drm: add initial DRM developer documentation

Add a DRM DocBook providing basic information about DRM interfaces, including
TTM, GEM, KMS and vblank infrastructure.  Intended to provide information to
new and existing developers about how to perform driver initialization,
implement mode setting and other DRM features.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>

1 files changed, 839 insertions, 0 deletions

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+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
+	"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
+
+<book id="drmDevelopersGuide">
+  <bookinfo>
+    <title>Linux DRM Developer's Guide</title>
+
+    <copyright>
+      <year>2008-2009</year>
+      <holder>
+	Intel Corporation (Jesse Barnes &lt;jesse.barnes@intel.com&gt;)
+      </holder>
+    </copyright>
+
+    <legalnotice>
+      <para>
+	The contents of this file may be used under the terms of the GNU
+	General Public License version 2 (the "GPL") as distributed in
+	the kernel source COPYING file.
+      </para>
+    </legalnotice>
+  </bookinfo>
+
+<toc></toc>
+
+  <!-- Introduction -->
+
+  <chapter id="drmIntroduction">
+    <title>Introduction</title>
+    <para>
+      The Linux DRM layer contains code intended to support the needs
+      of complex graphics devices, usually containing programmable
+      pipelines well suited to 3D graphics acceleration.  Graphics
+      drivers in the kernel can make use of DRM functions to make
+      tasks like memory management, interrupt handling and DMA easier,
+      and provide a uniform interface to applications.
+    </para>
+    <para>
+      A note on versions: this guide covers features found in the DRM
+      tree, including the TTM memory manager, output configuration and
+      mode setting, and the new vblank internals, in addition to all
+      the regular features found in current kernels.
+    </para>
+    <para>
+      [Insert diagram of typical DRM stack here]
+    </para>
+  </chapter>
+
+  <!-- Internals -->
+
+  <chapter id="drmInternals">
+    <title>DRM Internals</title>
+    <para>
+      This chapter documents DRM internals relevant to driver authors
+      and developers working to add support for the latest features to
+      existing drivers.
+    </para>
+    <para>
+      First, we'll go over some typical driver initialization
+      requirements, like setting up command buffers, creating an
+      initial output configuration, and initializing core services.
+      Subsequent sections will cover core internals in more detail,
+      providing implementation notes and examples.
+    </para>
+    <para>
+      The DRM layer provides several services to graphics drivers,
+      many of them driven by the application interfaces it provides
+      through libdrm, the library that wraps most of the DRM ioctls.
+      These include vblank event handling, memory
+      management, output management, framebuffer management, command
+      submission &amp; fencing, suspend/resume support, and DMA
+      services.
+    </para>
+    <para>
+      The core of every DRM driver is struct drm_device.  Drivers
+      will typically statically initialize a drm_device structure,
+      then pass it to drm_init() at load time.
+    </para>
+
+  <!-- Internals: driver init -->
+
+  <sect1>
+    <title>Driver initialization</title>
+    <para>
+      Before calling the DRM initialization routines, the driver must
+      first create and fill out a struct drm_device structure.
+    </para>
+    <programlisting>
+      static struct drm_driver driver = {
+	/* don't use mtrr's here, the Xserver or user space app should
+	 * deal with them for intel hardware.
