serial: implement receive FIFO flush via FCR

The FIFO Control Register (FCR) controls the behavior of the receive
and transmit FIFO buffers of the device. The current implementation
does not emulate this register, as FIFO buffers are always enabled.
However, there are two bits in this register that control flushing of
said FIFOS. The transmission FIFO is already flushed by the current
implementation on every write, but the receive FIFO is not. This is
problematic, as some driver implementations (e.g. FreeBSD's) rely on
being able to clear this buffer via the corresponding bit.

Implement the correct behavior when a driver sets this bit by clearing
`in_buffer`. Since there is no more data in the receive FIFO, the
data-ready bit in the Line Status Register (LSR) must be cleared as
well, in case it was set.

Fixes: rust-vmm#83
Signed-off-by: Carlos López <[email protected]>

vm-superio/CHANGELOG.md

@@ -1,5 +1,12 @@
 # Changelog
 
+# Upcoming version
+
+## Changed
+
+- Implemented receive FIFO flushing via the FCR register for the `Serial`
+  device ([#83](https://github.com/rust-vmm/vm-superio/issues/83)).
+
 # v0.8.0
 
 ## Changed

vm-superio/src/serial.rs

@@ -25,6 +25,7 @@ use crate::Trigger;
 const DATA_OFFSET: u8 = 0;
 const IER_OFFSET: u8 = 1;
 const IIR_OFFSET: u8 = 2;
+const FCR_OFFSET: u8 = IIR_OFFSET;
 const LCR_OFFSET: u8 = 3;
 const MCR_OFFSET: u8 = 4;
 const LSR_OFFSET: u8 = 5;
@@ -50,6 +51,8 @@ const IIR_NONE_BIT: u8 = 0b0000_0001;
 const IIR_THR_EMPTY_BIT: u8 = 0b0000_0010;
 const IIR_RDA_BIT: u8 = 0b0000_0100;
 
+const FCR_FLUSH_IN_BIT: u8 = 0b0000_0010;
+
 const LCR_DLAB_BIT: u8 = 0b1000_0000;
 
 const LSR_DATA_READY_BIT: u8 = 0b0000_0001;
@@ -625,7 +628,14 @@ impl<T: Trigger, EV: SerialEvents, W: Write> Serial<T, EV, W> {
             LCR_OFFSET => self.line_control = value,
             MCR_OFFSET => self.modem_control = value,
             SCR_OFFSET => self.scratch = value,
-            // We are not interested in writing to other offsets (such as FCR offset).
+            FCR_OFFSET => {
+                // Clear the receive FIFO
+                if value & FCR_FLUSH_IN_BIT != 0 {
+                    self.in_buffer.clear();
+                    self.clear_lsr_rda_bit();
+                    self.events.in_buffer_empty();
+                }
+            }
             _ => {}
         }
         Ok(())