Re: [PATCH] drm/doc: Clarify the dumb object interfaces

Hi Daniel,

Thank you for the patch.

On Friday 24 January 2014 17:58:40 Daniel Vetter wrote:

• This is _not_ a generic interface to create gem objects, but just an interface to make early boot services (like boot splash) with a generic KMS userspace driver possible. Hence it's better to move the documentation for this from the GEM section to the KMS section, next to the creation of framebuffer objects.

• Make it really clear that the returned handle isn't necessarily a GEM object (it can also be e.g. a TTM handle when running on top of vmwgfx).

• Add a paragraph to make it clear that this is just for unaccelarated userspace - gpu drivers need to have their own buffer object creation ioctl which is hardware specific.

v2: Clarify that the documentation doesn't just apply to GEM-based drivers only but is now generally valid, as suggested by David.

v3: Polish the intro sentence a bit and one s/objects/handles/ for clarification, both suggested by Laurent.

v4: More text polish from Laurent's review.

Cc: David Herrmann dh.herrmann@gmail.com Cc: Laurent Pinchart laurent.pinchart@ideasonboard.com Signed-off-by: Daniel Vetter daniel.vetter@ffwll.ch

Acked-by: Laurent Pinchart laurent.pinchart@ideasonboard.com

---

Documentation/DocBook/drm.tmpl | 131 ++++++++++++++++++++------------------ 1 file changed, 71 insertions(+), 60 deletions(-)

diff --git a/Documentation/DocBook/drm.tmpl b/Documentation/DocBook/drm.tmpl index ed1d6d289022..67cfe184749c 100644 --- a/Documentation/DocBook/drm.tmpl +++ b/Documentation/DocBook/drm.tmpl @@ -830,62 +830,6 @@ char *date;</synopsis> </para> </sect3> <sect3>

•    <title>Dumb GEM Objects</title>
  

•    <para>
  

•      The GEM API doesn't standardize GEM objects creation and leaves
  

it to - driver-specific ioctls. While not an issue for full-fledged graphics - stacks that include device-specific userspace components (in libdrm for - instance), this limit makes DRM-based early boot graphics unnecessarily - complex.

•    </para>
  

•    <para>
  

•      Dumb GEM objects partly alleviate the problem by providing a
  

standard - API to create dumb buffers suitable for scanout, which can then be used - to create KMS frame buffers.

•    </para>
  

•    <para>
  

•      To support dumb GEM objects drivers must implement the
  

•      <methodname>dumb_create</methodname>,
  

•      <methodname>dumb_destroy</methodname> and
  

•      <methodname>dumb_map_offset</methodname> operations.
  

•    </para>
  

•    <itemizedlist>
  

•      <listitem>
  

•        <synopsis>int (*dumb_create)(struct drm_file *file_priv, struct
  

drm_device *dev, - struct drm_mode_create_dumb *args);</synopsis> - <para>

•          The <methodname>dumb_create</methodname> operation creates a
  

GEM - object suitable for scanout based on the width, height and depth - from the struct <structname>drm_mode_create_dumb</structname> - argument. It fills the argument's <structfield>handle</structfield>, - <structfield>pitch</structfield> and <structfield>size</structfield> - fields with a handle for the newly created GEM object and its line

•          pitch and size in bytes.
  

•        </para>
  

•      </listitem>
  

•      <listitem>
  

•        <synopsis>int (*dumb_destroy)(struct drm_file *file_priv,
  

struct drm_device *dev, - uint32_t handle);</synopsis>

•        <para>
  

•          The <methodname>dumb_destroy</methodname> operation destroys
  

a dumb - GEM object created by <methodname>dumb_create</methodname>. - </para>

•      </listitem>
  

•      <listitem>
  

•        <synopsis>int (*dumb_map_offset)(struct drm_file *file_priv,
  

struct drm_device *dev, - uint32_t handle, uint64_t *offset);</synopsis> - <para>

•          The <methodname>dumb_map_offset</methodname> operation
  

associates an - mmap fake offset with the GEM object given by the handle and returns - it. Drivers must use the

•          <function>drm_gem_create_mmap_offset</function> function to
  

•          associate the fake offset as described in
  

•          <xref linkend="drm-gem-objects-mapping"/>.
  

