x86_64/debug: implement debug port detection

2 files changed, 10 insertions, 7 deletions

diff --git a/arch/x86_64/include/x86_64/debug/qemu_output.hpp b/arch/x86_64/include/x86_64/debug/qemu_output.hpp
index 6ff4da6..61b47d6 100644
--- a/arch/x86_64/include/x86_64/debug/qemu_output.hpp
+++ b/arch/x86_64/include/x86_64/debug/qemu_output.hpp
@@ -15,13 +15,13 @@ namespace teachos::debug::x86_64
   //! This device implements output to the port 0xE9 debug console present in QEMU in Bochs. It is designed to wrap a
   //! different output device (e.g. a VGA text output device) and forwards the written data accordingly.
   //!
-  //! @note Support for the device has to be enabled when the emulator is started. Since there is no reliable way to
-  //! detect the presence of the port, writes will still happen even if the device is not enabled in the emulator. It is
-  //! thus possible that the output is simply lost.
+  //! @note Support for the device has to be enabled when the emulator is started. The device will try to detect if the
+  //! port is available. If the port is detected, any output to the device will be written to port before being
+  //! forwarded to the lower device.
   struct qemu_output : cio::output_device
   {
     //! The port to write to.
-    using dbg = io::x86_64::port<0xE9, char, io::x86_64::port_write>;
+    using port = io::x86_64::port<0xE9, unsigned char, io::x86_64::port_write, io::x86_64::port_read>;
 
     //! Construct a new debug device wrapper for the given output device.
     //!
@@ -34,6 +34,8 @@ namespace teachos::debug::x86_64
   private:
     //! The device to forward the output to.
     output_device & m_lower;
+    //! Whether the device has been detected.
+    bool m_present;
   };
 
 }  // namespace teachos::debug::x86_64
diff --git a/arch/x86_64/src/debug/qemu_output.cpp b/arch/x86_64/src/debug/qemu_output.cpp
index 773306f..0af5bdc 100644
--- a/arch/x86_64/src/debug/qemu_output.cpp
+++ b/arch/x86_64/src/debug/qemu_output.cpp
@@ -2,6 +2,7 @@
 
 #include "kapi/cio.hpp"
 
+#include <algorithm>
 #include <string_view>
 
 namespace teachos::debug::x86_64
@@ -9,15 +10,15 @@ namespace teachos::debug::x86_64
 
   qemu_output::qemu_output(output_device & lower)
       : m_lower{lower}
+      , m_present{port::read() == port::address}
   {}
 
   auto qemu_output::write(cio::output_stream stream, std::string_view text) -> void
   {
-    for (auto c : text)
+    if (m_present)
     {
-      dbg::write(c);
+      std::ranges::for_each(text, port::write<port::value_type>);
     }
-
     m_lower.write(stream, text);
   }