of: Document timings subnode of nvidia,tegra-mc

The MC driver needs some timing-specific information to program the EMEM
during a rate change of the EMC clock.
Signed-off-by: NTomeu Vizoso <tomeu.vizoso@collabora.com>
Signed-off-by: NThierry Reding <treding@nvidia.com>

Documentation/devicetree/bindings/memory-controllers/nvidia,tegra-mc.txt

 NVIDIA Tegra Memory Controller device tree bindings
 ===================================================
 
+memory-controller node
+----------------------
+
 Required properties:
 - compatible: Should be "nvidia,tegra<chip>-mc"
 - reg: Physical base address and length of the controller's registers.
@@ -15,9 +18,49 @@ Required properties:
 This device implements an IOMMU that complies with the generic IOMMU binding.
 See ../iommu/iommu.txt for details.
 
-Example:
---------
+emc-timings subnode
+-------------------
+
+The node should contain a "emc-timings" subnode for each supported RAM type (see field RAM_CODE in
+register PMC_STRAPPING_OPT_A).
+
+Required properties for "emc-timings" nodes :
+- nvidia,ram-code : Should contain the value of RAM_CODE this timing set is used for.
+
+timing subnode
+--------------
+
+Each "emc-timings" node should contain a subnode for every supported EMC clock rate.
+
+Required properties for timing nodes :
+- clock-frequency : Should contain the memory clock rate in Hz.
+- nvidia,emem-configuration : Values to be written to the EMEM register block. For the Tegra124 SoC
+(see section "15.6.1 MC Registers" in the TRM), these are the registers whose values need to be
+specified, according to the board documentation:
+
+	MC_EMEM_ARB_CFG
+	MC_EMEM_ARB_OUTSTANDING_REQ
+	MC_EMEM_ARB_TIMING_RCD
+	MC_EMEM_ARB_TIMING_RP
+	MC_EMEM_ARB_TIMING_RC
+	MC_EMEM_ARB_TIMING_RAS
+	MC_EMEM_ARB_TIMING_FAW
+	MC_EMEM_ARB_TIMING_RRD
+	MC_EMEM_ARB_TIMING_RAP2PRE
+	MC_EMEM_ARB_TIMING_WAP2PRE
+	MC_EMEM_ARB_TIMING_R2R
+	MC_EMEM_ARB_TIMING_W2W
+	MC_EMEM_ARB_TIMING_R2W
+	MC_EMEM_ARB_TIMING_W2R
+	MC_EMEM_ARB_DA_TURNS
+	MC_EMEM_ARB_DA_COVERS
+	MC_EMEM_ARB_MISC0
+	MC_EMEM_ARB_MISC1
+	MC_EMEM_ARB_RING1_THROTTLE
 
+Example SoC include file:
+
+/ {
 	mc: memory-controller@0,70019000 {
 		compatible = "nvidia,tegra124-mc";
 		reg = <0x0 0x70019000 0x0 0x1000>;
@@ -34,3 +77,40 @@ Example:
 		...
 		iommus = <&mc TEGRA_SWGROUP_SDMMC1A>;
 	};
+};
+
+Example board file:
+
+/ {
+	memory-controller@0,70019000 {
+		emc-timings-3 {
+			nvidia,ram-code = <3>;
+
+			timing-12750000 {
+				clock-frequency = <12750000>;
+
+				nvidia,emem-configuration = <
+					0x40040001 /* MC_EMEM_ARB_CFG */
+					0x8000000a /* MC_EMEM_ARB_OUTSTANDING_REQ */
+					0x00000001 /* MC_EMEM_ARB_TIMING_RCD */
+					0x00000001 /* MC_EMEM_ARB_TIMING_RP */
+					0x00000002 /* MC_EMEM_ARB_TIMING_RC */
+					0x00000000 /* MC_EMEM_ARB_TIMING_RAS */
+					0x00000002 /* MC_EMEM_ARB_TIMING_FAW */
+					0x00000001 /* MC_EMEM_ARB_TIMING_RRD */
+					0x00000002 /* MC_EMEM_ARB_TIMING_RAP2PRE */
+					0x00000008 /* MC_EMEM_ARB_TIMING_WAP2PRE */
+					0x00000003 /* MC_EMEM_ARB_TIMING_R2R */
+					0x00000002 /* MC_EMEM_ARB_TIMING_W2W */
+					0x00000003 /* MC_EMEM_ARB_TIMING_R2W */
+					0x00000006 /* MC_EMEM_ARB_TIMING_W2R */
+					0x06030203 /* MC_EMEM_ARB_DA_TURNS */
+					0x000a0402 /* MC_EMEM_ARB_DA_COVERS */
+					0x77e30303 /* MC_EMEM_ARB_MISC0 */
+					0x70000f03 /* MC_EMEM_ARB_MISC1 */
+					0x001f0000 /* MC_EMEM_ARB_RING1_THROTTLE */
+				>;
+			};
+		};
+	};
+};