Documentation/gpu: add new gpu.rst converted from DocBook gpu.tmpl

This is the first step towards converting the DocBook gpu.tmpl to Sphinx
and reStructuredText, the new kernel documentation tool and markup.

Use Jon's "cheesy conversion script" in Documentation/sphinx/tmplcvt to
do the rough conversion. Do the manual edits in follow-up patches. Add a
new Documentation/gpu directories for the graphics related
documentation. (Hooray, now we can have directories based on topics
rather than tools under Documentation.)

We also won't remove the DocBook gpu.tmpl yet so it's easier to build
both and compare the results for parity.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/bc7b4f9ac037632e0c8469c079d21fad5eaa39a0.1466506505.git.jani.nikula@intel.com

2 files changed, 2602 insertions, 0 deletions

diff --git a/Documentation/gpu/index.rst b/Documentation/gpu/index.rst
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+==================================
+Linux GPU Driver Developer's Guide
+==================================
+
+:Author: Jesse Barnes Initial version
+:Author: Laurent Pinchart Driver internals
+:Author: Daniel Vetter Contributions all over the place
+:Author: Lukas Wunner vga_switcheroo documentation
+:Date:   2015-10-11
+
+This first part of the GPU Driver Developer's Guide documents core DRM
+code, helper libraries for writing drivers and generic userspace
+interfaces exposed by DRM drivers.
+
+Introduction
+============
+
+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 may
+make use of DRM functions to make tasks like memory management,
+interrupt handling and DMA easier, and provide a uniform interface to
+applications.
+
+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.
+
+[Insert diagram of typical DRM stack here]
+
+Style Guidelines
+----------------
+
+For consistency this documentation uses American English. Abbreviations
+are written as all-uppercase, for example: DRM, KMS, IOCTL, CRTC, and so
+on. To aid in reading, documentations make full use of the markup
+characters kerneldoc provides: @parameter for function parameters,
+@member for structure members, &structure to reference structures and
+function() for functions. These all get automatically hyperlinked if
+kerneldoc for the referenced objects exists. When referencing entries in
+function vtables please use ->vfunc(). Note that kerneldoc does not
+support referencing struct members directly, so please add a reference
+to the vtable struct somewhere in the same paragraph or at least
+section.
+
+Except in special situations (to separate locked from unlocked variants)
+locking requirements for functions aren't documented in the kerneldoc.
+Instead locking should be check at runtime using e.g.
+``WARN_ON(!mutex_is_locked(...));``. Since it's much easier to ignore
+documentation than runtime noise this provides more value. And on top of
+that runtime checks do need to be updated when the locking rules change,
+increasing the chances that they're correct. Within the documentation
+the locking rules should be explained in the relevant structures: Either
+in the comment for the lock explaining what it protects, or data fields
+need a note about which lock protects them, or both.
+
+Functions which have a non-\ ``void`` return value should have a section
+called "Returns" explaining the expected return values in different
+cases and their meanings. Currently there's no consensus whether that
+section name should be all upper-case or not, and whether it should end
+in a colon or not. Go with the file-local style. Other common section
+names are "Notes" with information for dangerous or tricky corner cases,
+and "FIXME" where the interface could be cleaned up.
+
+DRM Internals
+=============
+
+This chapter documents DRM internals relevant to driver authors and
+developers working to add support for the latest features to existing
+drivers.
+
+First, we go over some typical driver initialization requirements, like
+setting up command buffers, creating an initial output configuration,
+and initializing core services. Subsequent sections cover core internals
+in more detail, providing implementation notes and examples.
+
+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 & fencing, suspend/resume support, and
+DMA services.
+
+Driver Initialization
+---------------------
+
+At the core of every DRM driver is a :c:type:`struct drm_driver
+<drm_driver>` structure. Drivers typically statically initialize
+a drm_driver structure, and then pass it to
+:c:func:`drm_dev_alloc()` to allocate a device instance. After the
+device instance is fully initialized it can be registered (which makes
+it accessible from userspace) using :c:func:`drm_dev_register()`.
+
+The :c:type:`struct drm_driver <drm_driver>` structure
+contains static information that describes the driver and features it
+supports, and pointers to methods that the DRM core will call to
+implement the DRM API. We will first go through the :c:type:`struct
+drm_driver <drm_driver>` static information fields, and will
+then describe individual operations in details as they get used in later
+sections.
