[linux-sunxi] Re: [PATCH 3/4] simplefb: disable dt node upon remove

[Luc Verhaegen]

On Wed, Aug 13, 2014 at 11:45:24AM +0200, Luc Verhaegen wrote:
> On Wed, Aug 13, 2014 at 10:23:14AM +0100, Grant Likely wrote:
> > 
> > The majority of the DT code is based on the assumption of a static
> > tree. Pantelis has been working on being able to modify it at runtime
> > with overlays, but he has had to go through a lot of rework because it
> > is not a trivial task. When you get into modifying the DT, you need to
> > have a lot more understanding of the side effects to changing the
> > tree. The DT structure also has a lifecycle that can go beyond the
> > current lifecycle of the kernel. The kexec tool will extract the
> > current tree from the kernel, make the appropriate modifications, and
> > use that to boot the next kernel. Allowing any driver to modify the
> > tree has side effects beyond just the current kernel.
> > 
> > In this specific case, it will interact badly with the work Pantelis
> > is doing to make platform devices work with overlays. Modifying the
> > status property will cause the associated struct device to get removed
> > in the middle of probing a driver for that device! That will most
> > likely cause an oops.
> > 
> > Besides, Luc straight out *said*: "...even though it has no real value
> > today". In what circumstance is that justification for modifying the
> > tree?
> 
> With that sentence i meant that given the current state of things, it 
> has no real value.
> 
> It has no value currently as re-probing simplefb is not going to happen. 
> But it's not a big leap to turn simplefb into a proper module. Not that 
> that makes much sense, but that's never stopped anyone.
> 
> To me it seemed simple, dt is what drives simplefb, so dt then also 
> becomes responsible for making sure that simplefb or another driver does 
> not attempt to blindly use this info again. The way this is implemented 
> i do not care for in any way, i just knew that i could not do nothing 
> here, given the catastrophic effect disabling the clocks has on simplefb 
> on sunxi. Given the discussion that errupted here, i'd say that this 
> does need some resolution, and altering the dt is going to have to be 
> part of the solution.
> 
> In any case, i will gladly drop this patch, as it is not absolutely 
> necessary. But it should be very clear that there is no going back on 
> this dt node after the clocks were released once.
> 
> Luc Verhaegen.

What about approaching this from the other end? U-Boot could add a 
property named "once-only" or so.

Luc Verhaegen.