+	 */
+	.driver_features =
+	    DRIVER_USE_AGP | DRIVER_REQUIRE_AGP |
+	    DRIVER_HAVE_IRQ | DRIVER_IRQ_SHARED | DRIVER_MODESET,
+	.load = i915_driver_load,
+	.unload = i915_driver_unload,
+	.firstopen = i915_driver_firstopen,
+	.lastclose = i915_driver_lastclose,
+	.preclose = i915_driver_preclose,
+	.save = i915_save,
+	.restore = i915_restore,
+	.device_is_agp = i915_driver_device_is_agp,
+	.get_vblank_counter = i915_get_vblank_counter,
+	.enable_vblank = i915_enable_vblank,
+	.disable_vblank = i915_disable_vblank,
+	.irq_preinstall = i915_driver_irq_preinstall,
+	.irq_postinstall = i915_driver_irq_postinstall,
+	.irq_uninstall = i915_driver_irq_uninstall,
+	.irq_handler = i915_driver_irq_handler,
+	.reclaim_buffers = drm_core_reclaim_buffers,
+	.get_map_ofs = drm_core_get_map_ofs,
+	.get_reg_ofs = drm_core_get_reg_ofs,
+	.fb_probe = intelfb_probe,
+	.fb_remove = intelfb_remove,
+	.fb_resize = intelfb_resize,
+	.master_create = i915_master_create,
+	.master_destroy = i915_master_destroy,
+#if defined(CONFIG_DEBUG_FS)
+	.debugfs_init = i915_debugfs_init,
+	.debugfs_cleanup = i915_debugfs_cleanup,
+#endif
+	.gem_init_object = i915_gem_init_object,
+	.gem_free_object = i915_gem_free_object,
+	.gem_vm_ops = &amp;i915_gem_vm_ops,
+	.ioctls = i915_ioctls,
+	.fops = {
+		.owner = THIS_MODULE,
+		.open = drm_open,
+		.release = drm_release,
+		.ioctl = drm_ioctl,
+		.mmap = drm_mmap,
+		.poll = drm_poll,
+		.fasync = drm_fasync,
+#ifdef CONFIG_COMPAT
+		.compat_ioctl = i915_compat_ioctl,
+#endif
+		},
+	.pci_driver = {
+		.name = DRIVER_NAME,
+		.id_table = pciidlist,
+		.probe = probe,
+		.remove = __devexit_p(drm_cleanup_pci),
+		},
+	.name = DRIVER_NAME,
+	.desc = DRIVER_DESC,
+	.date = DRIVER_DATE,
+	.major = DRIVER_MAJOR,
+	.minor = DRIVER_MINOR,
+	.patchlevel = DRIVER_PATCHLEVEL,
+      };
+    </programlisting>
+    <para>
+      In the example above, taken from the i915 DRM driver, the driver
+      sets several flags indicating what core features it supports.
+      We'll go over the individual callbacks in later sections.  Since
+      flags indicate which features your driver supports to the DRM
+      core, you need to set most of them prior to calling drm_init().  Some,
+      like DRIVER_MODESET can be set later based on user supplied parameters,
+      but that's the exception rather than the rule.
+    </para>
+    <variablelist>
+      <title>Driver flags</title>
+      <varlistentry>
+	<term>DRIVER_USE_AGP</term>
+	<listitem><para>
+	    Driver uses AGP interface
+	</para></listitem>
+      </varlistentry>
+      <varlistentry>
+	<term>DRIVER_REQUIRE_AGP</term>
+	<listitem><para>
+	    Driver needs AGP interface to function.
+	</para></listitem>
+      </varlistentry>
+      <varlistentry>
+	<term>DRIVER_USE_MTRR</term>
+	<listitem>
+	  <para>
+	    Driver uses MTRR interface for mapping memory.  Deprecated.
+	  </para>
+	</listitem>
+      </varlistentry>
+      <varlistentry>
+	<term>DRIVER_PCI_DMA</term>
+	<listitem><para>
+	    Driver is capable of PCI DMA.  Deprecated.
+	</para></listitem>
+      </varlistentry>
+      <varlistentry>
+	<term>DRIVER_SG</term>
+	<listitem><para>
+	    Driver can perform scatter/gather DMA.  Deprecated.
+	</para></listitem>
+      </varlistentry>
+      <varlistentry>
+	<term>DRIVER_HAVE_DMA</term>
+	<listitem><para>Driver supports DMA.  Deprecated.</para></listitem>
+      </varlistentry>
+      <varlistentry>
+	<term>DRIVER_HAVE_IRQ</term><term>DRIVER_IRQ_SHARED</term>
+	<listitem>
+	  <para>
+	    DRIVER_HAVE_IRQ indicates whether the driver has a IRQ
+	    handler, DRIVER_IRQ_SHARED indicates whether the device &amp;
+	    handler support shared IRQs (note that this is required of
+	    PCI drivers).
+	  </para>
+	</listitem>
+      </varlistentry>
+      <varlistentry>
+	<term>DRIVER_DMA_QUEUE</term>
+	<listitem>
+	  <para>
+	    If the driver queues DMA requests and completes them
+	    asynchronously, this flag should be set.  Deprecated.