+
+Driver Information
+~~~~~~~~~~~~~~~~~~
+
+Driver Features
+^^^^^^^^^^^^^^^
+
+Drivers inform the DRM core about their requirements and supported
+features by setting appropriate flags in the driver_features field.
+Since those flags influence the DRM core behaviour since registration
+time, most of them must be set to registering the :c:type:`struct
+drm_driver <drm_driver>` instance.
+
+u32 driver_features;
+
+DRIVER_USE_AGP
+    Driver uses AGP interface, the DRM core will manage AGP resources.
+
+DRIVER_REQUIRE_AGP
+    Driver needs AGP interface to function. AGP initialization failure
+    will become a fatal error.
+
+DRIVER_PCI_DMA
+    Driver is capable of PCI DMA, mapping of PCI DMA buffers to
+    userspace will be enabled. Deprecated.
+
+DRIVER_SG
+    Driver can perform scatter/gather DMA, allocation and mapping of
+    scatter/gather buffers will be enabled. Deprecated.
+
+DRIVER_HAVE_DMA
+    Driver supports DMA, the userspace DMA API will be supported.
+    Deprecated.
+
+DRIVER_HAVE_IRQ; DRIVER_IRQ_SHARED
+    DRIVER_HAVE_IRQ indicates whether the driver has an IRQ handler
+    managed by the DRM Core. The core will support simple IRQ handler
+    installation when the flag is set. The installation process is
+    described in ?.
+
+    DRIVER_IRQ_SHARED indicates whether the device & handler support
+    shared IRQs (note that this is required of PCI drivers).
+
+DRIVER_GEM
+    Driver use the GEM memory manager.
+
+DRIVER_MODESET
+    Driver supports mode setting interfaces (KMS).
+
+DRIVER_PRIME
+    Driver implements DRM PRIME buffer sharing.
+
+DRIVER_RENDER
+    Driver supports dedicated render nodes.
+
+DRIVER_ATOMIC
+    Driver supports atomic properties. In this case the driver must
+    implement appropriate obj->atomic_get_property() vfuncs for any
+    modeset objects with driver specific properties.
+
+Major, Minor and Patchlevel
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+int major; int minor; int patchlevel;
+The DRM core identifies driver versions by a major, minor and patch
+level triplet. The information is printed to the kernel log at
+initialization time and passed to userspace through the
+DRM_IOCTL_VERSION ioctl.
+
+The major and minor numbers are also used to verify the requested driver
+API version passed to DRM_IOCTL_SET_VERSION. When the driver API
+changes between minor versions, applications can call
+DRM_IOCTL_SET_VERSION to select a specific version of the API. If the
+requested major isn't equal to the driver major, or the requested minor
+is larger than the driver minor, the DRM_IOCTL_SET_VERSION call will
+return an error. Otherwise the driver's set_version() method will be
+called with the requested version.
+
+Name, Description and Date
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+char \*name; char \*desc; char \*date;
+The driver name is printed to the kernel log at initialization time,
+used for IRQ registration and passed to userspace through
+DRM_IOCTL_VERSION.
+
+The driver description is a purely informative string passed to
+userspace through the DRM_IOCTL_VERSION ioctl and otherwise unused by
+the kernel.
+
+The driver date, formatted as YYYYMMDD, is meant to identify the date of
+the latest modification to the driver. However, as most drivers fail to
+update it, its value is mostly useless. The DRM core prints it to the
+kernel log at initialization time and passes it to userspace through the
+DRM_IOCTL_VERSION ioctl.
+
+Device Instance and Driver Handling
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. kernel-doc:: drivers/gpu/drm/drm_drv.c
+   :doc: driver instance overview
+
+.. kernel-doc:: drivers/gpu/drm/drm_drv.c
+   :export:
+
+Driver Load
+~~~~~~~~~~~
+
+IRQ Registration
+^^^^^^^^^^^^^^^^
+
+The DRM core tries to facilitate IRQ handler registration and
+unregistration by providing :c:func:`drm_irq_install()` and
+:c:func:`drm_irq_uninstall()` functions. Those functions only
+support a single interrupt per device, devices that use more than one
+IRQs need to be handled manually.
