tool: Move read_debug to trait so generic code can use it

Returns error for backends that don't support it.

tool/src/access/lpc/linux.rs

@@ -130,12 +130,6 @@ impl AccessLpcLinux {
         Ok(self.cmd.write(addr as u16, data)?)
     }
 
-    /// Read from the debug space
-    //TODO: better public interface
-    pub unsafe fn read_debug(&mut self, addr: u8) -> Result<u8, Error> {
-        Ok(self.dbg.read(addr as u16)?)
-    }
-
     /// Returns Ok if a command can be sent
     unsafe fn command_check(&mut self) -> Result<(), Error> {
         if self.read_cmd(SMFI_CMD_CMD)? == 0 {
@@ -180,4 +174,8 @@ impl Access for AccessLpcLinux {
     fn data_size(&self) -> usize {
         SMFI_CMD_SIZE - SMFI_CMD_DATA as usize
     }
+
+    unsafe fn read_debug(&mut self, addr: u8) -> Result<u8, Error> {
+        Ok(self.dbg.read(addr as u16)?)
+    }
 }

tool/src/access/lpc/sim.rs

@@ -71,12 +71,6 @@ impl AccessLpcSim {
         self.outb(SMFI_CMD_BASE + u16::from(addr), data)
     }
 
-    /// Read from the debug space
-    //TODO: better public interface
-    pub unsafe fn read_debug(&mut self, addr: u8) -> Result<u8, Error> {
-        self.inb(SMFI_DBG_BASE + u16::from(addr))
-    }
-
     /// Returns Ok if a command can be sent
     unsafe fn command_check(&mut self) -> Result<(), Error> {
         if self.read_cmd(SMFI_CMD_CMD)? == 0 {
@@ -121,4 +115,8 @@ impl Access for AccessLpcSim {
     fn data_size(&self) -> usize {
         SMFI_CMD_SIZE - SMFI_CMD_DATA as usize
     }
+
+    unsafe fn read_debug(&mut self, addr: u8) -> Result<u8, Error> {
+        self.inb(SMFI_DBG_BASE + u16::from(addr))
+    }
 }

tool/src/access/mod.rs

@@ -23,6 +23,12 @@ pub trait Access {
 
     /// The maximum size that can be provided for the data argument
     fn data_size(&self) -> usize;
+
+    /// Read from the debug space
+    //TODO: better public interface
+    unsafe fn read_debug(&mut self, _addr: u8) -> Result<u8, Error> {
+        Err(Error::NotSupported)
+    }
 }
 
 impl Access for &mut dyn Access {

tool/src/error.rs

@@ -3,6 +3,8 @@
 pub enum Error {
     /// Data length is too large
     DataLength(usize),
+    /// Operation not supported
+    NotSupported,
     /// A parameter was invalid
     Parameter,
     /// EC protocol returned an error result