+	  </para>
+	</listitem>
+      </varlistentry>
+      <varlistentry>
+	<term>DRIVER_FB_DMA</term>
+	<listitem>
+	  <para>
+	    Driver supports DMA to/from the framebuffer.  Deprecated.
+	  </para>
+	</listitem>
+      </varlistentry>
+      <varlistentry>
+	<term>DRIVER_MODESET</term>
+	<listitem>
+	  <para>
+	    Driver supports mode setting interfaces.
+	  </para>
+	</listitem>
+      </varlistentry>
+    </variablelist>
+    <para>
+      In this specific case, the driver requires AGP and supports
+      IRQs.  DMA, as we'll see, is handled by device specific ioctls
+      in this case.  It also supports the kernel mode setting APIs, though
+      unlike in the actual i915 driver source, this example unconditionally
+      exports KMS capability.
+    </para>
+  </sect1>
+
+  <!-- Internals: driver load -->
+
+  <sect1>
+    <title>Driver load</title>
+    <para>
+      In the previous section, we saw what a typical drm_driver
+      structure might look like.  One of the more important fields in
+      the structure is the hook for the load function.
+    </para>
+    <programlisting>
+      static struct drm_driver driver = {
+      	...
+      	.load = i915_driver_load,
+        ...
+      };
+    </programlisting>
+    <para>
+      The load function has many responsibilities: allocating a driver
+      private structure, specifying supported performance counters,
+      configuring the device (e.g. mapping registers &amp; command
+      buffers), initializing the memory manager, and setting up the
+      initial output configuration.
+    </para>
+    <para>
+      Note that the tasks performed at driver load time must not
+      conflict with DRM client requirements.  For instance, if user
+      level mode setting drivers are in use, it would be problematic
+      to perform output discovery &amp; configuration at load time.
+      Likewise, if pre-memory management aware user level drivers are
+      in use, memory management and command buffer setup may need to
+      be omitted.  These requirements are driver specific, and care
+      needs to be taken to keep both old and new applications and
+      libraries working.  The i915 driver supports the "modeset"
+      module parameter to control whether advanced features are
+      enabled at load time or in legacy fashion.  If compatibility is
+      a concern (e.g. with drivers converted over to the new interfaces
+      from the old ones), care must be taken to prevent incompatible
+      device initialization and control with the currently active
+      userspace drivers.
+    </para>
+
+    <sect2>
+      <title>Driver private &amp; performance counters</title>
+      <para>
+	The driver private hangs off the main drm_device structure and
+	can be used for tracking various device specific bits of
+	information, like register offsets, command buffer status,
+	register state for suspend/resume, etc.  At load time, a
+	driver can simply allocate one and set drm_device.dev_priv
+	appropriately; at unload the driver can free it and set
+	drm_device.dev_priv to NULL.
+      </para>
+      <para>
+	The DRM supports several counters which can be used for rough
+	performance characterization.  Note that the DRM stat counter
+	system is not often used by applications, and supporting
+	additional counters is completely optional.
+      </para>
+      <para>
+	These interfaces are deprecated and should not be used.  If performance
+	monitoring is desired, the developer should investigate and
+	potentially enhance the kernel perf and tracing infrastructure to export
+	GPU related performance information to performance monitoring
+	tools and applications.
+      </para>
+    </sect2>
+
+    <sect2>
+      <title>Configuring the device</title>
+      <para>
+	Obviously, device configuration will be device specific.
+	However, there are several common operations: finding a
+	device's PCI resources, mapping them, and potentially setting
+	up an IRQ handler.
+      </para>
+      <para>
+	Finding &amp; mapping resources is fairly straightforward.  The
+	DRM wrapper functions, drm_get_resource_start() and
+	drm_get_resource_len() can be used to find BARs on the given
+	drm_device struct.  Once those values have been retrieved, the
+	driver load function can call drm_addmap() to create a new
+	mapping for the BAR in question.  Note you'll probably want a
+	drm_local_map_t in your driver private structure to track any
+	mappings you create.
