[RFC resend] arm64: mt8173: Fix Acer Chromebooks mmsys probe problem

[Laurent Pinchart]

Hi Philipp,

On Thursday, 19 October 2017 16:01:54 EEST Philipp Zabel wrote:
> On Thu, 2017-10-19 at 13:26 +0200, Matthias Brugger wrote:
> > In theory the MMSYS device tree identifier is matches twice, by the clk
> > driver and the DRM subsystem. But the kernel only matches the first
> > driver for a device (clk) and discards the second one. This breaks
> > graphics on mt8173 and most probably on mt2701 as well.
> > 
> > MMSYS in Mediatek SoCs has some registers to control clock gates (which is
> > used in the clk driver) and some registers to enable the differnet blocks
> > of the display subsystem. The kernel uses the binding to load the central
> > comoponent of the distplay subsystem, which in place probes all the other
> > components and enables the present ones in the MMSYS.
> > 
> > We found us with the problem, that we need to change and therefor break
> > one
> > of the two bindings, or the DRM one or the clock driver one.
> > 
> > Apart from that the DRM subysystem does access the MMSYS registers via
> > relaxed reads/writes. But the it should to so via regmap, as the
> > registers are shared.
> > 
> > Possible solutions:
> > 1) We add a new mediatek,mt8173-mmsys-clk node, which lives as a
> > simple-mfd under the actual mmsys node. We change the clock driver to
> > probe on this binding. This would make sense as the clock gate register
> > live completly in the MMSYS configuration registers.
> 
> The reason why the drm driver matches against the mmsys node in the
> first place is that we wanted to avoid 2).

Why did you want to avoid 2) ?

> Also, mmsys is not a pure clock controller, as it also contains the
> display path configuration in its register space.

Which makes the mmsys related to display, but more in a syscon (combining 
clocks and routing, and I assume other miscellaneous features that wouldn't 
fit nicely in the other display-related IP cores) way than actually being part 
of the display subsystem. Or does mmsys only provide display-related features 
?

> > 2) As the nodes of the DRM subsystem just need some of the registers of
> > MMSYS we add a new binding mediatek,mt8173-dispsys which probes the
> > central component of the DRM system. It has only a handle to mt8173-mmsys
> > to access the registerspace via regmap functions.
> > 
> > In this patchset I implemented 2). Please take into account, that this is
> > a RFC. I had no time to actually test the verison on real HW. Some of the
> > register accesses should be done using regmap_update instead of
> > regmap_read + regmap_write.
> > 
> > This RFC shall only show how solution 2) would look like. We can use it as
> > discussion to see how we circumvent the actual situation.
> 
> Or we could leave the bindings untouched and create one platform device
> from the other or even set up the clocks from the drm driver?

Does mmsys provide features (such as clocks) to non-display IP cores ?

-- 
Regards,

Laurent Pinchart