diff --git a/typecd/cable.cc b/typecd/cable.cc
index 8b90617..c2760c1 100644
--- a/typecd/cable.cc
+++ b/typecd/cable.cc
@@ -138,4 +138,8 @@
   return usb_speed == kUSBSuperSpeed31Gen1 || usb_speed == kUSBSuperSpeed31Gen2;
 }
 
+bool Cable::DiscoveryComplete() {
+  return num_alt_modes_ == alt_modes_.size();
+}
+
 }  // namespace typecd
diff --git a/typecd/cable.h b/typecd/cable.h
index c623daa..b3a4e5f 100644
--- a/typecd/cable.h
+++ b/typecd/cable.h
@@ -66,6 +66,12 @@
   // Check whether the cable supports Thunderbolt3 speed requirements.
   bool TBT3PDIdentityCheck();
 
+  // Check whether SOP' PD discovery is complete (and we have all the PD
+  // information that the kernel can provide). To determine this, we check
+  // whether the number of registered altmodes equals the |num_alt_modes_| value
+  // which is read from sysfs.
+  bool DiscoveryComplete();
+
  private:
   // Map representing all SOP' alternate modes.
   // The key is the index of the alternate mode as determined
diff --git a/typecd/cable_test.cc b/typecd/cable_test.cc
index ee46e7f..85a76fe 100644
--- a/typecd/cable_test.cc
+++ b/typecd/cable_test.cc
@@ -6,6 +6,7 @@
 
 #include <string>
 
+#include <base/files/scoped_temp_dir.h>
 #include <base/strings/stringprintf.h>
 #include <gtest/gtest.h>
 
@@ -76,24 +77,33 @@
 // Check that calls of AddAltMode() done explicitly function correctly. Also
 // check that trying to add the same alt mode twice fails.
 TEST_F(CableTest, TestAltModeManualAddition) {
+  // Set up a temp dir.
+  base::ScopedTempDir scoped_temp_dir;
+  ASSERT_TRUE(scoped_temp_dir.CreateUniqueTempDir());
+  base::FilePath temp_dir = scoped_temp_dir.GetPath();
+
+  // Create the sysfs path for the cable and plug.
+  auto cable_path = temp_dir.Append(std::string("port0-cable"));
+  ASSERT_TRUE(base::CreateDirectory(cable_path));
   Cable cable((base::FilePath(kFakePort0CableSysPath)));
 
-  // Set up fake sysfs paths.
-  base::FilePath temp_dir;
-  ASSERT_TRUE(base::CreateNewTempDirectory("", &temp_dir));
+  // Create sysfs path for SOP' plug.
+  auto sop_plug_path = temp_dir.Append(std::string("port0-plug0"));
+  ASSERT_TRUE(base::CreateDirectory(sop_plug_path));
 
+  // TODO(b/172097194): Modify the test to check for DiscoveryComplete().
+
+  // Set up fake sysfs paths for alternate modes.
   std::string mode0_dirname =
       base::StringPrintf("port%d-plug0.%d", 0, kDPAltModeIndex);
-  auto mode0_path = temp_dir.Append(mode0_dirname);
+  auto mode0_path = sop_plug_path.Append(mode0_dirname);
   ASSERT_TRUE(CreateFakeAltMode(mode0_path, kDPSVID, kDPVDO, kDPVDOIndex));
-
   EXPECT_TRUE(cable.AddAltMode(mode0_path));
 
   std::string mode1_dirname =
       base::StringPrintf("port%d-plug0.%d", 0, kTBTAltModeIndex);
-  auto mode1_path = temp_dir.Append(mode1_dirname);
+  auto mode1_path = sop_plug_path.Append(mode1_dirname);
   ASSERT_TRUE(CreateFakeAltMode(mode1_path, kTBTSVID, kTBTVDO, kTBTVDOIndex));
-
   EXPECT_TRUE(cable.AddAltMode(mode1_path));
   // Trying to add an existing alt mode again should fail.
   EXPECT_FALSE(cable.AddAltMode(mode1_path));
diff --git a/typecd/port.cc b/typecd/port.cc
index e9eff1a..a5d95ce 100644
--- a/typecd/port.cc
+++ b/typecd/port.cc
@@ -308,4 +308,13 @@
   return false;
 }
 
+bool Port::IsCableDiscoveryComplete() {
+  if (!cable_) {
+    LOG(INFO) << "Trying to check discovery complete for a non-existent cable.";
+    return false;
+  }
+
+  return cable_->DiscoveryComplete();
+}
+
 }  // namespace typecd
diff --git a/typecd/port.h b/typecd/port.h
index d455662..16b2ecb 100644
--- a/typecd/port.h
+++ b/typecd/port.h
@@ -66,6 +66,13 @@
   // returns true.
   bool IsPartnerDiscoveryComplete();
 
+  // Return true when all PD discovery information (PD Identity VDOs, all
+  // Discover Mode data) for a cable has been processed.
+  //
+  // NOTE: Any mode entry decision logic should only run if this function
+  // returns true.
+  bool IsCableDiscoveryComplete();
+
  private:
   friend class PortTest;
   FRIEND_TEST(PortTest, TestBasicAdd);
diff --git a/typecd/port_manager.cc b/typecd/port_manager.cc
index 8b85c99..d94a387 100644
--- a/typecd/port_manager.cc
+++ b/typecd/port_manager.cc
@@ -143,8 +143,8 @@
     return;
   }
 
-  // TODO(b/152251292): Check for Cable Discovery complete too.
-  if (!port->IsPartnerDiscoveryComplete()) {
+  if (!port->IsPartnerDiscoveryComplete() ||
+      !port->IsCableDiscoveryComplete()) {
     LOG(WARNING)
         << "Can't switch modes Partner/Cable discovery not complete for port "
         << port_num;