sync off when
>> the kernel starts?
>
> And what happens if we simply ignore it? Is there a risk of anything
> burning beyond software repair? If not, ignore it. There is no need to
> support all incompatible mixes even if it means some people will see
> strange fireworks on their screens when not following the software
> requirements.
>

Then you have to define what is compatible.

eg. you can define that if you do have simplefb in u-boot you should
have it in kernel.

If you do not have simplefb in kernel the fb memory may get reused and
the clocks should get disabled as unused, eventually. Unless you set
the debug option not to disable them. And KMS might eventually take
over reprogramming or re-enabling the clocks again if you have it.

Or as pointed out earlier you might find this unacceptable and even
with simplefb compiled out the KMS driver would need enough simplefb
logic to analyze the simplefb DT node and reuse the clocks and memory
without reprogramming.

With both simplefb and KMS you need the KMS driver to take over after
simplefb. This is handled more or less gracefully with vesafb/offb/..
and whatever approach is used there might be of some use as example
and the flicker-freeness of that approach as Linux standard.

Thanks

Michal