virtio-queue: Add an is_read_only() method for DescriptorChain

Add an is_read_only method to the DescriptorChain struct.  This is
easier to read at a glance than `!is_write_only()`, which looks almost
the same as `is_write_only()`.

BUG=chromium:703939
TEST=none

Change-Id: I0cb3b17c30e38804a21d3e63e975157c96879cb4
Signed-off-by: Chirantan Ekbote <chirantan@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1067703
Reviewed-by: Zach Reizner <zachr@chromium.org>
Reviewed-by: Dylan Reid <dgreid@chromium.org>

1 files changed, 8 insertions, 0 deletions

diff --git a/devices/src/virtio/queue.rs b/devices/src/virtio/queue.rs
index 59b09b7..702c100 100644
--- a/devices/src/virtio/queue.rs
+++ b/devices/src/virtio/queue.rs
@@ -103,6 +103,14 @@ impl<'a> DescriptorChain<'a> {
         self.flags & VIRTQ_DESC_F_WRITE != 0
     }
 
+    /// If the driver designated this as a read only descriptor.
+    ///
+    /// If this is false, this descriptor is write only.
+    /// Read only means the emulated device can read and the driver can write.
+    pub fn is_read_only(&self) -> bool {
+        self.flags & VIRTQ_DESC_F_WRITE == 0
+    }
+
     /// Gets the next descriptor in this descriptor chain, if there is one.
     ///
     /// Note that this is distinct from the next descriptor chain returned by `AvailIter`, which is