arm64: dts: broadcom: bcmbca: bcm4908: Reserve CFE stub area

The CFE bootloader places a stub program in the first page of physical
memory to hold the secondary CPUs until the boot CPU writes the release
address, but does not splice a /reserved-memory node into the FDT to
protect it. If Linux overwrites this program before execution reaches
smp_prepare_cpus(), the secondary CPUs may become inaccessible.

This is only a problem with CFE, and then only until the secondary CPUs
are brought online. Ideally, there would be some hypothetical mechanism
we could use to indicate that this area of memory is sensitive only
during boot. But as there is none, and since it is such a small amount
of memory, it is easiest to reserve it unconditionally.

Therefore, add a /reserved-memory node to bcm4908.dtsi to protect the
first 4KiB of physical memory.

Signed-off-by: Sam Edwards <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Florian Fainelli <[email protected]>

Author: CFSworks

arch/arm64/boot/dts/broadcom/bcmbca/bcm4908.dtsi

@@ -68,6 +68,16 @@
 		};
 	};
 
+	reserved-memory {
+		#address-cells = <2>;
+		#size-cells = <2>;
+		ranges;
+
+		cfe-stub@0 {
+			reg = <0x0 0x0 0x0 0x1000>;
+		};
+	};
+
 	axi@81000000 {
 		compatible = "simple-bus";
 		#address-cells = <1>;