+
+Managed IRQ Registration
+''''''''''''''''''''''''
+
+:c:func:`drm_irq_install()` starts by calling the irq_preinstall
+driver operation. The operation is optional and must make sure that the
+interrupt will not get fired by clearing all pending interrupt flags or
+disabling the interrupt.
+
+The passed-in IRQ will then be requested by a call to
+:c:func:`request_irq()`. If the DRIVER_IRQ_SHARED driver feature
+flag is set, a shared (IRQF_SHARED) IRQ handler will be requested.
+
+The IRQ handler function must be provided as the mandatory irq_handler
+driver operation. It will get passed directly to
+:c:func:`request_irq()` and thus has the same prototype as all IRQ
+handlers. It will get called with a pointer to the DRM device as the
+second argument.
+
+Finally the function calls the optional irq_postinstall driver
+operation. The operation usually enables interrupts (excluding the
+vblank interrupt, which is enabled separately), but drivers may choose
+to enable/disable interrupts at a different time.
+
+:c:func:`drm_irq_uninstall()` is similarly used to uninstall an
+IRQ handler. It starts by waking up all processes waiting on a vblank
+interrupt to make sure they don't hang, and then calls the optional
+irq_uninstall driver operation. The operation must disable all hardware
+interrupts. Finally the function frees the IRQ by calling
+:c:func:`free_irq()`.
+
+Manual IRQ Registration
+'''''''''''''''''''''''
+
+Drivers that require multiple interrupt handlers can't use the managed
+IRQ registration functions. In that case IRQs must be registered and
+unregistered manually (usually with the :c:func:`request_irq()` and
+:c:func:`free_irq()` functions, or their devm_\* equivalent).
+
+When manually registering IRQs, drivers must not set the
+DRIVER_HAVE_IRQ driver feature flag, and must not provide the
+irq_handler driver operation. They must set the :c:type:`struct
+drm_device <drm_device>` irq_enabled field to 1 upon
+registration of the IRQs, and clear it to 0 after unregistering the
+IRQs.
+
+Memory Manager Initialization
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Every DRM driver requires a memory manager which must be initialized at
+load time. DRM currently contains two memory managers, the Translation
+Table Manager (TTM) and the Graphics Execution Manager (GEM). This
+document describes the use of the GEM memory manager only. See ? for
+details.
+
+Miscellaneous Device Configuration
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Another task that may be necessary for PCI devices 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 after the ROM has
+been mapped and any necessary information has been extracted, it should
+be unmapped; on many devices, the ROM address decoder is shared with
+other BARs, so leaving it mapped could cause undesired behaviour like
+hangs or memory corruption.
+
+Bus-specific Device Registration and PCI Support
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+A number of functions are provided to help with device registration. The
+functions deal with PCI and platform devices respectively and are only
+provided for historical reasons. These are all deprecated and shouldn't
+be used in new drivers. Besides that there's a few helpers for pci
+drivers.
+
+.. kernel-doc:: drivers/gpu/drm/drm_pci.c
+   :export:
+
+.. kernel-doc:: drivers/gpu/drm/drm_platform.c
+   :export:
+
+Memory management
+-----------------
+
+Modern Linux systems require large amount of graphics memory to store
+frame buffers, textures, vertices and other graphics-related data. Given
+the very dynamic nature of many of that data, managing graphics memory
+efficiently is thus crucial for the graphics stack and plays a central
+role in the DRM infrastructure.
+
+The DRM core includes two memory managers, namely Translation Table Maps
+(TTM) and Graphics Execution Manager (GEM). TTM was the first DRM memory
+manager to be developed and tried to be a one-size-fits-them all
+solution. It provides a single userspace API to accommodate the need of
+all hardware, supporting both Unified Memory Architecture (UMA) devices
+and devices with dedicated video RAM (i.e. most discrete video cards).
+This resulted in a large, complex piece of code that turned out to be
+hard to use for driver development.
+
+GEM started as an Intel-sponsored project in reaction to TTM's
+complexity. Its design philosophy is completely different: instead of
+providing a solution to every graphics memory-related problems, GEM
+identified common code between drivers and created a support library to
+share it. GEM has simpler initialization and execution requirements than
+TTM, but has no video RAM management capabilities and is thus limited to
+UMA devices.
