Primary node clarification for VK_EXT_physical_device_drm

The extension hinted that the primary node represents display
capabilities as its intended purpose, but the current text let it
ambiguous enough that there has been confusion and proposed
implementations which expose different devices for the primary node.

Add clarification notes to further define the scope of primary node
in the context of this extension.

Signed-off-by: Erico Nunes <[email protected]>

appendices/VK_EXT_physical_device_drm.adoc

@@ -25,12 +25,19 @@ Unlike the EGL extension, this extension does not expose a string containing
 the name of the device file and instead exposes device minor numbers.
 
 DRM defines multiple device node types.
-Each physical device may have one primary node and one render node
-associated.
+Each physical device is associated with a single underlying DRM device, which
+may have one primary node and one render node.
+The primary node, in the context of this extension, refers to the capability of
+that device node to be used as a display subsystem.
+Note that this is not the same definition as a "DRM primary device node" on
+Linux, which may exist even for devices without a display subsystem.
+
 Physical devices may have no primary node (e.g. if the device does not have
 a display subsystem), may have no render node (e.g. if it is a software
 rendering engine), or may have neither (e.g. if it is a software rendering
 engine without a display subsystem).
+Physical devices which don't have a display subsystem implemented as part of
+the same underlying DRM device report that as hasPrimary=false.
 
 To query DRM properties for a physical device, chain
 slink:VkPhysicalDeviceDrmPropertiesEXT to slink:VkPhysicalDeviceProperties2.

chapters/devsandqueues.adoc

@@ -838,7 +838,7 @@ include::{generated}/api/structs/VkPhysicalDeviceDrmPropertiesEXT.adoc[]
   * pname:pNext is `NULL` or a pointer to a structure extending this
     structure.
   * pname:hasPrimary is a boolean indicating whether the physical device has
-    a DRM primary node.
+    a DRM primary node that can be used as a display subsystem.
   * pname:hasRender is a boolean indicating whether the physical device has
     a DRM render node.
   * pname:primaryMajor is the DRM primary node major number, if any.