•        </para>
  

•      </listitem>
  

•    </itemizedlist>
  

•  </sect3>
  

•  <sect3>
     <title>Memory Coherency</title>
     <para>
       When mapped to the device or used in a command buffer, backing
  

pages @@ -968,9 +912,11 @@ int max_width, max_height;</synopsis> Frame buffers rely on the underneath memory manager for low-level memory operations. When creating a frame buffer applications pass a memory handle (or a list of memory handles for multi-planar formats) through - the <parameter>drm_mode_fb_cmd2</parameter> argument. This document - assumes that the driver uses GEM, those handles thus reference GEM - objects.

• the <parameter>drm_mode_fb_cmd2</parameter> argument. For drivers using
• GEM as their userspace buffer management interface this would be a GEM
• handle. Drivers are however free to use their own backing storage object
• handles, e.g. vmwgfx directly exposes special TTM handles to userspace
• and so expects TTM handles in the create ioctl and not GEM handles.

  </para> <para> Drivers must first validate the requested frame buffer parameters

passed @@ -992,7 +938,7 @@ int max_width, max_height;</synopsis> </para>

   <para>

• The initailization of the new framebuffer instance is finalized with a

• The initilization of the new framebuffer instance is finalized with a call to <function>drm_framebuffer_init</function> which takes a pointer to DRM frame buffer operations (struct <structname>drm_framebuffer_funcs</structname>). Note that this function

@@ -1052,6 +998,71 @@ int max_width, max_height;</synopsis> <function>drm_framebuffer_unregister_private</function>. </sect2> <sect2>

•  <title>Dumb Buffer Objects</title>
  

•  <para>
  

• The KMS API doesn't standardize backing storage object creation and
• leaves it to driver-specific ioctls. Furthermore actually creating a
• buffer object even for GEM-based drivers is done through a
• driver-specific ioctl - GEM only has a common userspace interface for
• sharing and destroying objects. While not an issue for full-fledged
• graphics stacks that include device-specific userspace components (in
• libdrm for instance), this limit makes DRM-based early boot graphics
• unnecessarily complex.

•  </para>
  

•  <para>
  

•    Dumb objects partly alleviate the problem by providing a standard
  

•    API to create dumb buffers suitable for scanout, which can then be
  

used + to create KMS frame buffers.

•  </para>
  

•  <para>
  

•    To support dumb objects drivers must implement the
  

•    <methodname>dumb_create</methodname>,
  

•    <methodname>dumb_destroy</methodname> and
  

•    <methodname>dumb_map_offset</methodname> operations.
  

•  </para>
  

•  <itemizedlist>
  

•    <listitem>
  

•      <synopsis>int (*dumb_create)(struct drm_file *file_priv, struct
  

drm_device *dev, + struct drm_mode_create_dumb *args);</synopsis> + <para>

•        The <methodname>dumb_create</methodname> operation creates a
  

driver + object (GEM or TTM handle) suitable for scanout based on the

•    width, height and depth from the struct
  

•    <structname>drm_mode_create_dumb</structname> argument. It fills the
  

•    argument's <structfield>handle</structfield>,
  

•    <structfield>pitch</structfield> and <structfield>size</structfield>
  

•    fields with a handle for the newly created object and its line
  

•        pitch and size in bytes.
  

•      </para>
  

•    </listitem>
  

•    <listitem>
  

•      <synopsis>int (*dumb_destroy)(struct drm_file *file_priv, struct
  

drm_device *dev, + uint32_t handle);</synopsis>

•      <para>
  

•        The <methodname>dumb_destroy</methodname> operation destroys a
  

dumb + object created by <methodname>dumb_create</methodname>. + </para>

•    </listitem>
  

•    <listitem>
  

•      <synopsis>int (*dumb_map_offset)(struct drm_file *file_priv,
  

struct drm_device *dev, + uint32_t handle, uint64_t *offset);</synopsis> + <para>

•        The <methodname>dumb_map_offset</methodname> operation
  

associates an + mmap fake offset with the object given by the handle and returns + it. Drivers must use the

•        <function>drm_gem_create_mmap_offset</function> function to
  

•        associate the fake offset as described in
  

•        <xref linkend="drm-gem-objects-mapping"/>.
  

•      </para>
  

•    </listitem>
  

•  </itemizedlist>
  

•  <para>
  

•    Note that dumb objects may not be used for gpu accelaration, as has
  

been + attempted on some ARM embedded platforms. Such drivers really must have + a hardware-specific ioctl to allocate suitable buffer objects.

•  </para>
  

• </sect2>

• <sect2> <title>Output Polling</title> <synopsis>void (*output_poll_changed)(struct drm_device

*dev);</synopsis> <para>