1 files changed, 24 insertions, 0 deletions
diff --git a/tools/serial.zig b/tools/serial.zig
index c13942c..da225fd 100644
--- a/tools/serial.zig
+++ b/tools/serial.zig
@@ -43,6 +43,10 @@ const Termios2 = extern struct {
     /// _IOR('T', 0x2A, struct termios2) and _IOW('T', 0x2B, struct termios2) for a 44-byte struct.
     const TCGETS2: u32 = 0x802C542A;
     const TCSETS2: u32 = 0x402C542B;
+    /// _IOW('T', 0x2C, ...): TCSETS2's draining sibling. Changing the divisor while bytes are
+    /// still in the kernel's output queue sends the tail of the old line at the new rate, which on
+    /// a mid-session baud switch corrupts exactly the handshake line the switch was announced by.
+    const TCSETSW2: u32 = 0x402C542C;
 
     /// CBAUD escape meaning "take the rate from ispeed/ospeed" (asm-generic/termbits.h).
     const BOTHER: u32 = 0o010000;
@@ -110,6 +114,26 @@ pub const Port = struct {
         return .{ .file = file, .io = io, .saved = saved };
     }
 
+    /// Re-rate an already-open port, leaving the raw-mode flags and the exclusive claim alone.
+    ///
+    /// This exists because the console and the flasher want different rates on the same wire. The
+    /// ROM loader auto-detects the host's rate from SYNC's 0x55 pattern, so `open` can simply pick
+    /// one; a running application cannot, because its UART divider was programmed by the
+    /// second-stage bootloader and the only way to change it is for the firmware to reprogram its
+    /// own divider and for the host to follow. That makes the switch a two-sided handshake, and
+    /// `TCSETSW2` is the half that has to be ordered: the firmware announces the change on the old
+    /// rate, and the announcement must have physically left the wire before either side moves.
+    pub fn setBaud(p: *Port, baud: Baud) !void {
+        var t: Termios2 = undefined;
+        if (@as(isize, @bitCast(linux.ioctl(p.file.handle, Termios2.TCGETS2, @intFromPtr(&t)))) < 0)
+            return error.NotATerminal;
+        t.cflag = (t.cflag & ~Termios2.CBAUD) | Termios2.BOTHER;
+        t.ispeed = baud.rate();
+        t.ospeed = baud.rate();
+        if (@as(isize, @bitCast(linux.ioctl(p.file.handle, Termios2.TCSETSW2, @intFromPtr(&t)))) < 0)
+            return error.SetAttrFailed;
+    }
+
     pub fn close(p: *Port) void {
         _ = linux.ioctl(p.file.handle, Termios2.TCSETS2, @intFromPtr(&p.saved));
         p.file.close(p.io);