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

[Thierry Reding]

On Thu, Aug 28, 2014 at 10:46:03PM +0200, Maxime Ripard wrote:
> On Thu, Aug 28, 2014 at 12:11:41PM +0200, Thierry Reding wrote:
[...]
> > Then since firmware already knows what it set up it can tell the
> > kernel to not touch those.
> 
> Somehow, I've been raised kernel-wise into thinking that you can never
> fully trust your firmware. Or at least that you should have a way to
> recover from any bug/bad behaviour from it.

If you don't trust your firmware then you shouldn't be taking over a
device that it's set up for you. Rather you should write a proper driver
that sets things up from the start.

This whole "we don't trust firmware" isn't going to work if we want to
have hand-off from firmware to kernel. And it's already been decided in
other threads that moving more code out of the kernel and into firmware
is a requirement (c.f. ARM64).

Also in this case you wrote the firmware, so why wouldn't you trust it?

> Moreover, the way I see it, there's a major flaw in having an
> attribute in the clock node: you don't even know if the clock is ever
> going to be used.
> 
> If simplefb is not compiled in, you won't claim the clocks, and they
> will be disabled, which is imho a good thing. This case wouldn't be
> covered with an attribute at the clock node, because you don't have a
> link to what device/feature actually uses it in the system, and so you
> have to make the assumption that it will be used. And you will end up
> with clocks with a rather high rate running for nothing.

That's completely hypothetical. If simplefb isn't enabled and the clock
isn't claimed there's probably still going to be some other driver
taking over eventually. If there isn't, what's the point of firmware
setting up display in the first case?

Thierry
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