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

[Thierry Reding]

On Wed, Aug 27, 2014 at 05:42:21PM +0200, Maxime Ripard wrote:
> On Wed, Aug 27, 2014 at 11:52:48AM +0200, Thierry Reding wrote:
> > On Wed, Aug 27, 2014 at 10:45:26AM +0200, Maxime Ripard wrote:
> > > On Wed, Aug 27, 2014 at 08:54:41AM +0200, Thierry Reding wrote:
> > > > On Tue, Aug 26, 2014 at 11:02:48PM +0200, Maxime Ripard wrote:
> > > > > On Tue, Aug 26, 2014 at 04:35:51PM +0200, Thierry Reding wrote:
> > [...]
> > > > > > > Mike Turquette repeatedly said that he was against such a DT property:
> > > > > > > https://lkml.org/lkml/2014/5/12/693
> > > > > > 
> > > > > > Mike says in that email that he's opposing the addition of a property
> > > > > > for clocks that is the equivalent of regulator-always-on. That's not
> > > > > > what this is about. If at all it'd be a property to mark a clock that
> > > > > > should not be disabled by default because it's essential.
> > > > > 
> > > > > It's just semantic. How is "a clock that should not be disabled by
> > > > > default because it's essential" not a clock that stays always on?
> > > > 
> > > > Because a clock that should not be disabled by default can be turned off
> > > > when appropriate. A clock that is always on can't be turned off.
> > > 
> > > If a clock is essential, then it should never be disabled. Or we don't
> > > share the same meaning of essential.
> > 
> > Essential for the particular use-case.
> 
> So, how would the clock driver would know about which use case we're
> in? How would it know about which display engine is currently running?
> How would it know about which video output is being set?
> 
> Currently, we have two separate display engines, which can each output
> either to 4 different outputs (HDMI, RGB/LVDS, 2 DSI). Each and every
> one of these combinations would require different clocks. What clocks
> will we put in the driver? All of them?

Ideally the solution wouldn't involve hard-coding this into the clock
driver at all. There should be a way for firmware to communicate to the
kernel that a given clock shouldn't be disabled. Then since firmware
already knows what it set up it can tell the kernel to not touch those.

Thierry
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