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

[CK Hu]

Hi,

On Thu, 2017-10-19 at 16:54 +0200, Philipp Zabel wrote:
> Hi Laurent,
> 
> On Thu, 2017-10-19 at 16:39 +0300, Laurent Pinchart wrote:
> > 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) ?
> 
> Because the "display-subsystem" node does not represent a real device,
> it's just there to probe the driver that stitches all the DISP
> components together.
> 
> > > 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 
> > ?
> 
> All devices in the 0x14000000 - 0x14ffffff memory range are part of the
> MMSYS system. That includes the MMSYS control or system configuration
> block at 0x14000000 - 0x14000fff as well as all the related MDP (media
> data path) and DISP (display data path) blocks that follow. The DISP
> blocks are purely display related, while the MDP blocks implement
> implement mem2mem functions like scaling and conversion.
> 
> > > > 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 ?
> 
> The MMSYS control block provides clocks for the DISP (display data path)
> and MDP (multimedia data path) blocks, as well as the routing between
> them, but not to anything outside of the MMSYS system.

According to register table of mediatek x20 mmsys [1] and Philipp's
statement, I think mmsys is neither clock-controller nor display
controller. It's a combination of multiple function. The four major
function is display's clock-control, mdp's clock-control, display's
routing, and mdp's routing. So we have two choice:

1) Centralize these multiple function control inside mmsys device: This
means there is only a mmsys device which contains function of
clock-control, display, and mdp.

2) Separate these multiple function to different device: mmsys is the
major device which owns the register resource but does nothing. The
function is controlled by three virtual device: mmsys-clock-controller,
display-controller, and mdp-controller.

I prefer 2) because these function seems independent. 

Regards,
CK

[1]
https://www.96boards.org/documentation/ConsumerEdition/MediaTekX20/AdditionalDocs/

> 
> regards
> Philipp