+<!-- !Fdrivers/gpu/drm/drm_bufs.c drm_get_resource_* -->
+<!-- !Finclude/drm/drmP.h drm_local_map_t -->
+      </para>
+      <para>
+	if compatibility with other operating systems isn't a concern
+	(DRM drivers can run under various BSD variants and OpenSolaris),
+	native Linux calls can be used for the above, e.g. pci_resource_*
+	and iomap*/iounmap.  See the Linux device driver book for more
+	info.
+      </para>
+      <para>
+	Once you have a register map, you can use the DRM_READn() and
+	DRM_WRITEn() macros to access the registers on your device, or
+	use driver specific versions to offset into your MMIO space
+	relative to a driver specific base pointer (see I915_READ for
+	example).
+      </para>
+      <para>
+	If your device supports interrupt generation, you may want to
+	setup an interrupt handler at driver load time as well.  This
+	is done using the drm_irq_install() function.  If your device
+	supports vertical blank interrupts, it should call
+	drm_vblank_init() to initialize the core vblank handling code before
+	enabling interrupts on your device.  This ensures the vblank related
+	structures are allocated and allows the core to handle vblank events.
+      </para>
+<!--!Fdrivers/char/drm/drm_irq.c drm_irq_install-->
+      <para>
+	Once your interrupt handler is registered (it'll use your
+	drm_driver.irq_handler as the actual interrupt handling
+	function), you can safely enable interrupts on your device,
+	assuming any other state your interrupt handler uses is also
+	initialized.
+      </para>
+      <para>
+	Another task that may be necessary during configuration is
+	mapping the video BIOS.  On many devices, the VBIOS describes
+	device configuration, LCD panel timings (if any), and contains
+	flags indicating device state.  Mapping the BIOS can be done
+	using the pci_map_rom() call, a convenience function that
+	takes care of mapping the actual ROM, whether it has been
+	shadowed into memory (typically at address 0xc0000) or exists
+	on the PCI device in the ROM BAR.  Note that once you've
+	mapped the ROM and extracted any necessary information, be
+	sure to unmap it; on many devices the ROM address decoder is
+	shared with other BARs, so leaving it mapped can cause
+	undesired behavior like hangs or memory corruption.
+<!--!Fdrivers/pci/rom.c pci_map_rom-->
+      </para>
+    </sect2>
+
+    <sect2>
+      <title>Memory manager initialization</title>
+      <para>
+	In order to allocate command buffers, cursor memory, scanout
+	buffers, etc., as well as support the latest features provided
+	by packages like Mesa and the X.Org X server, your driver
+	should support a memory manager.
+      </para>
+      <para>
+	If your driver supports memory management (it should!), you'll
+	need to set that up at load time as well.  How you intialize
+	it depends on which memory manager you're using, TTM or GEM.
+      </para>
+      <sect3>
+	<title>TTM initialization</title>
+	<para>
+	  TTM (for Translation Table Manager) manages video memory and
+	  aperture space for graphics devices. TTM supports both UMA devices
+	  and devices with dedicated video RAM (VRAM), i.e. most discrete
+	  graphics devices.  If your device has dedicated RAM, supporting
+	  TTM is desireable.  TTM also integrates tightly with your
+	  driver specific buffer execution function.  See the radeon
+	  driver for examples.
+	</para>
+	<para>
+	  The core TTM structure is the ttm_bo_driver struct.  It contains
+	  several fields with function pointers for initializing the TTM,
+	  allocating and freeing memory, waiting for command completion
+	  and fence synchronization, and memory migration.  See the
+	  radeon_ttm.c file for an example of usage.
+	</para>
+	<para>
+	  The ttm_global_reference structure is made up of several fields:
+	</para>
+	<programlisting>
+	  struct ttm_global_reference {
+	  	enum ttm_global_types global_type;
+	  	size_t size;
+	  	void *object;
+	  	int (*init) (struct ttm_global_reference *);
+	  	void (*release) (struct ttm_global_reference *);
+	  };
+	</programlisting>
+	<para>
+	  There should be one global reference structure for your memory
+	  manager as a whole, and there will be others for each object
+	  created by the memory manager at runtime.  Your global TTM should
+	  have a type of TTM_GLOBAL_TTM_MEM.  The size field for the global
+	  object should be sizeof(struct ttm_mem_global), and the init and
+	  release hooks should point at your driver specific init and
+	  release routines, which will probably eventually call
+	  ttm_mem_global_init and ttm_mem_global_release respectively.
