OpenTitanUARTDriver: fix missing unlock

Bug: 233102476

Writing >CIRCULAR_BUFFER_CAPACITY bytes to the uart will hang because
of a missing unlock of the tx_mutex.

While here add a shell test command that was used to debug the issue
(but don't enable it by default).

Change-Id: I3e35001a653fe2659fb1534e4100727b69f662a1
GitOrigin-RevId: 550b35c88328dcbd1f87dd8e0209b1faabf7f931

apps/system/components/DebugConsole/kata-shell/Cargo.toml

@@ -30,6 +30,7 @@ TEST_ML_COORDINATOR = []
 TEST_PANIC = []
 TEST_SECURITY_COORDINATOR = []
 TEST_TIMER_SERVICE = []
+TEST_UART = []
 
 [dependencies]
 crc = { version = "1.4.0", default_features = false }

apps/system/components/DebugConsole/kata-shell/src/lib.rs

@@ -44,6 +44,8 @@ mod test_panic;
 mod test_security_coordinator;
 #[cfg(feature = "TEST_TIMER_SERVICE")]
 mod test_timer_service;
+#[cfg(feature = "TEST_UART")]
+mod test_uart;
 
 extern "C" {
     static SELF_CNODE: seL4_CPtr;
@@ -144,6 +146,8 @@ pub fn repl<T: io::BufRead>(
     test_security_coordinator::add_cmds(&mut cmds);
     #[cfg(feature = "TEST_TIMER_SERVICE")]
     test_timer_service::add_cmds(&mut cmds);
+    #[cfg(feature = "TEST_UART")]
+    test_uart::add_cmds(&mut cmds);
 
     let mut line_reader = LineReader::new();
     loop {

apps/system/components/DebugConsole/kata-shell/src/test_uart.rs

@@ -0,0 +1,43 @@
+// UART driver shell test commands
+
+use core::fmt::Write;
+use crate::CmdFn;
+use crate::CommandError;
+use crate::HashMap;
+
+use kata_io as io;
+
+// NB: not exported by driver so may diverge
+const CIRCULAR_BUFFER_CAPACITY: usize = 512;
+
+pub fn add_cmds(cmds: &mut HashMap::<&str, CmdFn>) {
+    cmds.extend([
+        ("test_uart",    uart_command as CmdFn),
+    ]);
+}
+
+/// Exercise the UART driver.
+fn uart_command(
+    _args: &mut dyn Iterator<Item = &str>,
+    _input: &mut dyn io::BufRead,
+    output: &mut dyn io::Write,
+    _builtin_cpio: &[u8],
+) -> Result<(), CommandError> {
+    // Just like the kata shell prompt.
+    const TESTING: &str = "testing...";
+    output.write_str(TESTING)?;
+    output.write_str(&"\n")?;
+
+    // Fill the UART driver circular buffer.
+    let not_too_long = "ok".repeat(CIRCULAR_BUFFER_CAPACITY / 2);
+    output.write_str(&not_too_long)?;
+    output.write_str(&"\n")?;
+
+    // Overflow the UART driver circular buffer.
+    let too_long = "no".repeat((CIRCULAR_BUFFER_CAPACITY / 2) + 1);
+    output.write_str(&too_long)?;
+
+    writeln!(output, "Success!")?;
+
+    Ok(())
+}

apps/system/components/OpenTitanUARTDriver/src/driver.c

@@ -226,6 +226,7 @@ int write_write(size_t available) {
   while (cursor < cursor_limit) {
     LOCK(tx_mutex);
     if (circular_buffer_remaining(&tx_buf) == 0) {
+      UNLOCK(tx_mutex);
       break;
     }
     for (; cursor < cursor_limit; ++cursor) {