uefi: serial: fix core::fmt::Write impl

This implementation is more correct and closer to what users
typcially correct. Timeout errors can occur quite often on real
hardware, for example when sending data over a UART 16550. However,
in this case, it is more natural to retry the transmission as it
will succeed eventually.

For errors that are not Status::TIMEOUT errors, we will nevertheless
propagate an error.

This is the more expected and natural behavior.

Author: phip1611

uefi-test-runner/src/main.rs

@@ -10,6 +10,7 @@ extern crate alloc;
 
 use alloc::string::ToString;
 use alloc::vec::Vec;
+use core::fmt::Write;
 use uefi::mem::memory_map::MemoryMap;
 use uefi::prelude::*;
 use uefi::proto::console::serial::Serial;
@@ -115,7 +116,10 @@ fn send_request_helper(serial: &mut Serial, request: HostRequest) -> Result {
     serial.set_attributes(&io_mode)?;
 
     // Send a screenshot request to the host.
-    serial.write(request.as_bytes()).discard_errdata()?;
+    //serial.write(request.as_bytes()).discard_errdata()?;
+    serial
+        .write_str(request.as_str())
+        .expect("should write all bytes");
 
     // Wait for the host's acknowledgement before moving forward.
     let mut reply = [0; 3];

uefi/CHANGELOG.md

@@ -28,6 +28,7 @@
 - MSRV increased to 1.88.
 - `Serial::read()` and `Serial::write()` got small improvements to better
   match the spec. Read the documentation of these methods for further insights.
+- Fixed potential partial writes in `core::fmt::Write` impl of `Serial` protocol
 
 # uefi - v0.36.1 (2025-11-05)
 

uefi/src/proto/console/serial.rs

@@ -7,6 +7,7 @@ use crate::{Error, Result, ResultExt, Status, StatusExt};
 use core::fmt;
 use core::fmt::Write;
 use core::time::Duration;
+use log::error;
 use uefi::boot;
 use uefi_raw::protocol::console::serial::SerialIoProtocol;
 pub use uefi_raw::protocol::console::serial::{
@@ -316,6 +317,13 @@ impl Serial {
 
 impl Write for Serial {
     fn write_str(&mut self, s: &str) -> fmt::Result {
-        self.write(s.as_bytes()).map(|_| ()).map_err(|_| fmt::Error)
+        // We retry on Status::TIMEOUT but propagate other errors
+        self.write_exact(s.as_bytes()).map_err(|e| {
+            // Note: this may cause endless recursion in case the logger
+            // implementation writes to the serial protocol, and we end up in
+            // another error, and again, and again.
+            error!("failed to write to device: {e}");
+            fmt::Error
+        })
     }
 }