+	</para>
+	<para>
+	  Once your global TTM accounting structure is set up and initialized
+	  (done by calling ttm_global_item_ref on the global object you
+	  just created), you'll need to create a buffer object TTM to
+	  provide a pool for buffer object allocation by clients and the
+	  kernel itself.  The type of this object should be TTM_GLOBAL_TTM_BO,
+	  and its size should be sizeof(struct ttm_bo_global).  Again,
+	  driver specific init and release functions can be provided,
+	  likely eventually calling ttm_bo_global_init and
+	  ttm_bo_global_release, respectively.  Also like the previous
+	  object, ttm_global_item_ref is used to create an initial reference
+	  count for the TTM, which will call your initalization function.
+	</para>
+      </sect3>
+      <sect3>
+	<title>GEM initialization</title>
+	<para>
+	  GEM is an alternative to TTM, designed specifically for UMA
+	  devices.  It has simpler initialization and execution requirements
+	  than TTM, but has no VRAM management capability.  Core GEM
+	  initialization is comprised of a basic drm_mm_init call to create
+	  a GTT DRM MM object, which provides an address space pool for
+	  object allocation.  In a KMS configuration, the driver will
+	  need to allocate and initialize a command ring buffer following
+	  basic GEM initialization.  Most UMA devices have a so-called
+	  "stolen" memory region, which provides space for the initial
+	  framebuffer and large, contiguous memory regions required by the
+	  device.  This space is not typically managed by GEM, and must
+	  be initialized separately into its own DRM MM object.
+	</para>
+	<para>
+	  Initialization will be driver specific, and will depend on
+	  the architecture of the device.  In the case of Intel
+	  integrated graphics chips like 965GM, GEM initialization can
+	  be done by calling the internal GEM init function,
+	  i915_gem_do_init().  Since the 965GM is a UMA device
+	  (i.e. it doesn't have dedicated VRAM), GEM will manage
+	  making regular RAM available for GPU operations.  Memory set
+	  aside by the BIOS (called "stolen" memory by the i915
+	  driver) will be managed by the DRM memrange allocator; the
+	  rest of the aperture will be managed by GEM.
+	  <programlisting>
+	    /* Basic memrange allocator for stolen space (aka vram) */
+	    drm_memrange_init(&amp;dev_priv->vram, 0, prealloc_size);
+	    /* Let GEM Manage from end of prealloc space to end of aperture */
+	    i915_gem_do_init(dev, prealloc_size, agp_size);
+	  </programlisting>
+<!--!Edrivers/char/drm/drm_memrange.c-->
+	</para>
+	<para>
+	  Once the memory manager has been set up, we can allocate the
+	  command buffer.  In the i915 case, this is also done with a
+	  GEM function, i915_gem_init_ringbuffer().
+	</para>
+      </sect3>
+    </sect2>
+
+    <sect2>
+      <title>Output configuration</title>
+      <para>
+	The final initialization task is output configuration.  This involves
+	finding and initializing the CRTCs, encoders and connectors
+	for your device, creating an initial configuration and
+	registering a framebuffer console driver.
+      </para>
+      <sect3>
+	<title>Output discovery and initialization</title>
+	<para>
+	  Several core functions exist to create CRTCs, encoders and
+	  connectors, namely drm_crtc_init(), drm_connector_init() and
+	  drm_encoder_init(), along with several "helper" functions to
+	  perform common tasks.
+	</para>
+	<para>
+	  Connectors should be registered with sysfs once they've been
+	  detected and initialized, using the
+	  drm_sysfs_connector_add() function.  Likewise, when they're
+	  removed from the system, they should be destroyed with
+	  drm_sysfs_connector_remove().
