diff --git a/content/posts/expose-serial-port.md b/content/posts/expose-serial-port.md
new file mode 100644
index 0000000..a8c06b7
--- /dev/null
+++ b/content/posts/expose-serial-port.md
@@ -0,0 +1,197 @@
+---
+title: "How to expose a serial port to a custom AOSP running in Trout / Cuttlefish"
+date: 2024-09-20T16:34:36+02:00
+draft: false
+author: "Danil Garmanov"
+---
+
+Android running in a virtualized environment normally has no access to the hardware of the host machine, most of the hardware it "works" with is instead emulated.
+
+But sometimes one wants a way to attach external hardware to their custom Android without working with an actual physical Android device with a USB port.
+In our case we needed to attach a serial device.
+
+In this article we will demonstrate how one can expose a *serial port* to Trout.
+
+# crosvm
+Trout is based on [crosvm](https://github.com/google/crosvm), and crosvm already supports serial devices exposure using the `--serial` parameter of the `crosvm run` command.
+
+But we use crosvm indirectly, through Trout, when we start it using the `launch_cvd` command.
+And `launch_cvd` does not provide a way to pass values to the `--serial` param of `crosvm run`.
+
+So we will add it.
+
+# Modifying the source code of your AOSP fork
+*Disclaimer: the diffs are shown for Android 14, diffs for other versions of Android would be slightly different.*
+
+First, `cd device/google/cuttlefish`.
+
+Now, let's add new Cuttlefish config parameter, in the header file:
+```
+diff --git a/host/libs/config/cuttlefish_config.h b/host/libs/config/cuttlefish_config.h
+index 9a2af86cd..7c9bb5725 100644
+--- a/host/libs/config/cuttlefish_config.h
++++ b/host/libs/config/cuttlefish_config.h
+@@ -125,6 +125,9 @@ class CuttlefishConfig {
+   void set_gem5_debug_flags(const std::string& gem5_debug_flags);
+   std::string gem5_debug_flags() const;
+ 
++  void set_attach_serial_device(const std::string& attach_serial_device);
++  std::string attach_serial_device() const;
++
+   void set_enable_host_uwb(bool enable_host_uwb);
+   bool enable_host_uwb() const;
+ 
+```
+...and in the implementation file:
+```
+diff --git a/host/libs/config/cuttlefish_config.cpp b/host/libs/config/cuttlefish_config.cpp
+index 5c9e8dc36..2bdf2d6e4 100644
+--- a/host/libs/config/cuttlefish_config.cpp
++++ b/host/libs/config/cuttlefish_config.cpp
+@@ -188,6 +188,15 @@ void CuttlefishConfig::set_gem5_debug_flags(const std::string& gem5_debug_flags)
+   (*dictionary_)[kGem5DebugFlags] = gem5_debug_flags;
+ }
+ 
++static constexpr char kAttachSerialDevice[] = "attach_serial_device";
++void CuttlefishConfig::set_attach_serial_device(
++    const std::string& attach_serial_device) {
++  (*dictionary_)[kAttachSerialDevice] = attach_serial_device;
++}
++std::string CuttlefishConfig::attach_serial_device() const {
++  return (*dictionary_)[kAttachSerialDevice].asString();
++}
++
+ static constexpr char kWebRTCCertsDir[] = "webrtc_certs_dir";
+ void CuttlefishConfig::set_webrtc_certs_dir(const std::string& certs_dir) {
+   (*dictionary_)[kWebRTCCertsDir] = certs_dir;
+```
+
+Now let's add a new parameter to the `launch_cvd` command that would use this config change:
+```
+diff --git a/host/commands/assemble_cvd/flags.cc b/host/commands/assemble_cvd/flags.cc
+index c5ef0344a..e10615d72 100644
+--- a/host/commands/assemble_cvd/flags.cc
++++ b/host/commands/assemble_cvd/flags.cc
+@@ -202,6 +202,16 @@ DEFINE_string(
+     seccomp_policy_dir, CF_DEFAULTS_SECCOMP_POLICY_DIR,
+     "With sandbox'ed crosvm, overrieds the security comp policy directory");
+ 
++DEFINE_string(
++    attach_serial_device, "",
++    "Path to a serial device that should be attached to crosvm "
++    "(/dev/<something>). "
++    "QEMU is not supported.\n"
++    "To see the attached device name inside the guest machine, look for a "
