Implement pass through device

The number of PMP (Physical Memory Protection) registers is limited on any RISC-V platform. Currently, each memory-mapped virtual device requires a separate PMP entry. This commit introduces a solution that allows grouping all virtual devices under a single PMP entry by implementing a pass-through module in front of them.

src/device/empty.rs

@@ -0,0 +1,26 @@
+use crate::device::{DeviceAccess, VirtDevice, Width};
+
+// ———————————————————————————— Empty device ————————————————————————————— //
+
+pub struct EmptyDevice {}
+
+impl DeviceAccess for EmptyDevice {
+    fn read_device(&self, _: usize, _: Width) -> Result<usize, &'static str> {
+        panic!("Trying to read the empty device!");
+    }
+
+    fn write_device(&self, _: usize, _: Width, _: usize) -> Result<(), &'static str> {
+        panic!("Trying to write the empty device!");
+    }
+}
+
+impl EmptyDevice {
+    pub const fn new() -> VirtDevice {
+        VirtDevice {
+            start_addr: 0,
+            size: 0,
+            name: "",
+            device_interface: &EmptyDevice {},
+        }
+    }
+}

src/device/mod.rs

@@ -1,6 +1,10 @@
 //! Base device classes
 
+use crate::device::empty::EmptyDevice;
+use crate::platform::{Plat, Platform};
+
 pub mod clint;
+pub mod empty;
 pub mod tester;
 
 // ———————————————————————————— Virtual Devices ————————————————————————————— //
@@ -63,3 +67,47 @@ pub trait DeviceAccess: Sync + Send {
     fn write_device(&self, offset: usize, w_width: Width, value: usize)
         -> Result<(), &'static str>;
 }
+
+// ———————————————————————————— Pass Through module ————————————————————————————— //
+
+pub struct PassThroughModule {
+    devices: [VirtDevice; 2],
+    base_address: usize,
+}
+
+impl DeviceAccess for PassThroughModule {
+    fn read_device(&self, offset: usize, r_width: Width) -> Result<usize, &'static str> {
+        match find_matching_device(self.base_address + offset, &self.devices) {
+            Some(device) => device.device_interface.read_device(offset, r_width),
+            _ => Err("Mapping to non existent device"),
+        }
+    }
+
+    fn write_device(
+        &self,
+        offset: usize,
+        w_width: Width,
+        value: usize,
+    ) -> Result<(), &'static str> {
+        match find_matching_device(self.base_address + offset, &self.devices) {
+            Some(device) => device.device_interface.write_device(offset, w_width, value),
+            _ => Err("Mapping to non existent device"),
+        }
+    }
+}
+
+impl PassThroughModule {
+    pub const fn new(base_address: usize) -> PassThroughModule {
+        let empty_clint_device = EmptyDevice::new();
+        let empty_test_device = EmptyDevice::new();
+
+        PassThroughModule {
+            devices: [empty_clint_device, empty_test_device],
+            base_address: base_address,
+        }
+    }
+
+    pub fn attach_devices(&mut self) {
+        self.devices = Plat::create_virtual_devices()
+    }
+}