+	</para>
+	<programlisting>
+<![CDATA[
+void intel_crt_init(struct drm_device *dev)
+{
+	struct drm_connector *connector;
+	struct intel_output *intel_output;
+
+	intel_output = kzalloc(sizeof(struct intel_output), GFP_KERNEL);
+	if (!intel_output)
+		return;
+
+	connector = &intel_output->base;
+	drm_connector_init(dev, &intel_output->base,
+			   &intel_crt_connector_funcs, DRM_MODE_CONNECTOR_VGA);
+
+	drm_encoder_init(dev, &intel_output->enc, &intel_crt_enc_funcs,
+			 DRM_MODE_ENCODER_DAC);
+
+	drm_mode_connector_attach_encoder(&intel_output->base,
+					  &intel_output->enc);
+
+	/* Set up the DDC bus. */
+	intel_output->ddc_bus = intel_i2c_create(dev, GPIOA, "CRTDDC_A");
+	if (!intel_output->ddc_bus) {
+		dev_printk(KERN_ERR, &dev->pdev->dev, "DDC bus registration "
+			   "failed.\n");
+		return;
+	}
+
+	intel_output->type = INTEL_OUTPUT_ANALOG;
+	connector->interlace_allowed = 0;
+	connector->doublescan_allowed = 0;
+
+	drm_encoder_helper_add(&intel_output->enc, &intel_crt_helper_funcs);
+	drm_connector_helper_add(connector, &intel_crt_connector_helper_funcs);
+
+	drm_sysfs_connector_add(connector);
+}
+]]>
+	</programlisting>
+	<para>
+	  In the example above (again, taken from the i915 driver), a
+	  CRT connector and encoder combination is created.  A device
+	  specific i2c bus is also created, for fetching EDID data and
+	  performing monitor detection.  Once the process is complete,
+	  the new connector is regsitered with sysfs, to make its
+	  properties available to applications.
+	</para>
+	<sect4>
+	  <title>Helper functions and core functions</title>
+	  <para>
+	    Since many PC-class graphics devices have similar display output
+	    designs, the DRM provides a set of helper functions to make
+	    output management easier.  The core helper routines handle
+	    encoder re-routing and disabling of unused functions following
+	    mode set.  Using the helpers is optional, but recommended for
+	    devices with PC-style architectures (i.e. a set of display planes
+	    for feeding pixels to encoders which are in turn routed to
+	    connectors).  Devices with more complex requirements needing
+	    finer grained management can opt to use the core callbacks
+	    directly.
+	  </para>
+	  <para>
+	    [Insert typical diagram here.]  [Insert OMAP style config here.]
+	  </para>
+	</sect4>
+	<para>
+	  For each encoder, CRTC and connector, several functions must
+	  be provided, depending on the object type.  Encoder objects
+	  need should provide a DPMS (basically on/off) function, mode fixup
+	  (for converting requested modes into native hardware timings),
+	  and prepare, set and commit functions for use by the core DRM
+	  helper functions.  Connector helpers need to provide mode fetch and
+	  validity functions as well as an encoder matching function for
+	  returing an ideal encoder for a given connector.  The core
+	  connector functions include a DPMS callback, (deprecated)
+	  save/restore routines, detection, mode probing, property handling,
+	  and cleanup functions.
+	</para>
+<!--!Edrivers/char/drm/drm_crtc.h-->
+<!--!Edrivers/char/drm/drm_crtc.c-->
+<!--!Edrivers/char/drm/drm_crtc_helper.c-->
+      </sect3>
+    </sect2>
+  </sect1>
+
+  <!-- Internals: vblank handling -->
+
+  <sect1>
+    <title>VBlank event handling</title>
+    <para>
+      The DRM core exposes two vertical blank related ioctls:
+      DRM_IOCTL_WAIT_VBLANK and DRM_IOCTL_MODESET_CTL.
+<!--!Edrivers/char/drm/drm_irq.c-->
+    </para>
+    <para>
+      DRM_IOCTL_WAIT_VBLANK takes a struct drm_wait_vblank structure
+      as its argument, and is used to block or request a signal when a
+      specified vblank event occurs.
+    </para>
+    <para>
+      DRM_IOCTL_MODESET_CTL should be called by application level
+      drivers before and after mode setting, since on many devices the
+      vertical blank counter will be reset at that time.  Internally,
+      the DRM snapshots the last vblank count when the ioctl is called
+      with the _DRM_PRE_MODESET command so that the counter won't go
+      backwards (which is dealt with when _DRM_POST_MODESET is used).