++    "corresponding log "
++    "message with the 'SERIAL_PORT' prefix during the VM start.\n"
++    "The device name is stable, but it may change on AOSP updates.");
++
+ DEFINE_vec(start_webrtc, cuttlefish::BoolToString(CF_DEFAULTS_START_WEBRTC),
+             "Whether to start the webrtc process.");
+ 
+@@ -818,6 +828,8 @@ Result<CuttlefishConfig> InitializeCuttlefishConfiguration(
+ 
+   tmp_config_obj.set_gem5_debug_flags(FLAGS_gem5_debug_flags);
+ 
++  tmp_config_obj.set_attach_serial_device(FLAGS_attach_serial_device);
++
+   // streaming, webrtc setup
+   tmp_config_obj.set_webrtc_certs_dir(FLAGS_webrtc_certs_dir);
+   tmp_config_obj.set_sig_server_secure(FLAGS_webrtc_sig_server_secure);
+```
+
+Cuttlefish has already quite a few defined serial ports, let's increase their number by one:
+```
+diff --git a/host/libs/vm_manager/vm_manager.h b/host/libs/vm_manager/vm_manager.h
+index 4b116bfd9..036a2d3eb 100644
+--- a/host/libs/vm_manager/vm_manager.h
++++ b/host/libs/vm_manager/vm_manager.h
+@@ -54,7 +54,8 @@ class VmManager {
+   // - /dev/hvc9 = uwb
+   // - /dev/hvc10 = oemlock
+   // - /dev/hvc11 = keymint
+-  static const int kDefaultNumHvcs = 12;
++  static const int kDefaultNumHvcs =
++    13; // NOTE: on merge conflicts do +1 to the upstream's value
+ 
+   // This is the number of virtual disks (block devices) that should be
+   // configured by the VmManager. Related to the description above regarding
+```
+
+And finally let's actually forward the new parameter to `crosvm`:
+```
+diff --git a/host/libs/vm_manager/crosvm_manager.cpp b/host/libs/vm_manager/crosvm_manager.cpp
+index 100b71e56..d81b5eb08 100644
+--- a/host/libs/vm_manager/crosvm_manager.cpp
++++ b/host/libs/vm_manager/crosvm_manager.cpp
+@@ -449,6 +449,18 @@ Result<std::vector<MonitorCommand>> CrosvmManager::StartCommands(
+   for (auto i = 0; i < VmManager::kMaxDisks - disk_num; i++) {
+     crosvm_cmd.AddHvcSink();
+   }
++
++  if (!config.attach_serial_device().empty()) {
++    crosvm_cmd.AddHvcReadWrite(config.attach_serial_device(),
++                               config.attach_serial_device());
++    LOG(INFO) << "SERIAL_PORT: attaching serial port to crosvm: "
++              << config.attach_serial_device()
++              << ", most likely device name inside the guest machine: /dev/hvc"
++              << crosvm_cmd.HvcNum() - 1;
++  } else {
++    crosvm_cmd.AddHvcSink();
++  }
++
+   CF_EXPECT(crosvm_cmd.HvcNum() + disk_num ==
+                 VmManager::kMaxDisks + VmManager::kDefaultNumHvcs,
+             "HVC count (" << crosvm_cmd.HvcNum() << ") + disk count ("
+```
+
+... but not to QEMU, which is mostly used as AVD for normal (phone) Android apps (it might actually work, but we haven't test it since our focus is Trout / crossvm):
+```
+diff --git a/host/libs/vm_manager/qemu_manager.cpp b/host/libs/vm_manager/qemu_manager.cpp
+index a2744a35d..a0ae1286b 100644
+--- a/host/libs/vm_manager/qemu_manager.cpp
++++ b/host/libs/vm_manager/qemu_manager.cpp
+@@ -556,6 +556,8 @@ Result<std::vector<MonitorCommand>> QemuManager::StartCommands(
+     add_hvc_sink();
+   }
+ 
++  add_hvc_sink();  // attach_serial_device, not supported on QEMU
++
+   CF_EXPECT(
+       hvc_num + disk_num == VmManager::kMaxDisks + VmManager::kDefaultNumHvcs,
+       "HVC count (" << hvc_num << ") + disk count (" << disk_num << ") "
+```
+
+And the last thing we need to do is setting up proper access rights:
+```
+diff --git a/shared/config/ueventd.rc b/shared/config/ueventd.rc
+index c604e3d23..dd7da4de3 100644
+--- a/shared/config/ueventd.rc
++++ b/shared/config/ueventd.rc
+@@ -44,5 +44,13 @@
+ # keymint / Rust
+ /dev/hvc11 0666 system system
+ 
++# Serial Port integration for the -attach_serial_device
++# param of the launch_cvd command.
