first draft

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Editor/Material/Lit/LitUI.cs

             public static GUIContent metallicText = new GUIContent("Metallic", "Metallic scale factor");
             public static GUIContent smoothnessText = new GUIContent("Smoothness", "Smoothness scale factor");
             public static GUIContent smoothnessRemappingText = new GUIContent("Smoothness Remapping", "Smoothness remapping");
+            public static GUIContent useRoughnessText = new GUIContent("Use Roughness map", "Enabling this will invert a Roughness map to be used in a Smoothness slot");            
             public static GUIContent aoRemappingText = new GUIContent("AmbientOcclusion Remapping", "AmbientOcclusion remapping");
             public static GUIContent maskMapSText = new GUIContent("Mask Map - M(R), AO(G), D(B), S(A)", "Mask map");
             public static GUIContent maskMapSpecularText = new GUIContent("Mask Map - AO(G), D(B), S(A)", "Mask map");
         protected const string kSmoothnessRemapMin = "_SmoothnessRemapMin";
         protected MaterialProperty[] smoothnessRemapMax = new MaterialProperty[kMaxLayerCount];
         protected const string kSmoothnessRemapMax = "_SmoothnessRemapMax";
+        protected MaterialProperty[] useRoughness = new MaterialProperty[kMaxLayerCount];
+        protected const string kUseRoughness = "_UseRoughness";
         protected MaterialProperty[] aoRemapMin = new MaterialProperty[kMaxLayerCount];
         protected const string kAORemapMin = "_AORemapMin";
         protected MaterialProperty[] aoRemapMax = new MaterialProperty[kMaxLayerCount];
                 smoothness[i] = FindProperty(string.Format("{0}{1}", kSmoothness, m_PropertySuffixes[i]), props);
                 smoothnessRemapMin[i] = FindProperty(string.Format("{0}{1}", kSmoothnessRemapMin, m_PropertySuffixes[i]), props);
                 smoothnessRemapMax[i] = FindProperty(string.Format("{0}{1}", kSmoothnessRemapMax, m_PropertySuffixes[i]), props);
+                useRoughness[i] = FindProperty(string.Format("{0}{1}", kUseRoughness, m_PropertySuffixes[i]), props); 
                 aoRemapMin[i] = FindProperty(string.Format("{0}{1}", kAORemapMin, m_PropertySuffixes[i]), props);
                 aoRemapMax[i] = FindProperty(string.Format("{0}{1}", kAORemapMax, m_PropertySuffixes[i]), props);
                 maskMap[i] = FindProperty(string.Format("{0}{1}", kMaskMap, m_PropertySuffixes[i]), props);
             if(maskMap[layerIndex].textureValue == null)
             {
                 m_MaterialEditor.ShaderProperty(smoothness[layerIndex], Styles.smoothnessText);
+                m_MaterialEditor.TexturePropertySingleLine(((BaseLitGUI.MaterialId)materialID.floatValue == BaseLitGUI.MaterialId.LitSpecular) ? Styles.maskMapSpecularText : Styles.maskMapSText, maskMap[layerIndex]);
+                m_MaterialEditor.TexturePropertySingleLine(((BaseLitGUI.MaterialId)materialID.floatValue == BaseLitGUI.MaterialId.LitSpecular) ? Styles.maskMapSpecularText : Styles.maskMapSText, maskMap[layerIndex]);
+
+                EditorGUI.indentLevel++;
+                m_MaterialEditor.ShaderProperty(useRoughness[layerIndex], Styles.useRoughnessText);
-                    smoothnessRemapMin[layerIndex].floatValue = remapMin;
-                    smoothnessRemapMax[layerIndex].floatValue = remapMax;
+                    // If we use roughness map instead of smoothness map, just invert the range remapping
+                    if (useRoughness[layerIndex].floatValue > 0.0)
+                    {
+                        smoothnessRemapMin[layerIndex].floatValue = remapMax;
+                        smoothnessRemapMax[layerIndex].floatValue = remapMin;
+                    }
+                    else
+                    {
+                        smoothnessRemapMin[layerIndex].floatValue = remapMin;
+                        smoothnessRemapMax[layerIndex].floatValue = remapMax;
+                    }
                 }
 
