Kernel/riscv64: Get the kernel command line from the device tree

This also hides the fdt dump by default now,
it can be activated by adding `dump_fdt` to the kernel command line

Kernel/Arch/init.cpp

@@ -218,8 +218,11 @@ extern "C" [[noreturn]] UNMAP_AFTER_INIT void init([[maybe_unused]] BootInfo con
     // FIXME: Read the /chosen/bootargs property.
     kernel_cmdline = RPi::Mailbox::the().query_kernel_command_line(s_command_line_buffer);
 #elif ARCH(RISCV64)
-    // FIXME: Take this from the flattened device tree (/chosen/bootargs)
-    kernel_cmdline = "serial_debug"sv;
+    auto maybe_command_line = get_command_line_from_fdt();
+    if (maybe_command_line.is_error())
+        kernel_cmdline = "serial_debug"sv;
+    else
+        kernel_cmdline = maybe_command_line.value();
 #endif
 
     setup_serial_debug();
@@ -290,7 +293,8 @@ extern "C" [[noreturn]] UNMAP_AFTER_INIT void init([[maybe_unused]] BootInfo con
 #if ARCH(RISCV64)
     MUST(unflatten_fdt());
 
-    dump_fdt();
+    if (kernel_command_line().contains("dump_fdt"sv))
+        dump_fdt();
 #endif
 
     // Initialize TimeManagement before using randomness!

Kernel/Arch/riscv64/CPU.cpp

@@ -32,6 +32,13 @@ void dump_fdt()
     MUST(DeviceTree::dump(header, fdt));
 }
 
+ErrorOr<StringView> get_command_line_from_fdt()
+{
+    auto& header = *bit_cast<DeviceTree::FlattenedDeviceTreeHeader*>(&s_fdt_storage[0]);
+    auto fdt = ReadonlyBytes(s_fdt_storage, header.totalsize);
+    return TRY(DeviceTree::slow_get_property("/chosen/bootargs"sv, header, fdt)).as_string();
+}
+
 }
 
 DeviceTree::DeviceTree const& DeviceTree::get()

Kernel/Arch/riscv64/CPU.h

@@ -24,6 +24,7 @@ extern BootInfo s_boot_info;
 
 ErrorOr<void> unflatten_fdt();
 void dump_fdt();
+ErrorOr<StringView> get_command_line_from_fdt();
 }
 
 namespace DeviceTree {