++# Please update this name "/dev/hvcN" to "/dev/hvc{N+M}"
++# when kDefaultNumHvcs in 'host/libs/vm_manager/vm_manager.h' is updated.
++# The 'M' value here would be the delta between the new and the old values
++# of kDefaultNumHvcs.
++/dev/hvc12 0666 system system
++
+ # Factory Reset Protection
+ /dev/block/by-name/frp 0660 system system
+```
+
+You can find the full diff as one file [here](./full.diff).
+
+# Conclusion
+And that was basically it - now you can use the exposed port (`/dev/hvc12` in the case of the diff above) from inside of Android.
+Don't forget to use the new parameter when running `launch_cvd`, e.g.:
+```
+launch_cvd ... -attach_serial_device /dev/ttyACM0
+```
+Replace `/dev/ttyACM0` with the actual path of the serial device on your host machine.
+
+You can do a quick test by cross-compiling and adb-pushing a small binary to Android, which would attach to the serial port, and then executing the binary under `su`. Or just connect to the port from any system service.
diff --git a/content/posts/expose-serial-port/full.diff b/content/posts/expose-serial-port/full.diff
new file mode 100644
index 0000000..253f402
--- /dev/null
+++ b/content/posts/expose-serial-port/full.diff
@@ -0,0 +1,132 @@
+diff --git a/host/commands/assemble_cvd/flags.cc b/host/commands/assemble_cvd/flags.cc
+index c5ef0344a..e10615d72 100644
+--- a/host/commands/assemble_cvd/flags.cc
++++ b/host/commands/assemble_cvd/flags.cc
+@@ -202,6 +202,16 @@ DEFINE_string(
+     seccomp_policy_dir, CF_DEFAULTS_SECCOMP_POLICY_DIR,
+     "With sandbox'ed crosvm, overrieds the security comp policy directory");
+ 
++DEFINE_string(
++    attach_serial_device, "",
++    "Path to a serial device that should be attached to crosvm "
++    "(/dev/<something>). "
++    "QEMU is not supported.\n"
++    "To see the attached device name inside the guest machine, look for a "
++    "corresponding log "
++    "message with the 'SERIAL_PORT' prefix during the VM start.\n"
++    "The device name is stable, but it may change on AOSP updates.");
++
+ DEFINE_vec(start_webrtc, cuttlefish::BoolToString(CF_DEFAULTS_START_WEBRTC),
+             "Whether to start the webrtc process.");
+ 
+@@ -818,6 +828,8 @@ Result<CuttlefishConfig> InitializeCuttlefishConfiguration(
+ 
+   tmp_config_obj.set_gem5_debug_flags(FLAGS_gem5_debug_flags);
+ 
++  tmp_config_obj.set_attach_serial_device(FLAGS_attach_serial_device);
++
+   // streaming, webrtc setup
+   tmp_config_obj.set_webrtc_certs_dir(FLAGS_webrtc_certs_dir);
+   tmp_config_obj.set_sig_server_secure(FLAGS_webrtc_sig_server_secure);
+diff --git a/host/libs/config/cuttlefish_config.cpp b/host/libs/config/cuttlefish_config.cpp
+index 5c9e8dc36..2bdf2d6e4 100644
+--- a/host/libs/config/cuttlefish_config.cpp
++++ b/host/libs/config/cuttlefish_config.cpp
+@@ -188,6 +188,15 @@ void CuttlefishConfig::set_gem5_debug_flags(const std::string& gem5_debug_flags)
+   (*dictionary_)[kGem5DebugFlags] = gem5_debug_flags;
+ }
+ 
++static constexpr char kAttachSerialDevice[] = "attach_serial_device";
++void CuttlefishConfig::set_attach_serial_device(
++    const std::string& attach_serial_device) {
++  (*dictionary_)[kAttachSerialDevice] = attach_serial_device;
++}
++std::string CuttlefishConfig::attach_serial_device() const {
++  return (*dictionary_)[kAttachSerialDevice].asString();
++}
++
+ static constexpr char kWebRTCCertsDir[] = "webrtc_certs_dir";
+ void CuttlefishConfig::set_webrtc_certs_dir(const std::string& certs_dir) {
+   (*dictionary_)[kWebRTCCertsDir] = certs_dir;
