ARM: dts: bcm2711: Move emmc2 into its own bus

Depending on bcm2711's revision its emmc2 controller might have
different DMA constraints. Raspberry Pi 4's firmware will take care of
updating those, but only if a certain alias is found in the device tree.
So, move emmc2 into its own bus, so as not to pollute other devices with
dma-ranges changes and create the emmc2bus alias.

Based in Phil ELwell's downstream implementation.
Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Link: https://lore.kernel.org/r/20200304132437.20164-1-nsaenzjulienne@suse.de

arch/arm/boot/dts/bcm2711-rpi-4-b.dts

@@ -20,6 +20,7 @@ memory@0 {
 	};
 
 	aliases {
+		emmc2bus = &emmc2bus;
 		ethernet0 = &genet;
 	};
 

arch/arm/boot/dts/bcm2711.dtsi

@@ -241,17 +241,32 @@ pwm1: pwm@7e20c800 {
 			status = "disabled";
 		};
 
+		hvs@7e400000 {
+			interrupts = <GIC_SPI 97 IRQ_TYPE_LEVEL_HIGH>;
+		};
+	};
+
+	/*
+	 * emmc2 has different DMA constraints based on SoC revisions. It was
+	 * moved into its own bus, so as for RPi4's firmware to update them.
+	 * The firmware will find whether the emmc2bus alias is defined, and if
+	 * so, it'll edit the dma-ranges property below accordingly.
+	 */
+	emmc2bus: emmc2bus {
+		compatible = "simple-bus";
+		#address-cells = <2>;
+		#size-cells = <1>;
+
+		ranges = <0x0 0x7e000000  0x0 0xfe000000  0x01800000>;
+		dma-ranges = <0x0 0xc0000000  0x0 0x00000000  0x40000000>;
+
 		emmc2: emmc2@7e340000 {
 			compatible = "brcm,bcm2711-emmc2";
-			reg = <0x7e340000 0x100>;
+			reg = <0x0 0x7e340000 0x100>;
 			interrupts = <GIC_SPI 126 IRQ_TYPE_LEVEL_HIGH>;
 			clocks = <&clocks BCM2711_CLOCK_EMMC2>;
 			status = "disabled";
 		};
-
-		hvs@7e400000 {
-			interrupts = <GIC_SPI 97 IRQ_TYPE_LEVEL_HIGH>;
-		};
 	};
 
 	arm-pmu {