                 float aoMin = aoRemapMin[layerIndex].floatValue;
                     aoRemapMin[layerIndex].floatValue = aoMin;
                     aoRemapMax[layerIndex].floatValue = aoMax;
                 }
+                EditorGUI.indentLevel--;
-
-            m_MaterialEditor.TexturePropertySingleLine(((BaseLitGUI.MaterialId)materialID.floatValue == BaseLitGUI.MaterialId.LitSpecular) ? Styles.maskMapSpecularText : Styles.maskMapSText, maskMap[layerIndex]);
 
             m_MaterialEditor.ShaderProperty(normalMapSpace[layerIndex], Styles.normalMapSpaceText);
 

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Material/LayeredLit/LayeredLit.shader

         _SmoothnessRemapMax2("SmoothnessRemapMax2", Range(0.0, 1.0)) = 1.0
         _SmoothnessRemapMax3("SmoothnessRemapMax3", Range(0.0, 1.0)) = 1.0
 
+        [ToggleUI] _UseRoughness0("Use Roughness", Float) = 0.0
+        [ToggleUI] _UseRoughness1("Use Roughness", Float) = 0.0
+        [ToggleUI] _UseRoughness2("Use Roughness", Float) = 0.0
+        [ToggleUI] _UseRoughness3("Use Roughness", Float) = 0.0
+
         _AORemapMin0("AORemapMin0", Range(0.0, 1.0)) = 0.0
         _AORemapMin1("AORemapMin1", Range(0.0, 1.0)) = 0.0
         _AORemapMin2("AORemapMin2", Range(0.0, 1.0)) = 0.0

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Material/LayeredLit/LayeredLitTessellation.shader

         _SmoothnessRemapMax2("SmoothnessRemapMax2", Range(0.0, 1.0)) = 1.0
         _SmoothnessRemapMax3("SmoothnessRemapMax3", Range(0.0, 1.0)) = 1.0
 
+        [ToggleUI] _UseRoughness0("Use Roughness", Float) = 0.0
+        [ToggleUI] _UseRoughness1("Use Roughness", Float) = 0.0
+        [ToggleUI] _UseRoughness2("Use Roughness", Float) = 0.0
+        [ToggleUI] _UseRoughness3("Use Roughness", Float) = 0.0
+
         _AORemapMin0("AORemapMin0", Range(0.0, 1.0)) = 0.0
         _AORemapMin1("AORemapMin1", Range(0.0, 1.0)) = 0.0
         _AORemapMin2("AORemapMin2", Range(0.0, 1.0)) = 0.0

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Material/Lit/Lit.shader

         _MaskMap("MaskMap", 2D) = "white" {}
         _SmoothnessRemapMin("SmoothnessRemapMin", Float) = 0.0
         _SmoothnessRemapMax("SmoothnessRemapMax", Float) = 1.0
+        [ToggleUI] _UseRoughness("Use Roughness", Float) = 0.0
         _AORemapMin("AORemapMin", Float) = 0.0
         _AORemapMax("AORemapMax", Float) = 1.0
 

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Material/Lit/LitTessellation.shader

         _MaskMap("MaskMap", 2D) = "white" {}
         _SmoothnessRemapMin("SmoothnessRemapMin", Float) = 0.0
         _SmoothnessRemapMax("SmoothnessRemapMax", Float) = 1.0
+        [ToggleUI] _UseRoughness("Use Roughness", Float) = 0.0
         _AORemapMin("AORemapMin", Float) = 0.0
         _AORemapMax("AORemapMax", Float) = 1.0