+diff --git a/host/libs/config/cuttlefish_config.h b/host/libs/config/cuttlefish_config.h
+index 9a2af86cd..7c9bb5725 100644
+--- a/host/libs/config/cuttlefish_config.h
++++ b/host/libs/config/cuttlefish_config.h
+@@ -125,6 +125,9 @@ class CuttlefishConfig {
+   void set_gem5_debug_flags(const std::string& gem5_debug_flags);
+   std::string gem5_debug_flags() const;
+ 
++  void set_attach_serial_device(const std::string& attach_serial_device);
++  std::string attach_serial_device() const;
++
+   void set_enable_host_uwb(bool enable_host_uwb);
+   bool enable_host_uwb() const;
+ 
+diff --git a/host/libs/vm_manager/crosvm_manager.cpp b/host/libs/vm_manager/crosvm_manager.cpp
+index 100b71e56..d81b5eb08 100644
+--- a/host/libs/vm_manager/crosvm_manager.cpp
++++ b/host/libs/vm_manager/crosvm_manager.cpp
+@@ -449,6 +449,18 @@ Result<std::vector<MonitorCommand>> CrosvmManager::StartCommands(
+   for (auto i = 0; i < VmManager::kMaxDisks - disk_num; i++) {
+     crosvm_cmd.AddHvcSink();
+   }
++
++  if (!config.attach_serial_device().empty()) {
++    crosvm_cmd.AddHvcReadWrite(config.attach_serial_device(),
++                               config.attach_serial_device());
++    LOG(INFO) << "SERIAL_PORT: attaching serial port to crosvm: "
++              << config.attach_serial_device()
++              << ", most likely device name inside the guest machine: /dev/hvc"
++              << crosvm_cmd.HvcNum() - 1;
++  } else {
++    crosvm_cmd.AddHvcSink();
++  }
++
+   CF_EXPECT(crosvm_cmd.HvcNum() + disk_num ==
+                 VmManager::kMaxDisks + VmManager::kDefaultNumHvcs,
+             "HVC count (" << crosvm_cmd.HvcNum() << ") + disk count ("
+diff --git a/host/libs/vm_manager/qemu_manager.cpp b/host/libs/vm_manager/qemu_manager.cpp
+index a2744a35d..a0ae1286b 100644
+--- a/host/libs/vm_manager/qemu_manager.cpp
++++ b/host/libs/vm_manager/qemu_manager.cpp
+@@ -556,6 +556,8 @@ Result<std::vector<MonitorCommand>> QemuManager::StartCommands(
+     add_hvc_sink();
+   }
+ 
++  add_hvc_sink();  // attach_serial_device, not supported on QEMU
++
+   CF_EXPECT(
+       hvc_num + disk_num == VmManager::kMaxDisks + VmManager::kDefaultNumHvcs,
+       "HVC count (" << hvc_num << ") + disk count (" << disk_num << ") "
+diff --git a/host/libs/vm_manager/vm_manager.h b/host/libs/vm_manager/vm_manager.h
+index 4b116bfd9..036a2d3eb 100644
+--- a/host/libs/vm_manager/vm_manager.h
++++ b/host/libs/vm_manager/vm_manager.h
+@@ -54,7 +54,8 @@ class VmManager {
+   // - /dev/hvc9 = uwb
+   // - /dev/hvc10 = oemlock
+   // - /dev/hvc11 = keymint
+-  static const int kDefaultNumHvcs = 12;
++  static const int kDefaultNumHvcs =
++    13; // NOTE: on merge conflicts do +1 to the upstream's value
+ 
+   // This is the number of virtual disks (block devices) that should be
+   // configured by the VmManager. Related to the description above regarding
+diff --git a/shared/config/ueventd.rc b/shared/config/ueventd.rc
+index c604e3d23..dd7da4de3 100644
+--- a/shared/config/ueventd.rc
++++ b/shared/config/ueventd.rc
+@@ -44,5 +44,13 @@
+ # keymint / Rust
+ /dev/hvc11 0666 system system
+ 
++# Serial Port integration for the -attach_serial_device
++# param of the launch_cvd command.
++# Please update this name "/dev/hvcN" to "/dev/hvc{N+M}"
++# when kDefaultNumHvcs in 'host/libs/vm_manager/vm_manager.h' is updated.
++# The 'M' value here would be the delta between the new and the old values
++# of kDefaultNumHvcs.
++/dev/hvc12 0666 system system
++
+ # Factory Reset Protection
+ /dev/block/by-name/frp 0660 system system