path: root/Documentation/DocBook/drm.tmpl

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<!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