drm/doc: Clarify PRIME documentation

PRIME fds aren't actually GEM fds but are (like the modeset API)
independent of the underlying buffer manager, as long as that one uses
uint32_t as handles. So move that entire section out of the GEM
section and reword it a bit to clarify which parts of PRIME are
generic, and which are the mandatory pieces for GEM drivers to
correctly implement the GEM lifetime rules. The rewording mostly
consists of not mixing up GEM, PRIME and DRM.

I've considered adding some blurbs to the GEM object lifetime section
about interactions with dma-bufs, but then dropped that. As long as
drivers use the right helpers they should have this all implemented
correctly and hence can be regarded as an implementation detail of the
PRIME/GEM helpers. So no need to confuse driver writers with those
tricky interactions.

Reviewed-by: Alex Deucher <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>

Documentation/DocBook/drm.tmpl

···
           respectively. The conversion is handled by the DRM core without any
           driver-specific support.
         </para>
-        <para>
-          Similar to global names, GEM file descriptors are also used to share GEM
-          objects across processes. They offer additional security: as file
-          descriptors must be explicitly sent over UNIX domain sockets to be shared
-          between applications, they can't be guessed like the globally unique GEM
-          names.
-        </para>
-        <para>
-          Drivers that support GEM file descriptors, also known as the DRM PRIME
-          API, must set the DRIVER_PRIME bit in the struct
-          <structname>drm_driver</structname>
-          <structfield>driver_features</structfield> field, and implement the
-          <methodname>prime_handle_to_fd</methodname> and
-          <methodname>prime_fd_to_handle</methodname> operations.
-        </para>
-        <para>
-          <synopsis>int (*prime_handle_to_fd)(struct drm_device *dev,
-                            struct drm_file *file_priv, uint32_t handle,
-                            uint32_t flags, int *prime_fd);
-  int (*prime_fd_to_handle)(struct drm_device *dev,
-                            struct drm_file *file_priv, int prime_fd,
-                            uint32_t *handle);</synopsis>
-          Those two operations convert a handle to a PRIME file descriptor and
-          vice versa. Drivers must use the kernel dma-buf buffer sharing framework
-          to manage the PRIME file descriptors.
-        </para>
-        <para>
-          While non-GEM drivers must implement the operations themselves, GEM
-          drivers must use the <function>drm_gem_prime_handle_to_fd</function>
-          and <function>drm_gem_prime_fd_to_handle</function> helper functions.
-          Those helpers rely on the driver
-          <methodname>gem_prime_export</methodname> and
-          <methodname>gem_prime_import</methodname> operations to create a dma-buf
-          instance from a GEM object (dma-buf exporter role) and to create a GEM
-          object from a dma-buf instance (dma-buf importer role).
-        </para>
-        <para>
-          <synopsis>struct dma_buf * (*gem_prime_export)(struct drm_device *dev,
-                                       struct drm_gem_object *obj,
-                                       int flags);
-  struct drm_gem_object * (*gem_prime_import)(struct drm_device *dev,
-                                              struct dma_buf *dma_buf);</synopsis>
-          These two operations are mandatory for GEM drivers that support DRM
-          PRIME.
-        </para>
-        <sect4>
-          <title>DRM PRIME Helper Functions Reference</title>
-!Pdrivers/gpu/drm/drm_prime.c PRIME Helpers
-        </sect4>
+	<para>
+	  GEM also supports buffer sharing with dma-buf file descriptors through
+	  PRIME. GEM-based drivers must use the provided helpers functions to
+	  implement the exporting and importing correctly. See <xref linkend="drm-prime-support" />.
+	  Since sharing file descriptors is inherently more secure than the
+	  easily guessable and global GEM names it is the preferred buffer
+	  sharing mechanism. Sharing buffers through GEM names is only supported
+	  for legacy userspace. Furthermore PRIME also allows cross-device
+	  buffer sharing since it is based on dma-bufs.
+	</para>
       </sect3>
       <sect3 id="drm-gem-objects-mapping">
         <title>GEM Objects Mapping</title>
···
           abstracted from the client in libdrm.
         </para>
       </sect3>
-      <sect2>
+      <sect3>
         <title>GEM Function Reference</title>
 !Edrivers/gpu/drm/drm_gem.c
-      </sect2>
+      </sect3>
       </sect2>
       <sect2>
 	<title>VMA Offset Manager</title>
 !Pdrivers/gpu/drm/drm_vma_manager.c vma offset manager
 !Edrivers/gpu/drm/drm_vma_manager.c
 !Iinclude/drm/drm_vma_manager.h
+      </sect2>
+      <sect2 id="drm-prime-support">
+	<title>PRIME Buffer Sharing</title>
+	<para>
+	  PRIME is the cross device buffer sharing framework in drm, originally
+	  created for the OPTIMUS range of multi-gpu platforms. To userspace
+	  PRIME buffers are dma-buf based file descriptors.
+	</para>
+	<sect3>
+	  <title>Overview and Driver Interface</title>
+	  <para>
+	    Similar to GEM global names, PRIME file descriptors are
+	    also used to share buffer objects across processes. They offer
+	    additional security: as file descriptors must be explicitly sent over
+	    UNIX domain sockets to be shared between applications, they can't be
+	    guessed like the globally unique GEM names.
+	  </para>
+	  <para>
+	    Drivers that support the PRIME
+	    API must set the DRIVER_PRIME bit in the struct
+	    <structname>drm_driver</structname>
+	    <structfield>driver_features</structfield> field, and implement the
+	    <methodname>prime_handle_to_fd</methodname> and
+	    <methodname>prime_fd_to_handle</methodname> operations.
+	  </para>
+	  <para>
+	    <synopsis>int (*prime_handle_to_fd)(struct drm_device *dev,
+			  struct drm_file *file_priv, uint32_t handle,
+			  uint32_t flags, int *prime_fd);
+int (*prime_fd_to_handle)(struct drm_device *dev,
+			  struct drm_file *file_priv, int prime_fd,
+			  uint32_t *handle);</synopsis>
+	    Those two operations convert a handle to a PRIME file descriptor and
+	    vice versa. Drivers must use the kernel dma-buf buffer sharing framework
+	    to manage the PRIME file descriptors. Similar to the mode setting
+	    API PRIME is agnostic to the underlying buffer object manager, as
+	    long as handles are 32bit unsinged integers.
+	  </para>
+	  <para>
+	    While non-GEM drivers must implement the operations themselves, GEM
+	    drivers must use the <function>drm_gem_prime_handle_to_fd</function>
+	    and <function>drm_gem_prime_fd_to_handle</function> helper functions.
+	    Those helpers rely on the driver
+	    <methodname>gem_prime_export</methodname> and
+	    <methodname>gem_prime_import</methodname> operations to create a dma-buf
+	    instance from a GEM object (dma-buf exporter role) and to create a GEM
+	    object from a dma-buf instance (dma-buf importer role).
+	  </para>
+	  <para>
+	    <synopsis>struct dma_buf * (*gem_prime_export)(struct drm_device *dev,
+				     struct drm_gem_object *obj,
+				     int flags);
+struct drm_gem_object * (*gem_prime_import)(struct drm_device *dev,
+					    struct dma_buf *dma_buf);</synopsis>
+	    These two operations are mandatory for GEM drivers that support
+	    PRIME.
+	  </para>
+	</sect3>
+        <sect3>
+          <title>PRIME Helper Functions Reference</title>
+!Pdrivers/gpu/drm/drm_prime.c PRIME Helpers
+        </sect3>
       </sect2>
   </sect1>