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

[Laurent Pinchart]

Hi Philipp,

On Thursday, 19 October 2017 17:54:16 EEST Philipp Zabel wrote:
> On Thu, 2017-10-19 at 16:39 +0300, Laurent Pinchart wrote:
> > 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.

I'm not a big supported of such DT nodes for "logical" devices either. When 
possible I prefer describing the relationship between display IP cores using 
OF graph DT bindings.

In some cases (and I don't know if the Mediatek display is one of them) there 
is no single IP core that can be considered as a master from a display point 
of view. This is inconvenient for device drivers as there's no clear place for 
an entry point. I could thus be convinced to accept DT bindings for a logical 
display DT node when there's really no other good choice.

> >> 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.

Without more information about the hardware it's hard to tell whether the DT 
nodes for the DISP and MDP IP cores should be children of the MMSYS DT node or 
not. In any case, it looks like the MMSYS is a syscon that provides 
miscellaneous functions not fitting anywhere else.

On simple solution, which shouldn't require DT changes, would be to merge the 
MMSYS clock driver into the mediatek DRM driver, and register the MMSYS clocks 
at probe time instead of using CLK_OF_DECLARE.

Another option would be to handle MMSYS as an MFD. That shouldn't require DT 
changes either.

> >>> 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.

-- 
Regards,

Laurent Pinchart