[PATCH 03/15] dt-bindings: mfd: syscon: Add ti,am625-dss-dpi0-clk-ctrl compatible

[Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>]

The DPI output pipeline in K3 SoCs contains the display subsystem (DSS)
which produces the in-SoC parallel video signal, and a DPI block which
adjusts the signal to the external MIPI DPI output.

The DSS IP has registers to configure whether the data and sync signals
are driven on rising or falling clock edge, and on some SoCs these are
automatically conveyed to the DPI block which needs that configuration
to properly output the MIPI DPI signal.

However, on some SoCs the DPI block configuration has to be done
manually, using an extra register outside the DSS, DPI0_CLK_CTRL in
MAIN_CTRL_MMR_CFG0 block, which controls the DPI block's behavior. Note
that while the register is named "CLK_CTRL", it's not really related to
clocks, but the sync and data signals.

Currently the DPI0_CLK_CTRL is never written, so it's always 0, meaning
the data and sync are always driven on a rising clock edge regardless of
the DSS configuration.

DPI0_CLK_CTRL register seems to be an independent "quirk" register,
inside MAIN_CTRL_MMR_CFG0 block, which contains general purpose system
registers. The registers surrounding DPI0_CLK_CTRL seem to be controlled
by the system firmware or linux clock drivers. So, it is just this
single register we can map, and we can't create a syscon node for the
whole (or big parts of) MAIN_CTRL_MMR_CFG0.

I see two options to handle the register:

1) We could add that single register to the DSS binding as a new reg
   block. That feels wrong, as it's not a DSS register.
2) Add it as a syscon node, which can then be used by tidss driver.
   It is a bit silly to create a syscon node for a single 32-bit
   register, though.

Neither option feels good, but I do lean towards the latter approach.

Add 'ti,am625-dss-dpi0-clk-ctrl' compatible to syscon.yaml so we can add
the syscon node for the register.

Original patch from Louis Chauvet <louis.chauvet@bootlin.com>

Link: https://e2e.ti.com/support/processors-group/processors/f/processors-forum/1228207/am62a7-rgb-display-flickering-and-pixel-issue/
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
---
 Documentation/devicetree/bindings/mfd/syscon.yaml | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/Documentation/devicetree/bindings/mfd/syscon.yaml b/Documentation/devicetree/bindings/mfd/syscon.yaml
index e57add2bacd3..415cc21fd328 100644
--- a/Documentation/devicetree/bindings/mfd/syscon.yaml
+++ b/Documentation/devicetree/bindings/mfd/syscon.yaml
@@ -126,6 +126,7 @@ select:
           - ti,am62-opp-efuse-table
           - ti,am62-usb-phy-ctrl
           - ti,am625-dss-oldi-io-ctrl
+          - ti,am625-dss-dpi0-clk-ctrl
           - ti,am62p-cpsw-mac-efuse
           - ti,am654-dss-oldi-io-ctrl
           - ti,j784s4-acspcie-proxy-ctrl
@@ -239,6 +240,7 @@ properties:
               - ti,am62-opp-efuse-table
               - ti,am62-usb-phy-ctrl
               - ti,am625-dss-oldi-io-ctrl
+              - ti,am625-dss-dpi0-clk-ctrl
               - ti,am62p-cpsw-mac-efuse
               - ti,am654-dss-oldi-io-ctrl
               - ti,j784s4-acspcie-proxy-ctrl

-- 
2.43.0