+    </para>
+    <para>
+      To support the functions above, the DRM core provides several
+      helper functions for tracking vertical blank counters, and
+      requires drivers to provide several callbacks:
+      get_vblank_counter(), enable_vblank() and disable_vblank().  The
+      core uses get_vblank_counter() to keep the counter accurate
+      across interrupt disable periods.  It should return the current
+      vertical blank event count, which is often tracked in a device
+      register.  The enable and disable vblank callbacks should enable
+      and disable vertical blank interrupts, respectively.  In the
+      absence of DRM clients waiting on vblank events, the core DRM
+      code will use the disable_vblank() function to disable
+      interrupts, which saves power.  They'll be re-enabled again when
+      a client calls the vblank wait ioctl above.
+    </para>
+    <para>
+      Devices that don't provide a count register can simply use an
+      internal atomic counter incremented on every vertical blank
+      interrupt, and can make their enable and disable vblank
+      functions into no-ops.
+    </para>
+  </sect1>
+
+  <sect1>
+    <title>Memory management</title>
+    <para>
+      The memory manager lies at the heart of many DRM operations, and
+      is also required to support advanced client features like OpenGL
+      pbuffers.  The DRM currently contains two memory managers, TTM
+      and GEM.
+    </para>
+
+    <sect2>
+      <title>The Translation Table Manager (TTM)</title>
+      <para>
+	TTM was developed by Tungsten Graphics, primarily by Thomas
+	Hellström, and is intended to be a flexible, high performance
+	graphics memory manager.
+      </para>
+      <para>
+	Drivers wishing to support TTM must fill out a drm_bo_driver
+	structure.
+      </para>
+      <para>
+	TTM design background and information belongs here.
+      </para>
+    </sect2>
+
+    <sect2>
+      <title>The Graphics Execution Manager (GEM)</title>
+      <para>
+	GEM is an Intel project, authored by Eric Anholt and Keith
+	Packard.  It provides simpler interfaces than TTM, and is well
+	suited for UMA devices.
+      </para>
+      <para>
+	GEM-enabled drivers must provide gem_init_object() and
+	gem_free_object() callbacks to support the core memory
+	allocation routines.  They should also provide several driver
+	specific ioctls to support command execution, pinning, buffer
+	read &amp; write, mapping, and domain ownership transfers.
+      </para>
+      <para>
+	On a fundamental level, GEM involves several operations: memory
+	allocation and freeing, command execution, and aperture management
+	at command execution time.  Buffer object allocation is relatively
+	straightforward and largely provided by Linux's shmem layer, which
+	provides memory to back each object.  When mapped into the GTT
+	or used in a command buffer, the backing pages for an object are
+	flushed to memory and marked write combined so as to be coherent
+	with the GPU.  Likewise, when the GPU finishes rendering to an object,
+	if the CPU accesses it, it must be made coherent with the CPU's view
+	of memory, usually involving GPU cache flushing of various kinds.
+	This core CPU&lt;-&gt;GPU coherency management is provided by the GEM
+	set domain function, which evaluates an object's current domain and
+	performs any necessary flushing or synchronization to put the object
+	into the desired coherency domain (note that the object may be busy,
+	i.e. an active render target; in that case the set domain function
+	will block the client and wait for rendering to complete before
+	performing any necessary flushing operations).
+      </para>
+      <para>
+	Perhaps the most important GEM function is providing a command
+	execution interface to clients.  Client programs construct command
+	buffers containing references to previously allocated memory objects
+	and submit them to GEM.  At that point, GEM will take care to bind
+	all the objects into the GTT, execute the buffer, and provide
+	necessary synchronization between clients accessing the same buffers.
+	This often involves evicting some objects from the GTT and re-binding
+	others (a fairly expensive operation), and providing relocation
+	support which hides fixed GTT offsets from clients.  Clients must
+	take care not to submit command buffers that reference more objects
+	than can fit in the GTT or GEM will reject them and no rendering
+	will occur.  Similarly, if several objects in the buffer require
+	fence registers to be allocated for correct rendering (e.g. 2D blits
+	on pre-965 chips), care must be taken not to require more fence
+	registers than are available to the client.  Such resource management
+	should be abstracted from the client in libdrm.
