drm: Add error for multi-use of a single crtc

src/backend/drm/backend.rs

@@ -196,6 +196,10 @@ impl DrmBackendInternal {
             })
         });
     }
+
+    pub(crate) fn is_crtc(&self, crtc: crtc::Handle) -> bool {
+        crtc == self.crtc
+    }
 }
 
 impl DrmBackend {

src/backend/drm/error.rs

@@ -23,6 +23,8 @@ pub enum Error {
     Io(IoError),
     /// Selected an invalid Mode
     Mode(ModeError),
+    /// Error related to the selected crtc
+    Crtc(CrtcError),
 }
 
 impl fmt::Display for Error {
@@ -37,6 +39,7 @@ impl fmt::Display for Error {
                 &Error::Gbm(ref x) => x as &error::Error,
                 &Error::Io(ref x) => x as &error::Error,
                 &Error::Mode(ref x) => x as &error::Error,
+                &Error::Crtc(ref x) => x as &error::Error,
             }
         )
     }
@@ -55,6 +58,7 @@ impl error::Error for Error {
             &Error::Gbm(ref x) => Some(x as &error::Error),
             &Error::Io(ref x) => Some(x as &error::Error),
             &Error::Mode(ref x) => Some(x as &error::Error),
+            &Error::Crtc(ref x) => Some(x as &error::Error),
         }
     }
 }
@@ -98,6 +102,12 @@ impl From<ModeError> for Error {
     }
 }
 
+impl From<CrtcError> for Error {
+    fn from(err: CrtcError) -> Error {
+        Error::Crtc(err)
+    }
+}
+
 impl<H> From<TryNewError<Error, H>> for Error {
     fn from(err: TryNewError<Error, H>) -> Error {
         err.0
@@ -138,3 +148,34 @@ impl error::Error for ModeError {
         }
     }
 }
+
+/// Errors related to the selected crtc
+#[derive(Debug)]
+pub enum CrtcError {
+    /// Selected crtc is already in use by another `DrmBackend`
+    AlreadyInUse
+}
+
+impl fmt::Display for CrtcError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "{}", self.description())?;
+        if let Some(cause) = self.cause() {
+            write!(f, "\tCause: {}", cause)?;
+        }
+        Ok(())
+    }
+}
+
+impl error::Error for CrtcError {
+    fn description(&self) -> &str {
+        match self {
+            &CrtcError::AlreadyInUse => "Crtc is already in use by another DrmBackend",
+        }
+    }
+
+    fn cause(&self) -> Option<&error::Error> {
+        match self {
+            _ => None,
+        }
+    }
+}

src/backend/drm/mod.rs

@@ -164,7 +164,7 @@ mod error;
 
 pub use self::backend::{DrmBackend, Id};
 use self::backend::DrmBackendInternal;
-pub use self::error::{Error as DrmError, ModeError};
+pub use self::error::{Error as DrmError, ModeError, CrtcError};
 
 /// Internal struct as required by the drm crate
 #[derive(Debug)]
@@ -349,12 +349,18 @@ impl<H: DrmHandler + 'static> DrmDevice<H> {
     where
         I: Into<Vec<connector::Handle>>,
     {
+        for backend in self.backends.iter() {
+            if let Some(backend) = backend.upgrade() {
+                if backend.borrow().is_crtc(crtc) {
+                    return Err(DrmError::Crtc(CrtcError::AlreadyInUse));
+                }
+            }
+        }
+
         let logger = self.logger
             .new(o!("drm" => "backend", "crtc" => format!("{:?}", crtc)));
         let own_id = self.backends.len();
 
-        // TODO: Make sure we do not initialize the same crtc multiple times
-        // (check weak pointers and return an error otherwise)
         let backend = Rc::new(RefCell::new(DrmBackendInternal::new(
             self.context.clone(),
             crtc,