+      </para>
+    </sect2>
+
+  </sect1>
+
+  <!-- Output management -->
+  <sect1>
+    <title>Output management</title>
+    <para>
+      At the core of the DRM output management code is a set of
+      structures representing CRTCs, encoders and connectors.
+    </para>
+    <para>
+      A CRTC is an abstraction representing a part of the chip that
+      contains a pointer to a scanout buffer.  Therefore, the number
+      of CRTCs available determines how many independent scanout
+      buffers can be active at any given time.  The CRTC structure
+      contains several fields to support this: a pointer to some video
+      memory, a display mode, and an (x, y) offset into the video
+      memory to support panning or configurations where one piece of
+      video memory spans multiple CRTCs.
+    </para>
+    <para>
+      An encoder takes pixel data from a CRTC and converts it to a
+      format suitable for any attached connectors.  On some devices,
+      it may be possible to have a CRTC send data to more than one
+      encoder.  In that case, both encoders would receive data from
+      the same scanout buffer, resulting in a "cloned" display
+      configuration across the connectors attached to each encoder.
+    </para>
+    <para>
+      A connector is the final destination for pixel data on a device,
+      and usually connects directly to an external display device like
+      a monitor or laptop panel.  A connector can only be attached to
+      one encoder at a time.  The connector is also the structure
+      where information about the attached display is kept, so it
+      contains fields for display data, EDID data, DPMS &amp;
+      connection status, and information about modes supported on the
+      attached displays.
+    </para>
+<!--!Edrivers/char/drm/drm_crtc.c-->
+  </sect1>
+
+  <sect1>
+    <title>Framebuffer management</title>
+    <para>
+      In order to set a mode on a given CRTC, encoder and connector
+      configuration, clients need to provide a framebuffer object which
+      will provide a source of pixels for the CRTC to deliver to the encoder(s)
+      and ultimately the connector(s) in the configuration.  A framebuffer
+      is fundamentally a driver specific memory object, made into an opaque
+      handle by the DRM addfb function.  Once an fb has been created this
+      way it can be passed to the KMS mode setting routines for use in
+      a configuration.
+    </para>
+  </sect1>
+
+  <sect1>
+    <title>Command submission &amp; fencing</title>
+    <para>
+      This should cover a few device specific command submission
+      implementations.
+    </para>
+  </sect1>
+
+  <sect1>
+    <title>Suspend/resume</title>
+    <para>
+      The DRM core provides some suspend/resume code, but drivers
+      wanting full suspend/resume support should provide save() and
+      restore() functions.  These will be called at suspend,
+      hibernate, or resume time, and should perform any state save or
+      restore required by your device across suspend or hibernate
+      states.
+    </para>
+  </sect1>
+
+  <sect1>
+    <title>DMA services</title>
+    <para>
+      This should cover how DMA mapping etc. is supported by the core.
+      These functions are deprecated and should not be used.
+    </para>
+  </sect1>
+  </chapter>
+
+  <!-- External interfaces -->
+
+  <chapter id="drmExternals">
+    <title>Userland interfaces</title>
+    <para>
+      The DRM core exports several interfaces to applications,
+      generally intended to be used through corresponding libdrm
+      wrapper functions.  In addition, drivers export device specific
+      interfaces for use by userspace drivers &amp; device aware
+      applications through ioctls and sysfs files.
+    </para>
+    <para>
+      External interfaces include: memory mapping, context management,
+      DMA operations, AGP management, vblank control, fence
+      management, memory management, and output management.
+    </para>
+    <para>
+      Cover generic ioctls and sysfs layout here.  Only need high
+      level info, since man pages will cover the rest.
+    </para>
+  </chapter>
+
+  <!-- API reference -->
+
+  <appendix id="drmDriverApi">
+    <title>DRM Driver API</title>
+    <para>
+      Include auto-generated API reference here (need to reference it
+      from paragraphs above too).
+    </para>
+  </appendix>
+
+</book>