+
+The Translation Table Manager (TTM)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+TTM design background and information belongs here.
+
+TTM initialization
+^^^^^^^^^^^^^^^^^^
+
+    **Warning**
+
+    This section is outdated.
+
+Drivers wishing to support TTM must fill out a drm_bo_driver
+structure. The structure 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.
+
+The ttm_global_reference structure is made up of several fields:
+
+::
+
+              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 *);
+              };
+
+
+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 probably
+eventually call ttm_mem_global_init and ttm_mem_global_release,
+respectively.
+
+Once your global TTM accounting structure is set up and initialized by
+calling ttm_global_item_ref() on it, you 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 may
+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 initialization function.
+
+The Graphics Execution Manager (GEM)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The GEM design approach has resulted in a memory manager that doesn't
+provide full coverage of all (or even all common) use cases in its
+userspace or kernel API. GEM exposes a set of standard memory-related
+operations to userspace and a set of helper functions to drivers, and
+let drivers implement hardware-specific operations with their own
+private API.
+
+The GEM userspace API is described in the `GEM - the Graphics Execution
+Manager <http://lwn.net/Articles/283798/>`__ article on LWN. While
+slightly outdated, the document provides a good overview of the GEM API
+principles. Buffer allocation and read and write operations, described
+as part of the common GEM API, are currently implemented using
+driver-specific ioctls.
+
+GEM is data-agnostic. It manages abstract buffer objects without knowing
+what individual buffers contain. APIs that require knowledge of buffer
+contents or purpose, such as buffer allocation or synchronization
+primitives, are thus outside of the scope of GEM and must be implemented
+using driver-specific ioctls.
+
+On a fundamental level, GEM involves several operations:
+
+-  Memory allocation and freeing
+-  Command execution
+-  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.
+
+Device-specific operations, such as command execution, pinning, buffer
+read & write, mapping, and domain ownership transfers are left to
+driver-specific ioctls.
+
+GEM Initialization
+^^^^^^^^^^^^^^^^^^
+
+Drivers that use GEM must set the DRIVER_GEM bit in the struct
+:c:type:`struct drm_driver <drm_driver>` driver_features
+field. The DRM core will then automatically initialize the GEM core
+before calling the load operation. Behind the scene, this will create a
+DRM Memory Manager object which provides an address space pool for
+object allocation.
+
+In a KMS configuration, drivers need to allocate and initialize a
+command ring buffer following core GEM initialization if required by the
+hardware. UMA devices usually have what is called a "stolen" memory
+region, which provides space for the initial framebuffer and large,
+contiguous memory regions required by the device. This space is
+typically not managed by GEM, and must be initialized separately into
+its own DRM MM object.
+
+GEM Objects Creation
+^^^^^^^^^^^^^^^^^^^^
+
+GEM splits creation of GEM objects and allocation of the memory that
+backs them in two distinct operations.
+
+GEM objects are represented by an instance of struct :c:type:`struct
+drm_gem_object <drm_gem_object>`. Drivers usually need to
+extend GEM objects with private information and thus create a
+driver-specific GEM object structure type that embeds an instance of
+struct :c:type:`struct drm_gem_object <drm_gem_object>`.
+
+To create a GEM object, a driver allocates memory for an instance of its
+specific GEM object type and initializes the embedded struct
+:c:type:`struct drm_gem_object <drm_gem_object>` with a call
+to :c:func:`drm_gem_object_init()`. The function takes a pointer
+to the DRM device, a pointer to the GEM object and the buffer object
+size in bytes.
+
+GEM uses shmem to allocate anonymous pageable memory.
+:c:func:`drm_gem_object_init()` will create an shmfs file of the
+requested size and store it into the struct :c:type:`struct
+drm_gem_object <drm_gem_object>` filp field. The memory is
+used as either main storage for the object when the graphics hardware
+uses system memory directly or as a backing store otherwise.
+
+Drivers are responsible for the actual physical pages allocation by
+calling :c:func:`shmem_read_mapping_page_gfp()` for each page.
+Note that they can decide to allocate pages when initializing the GEM
+object, or to delay allocation until the memory is needed (for instance
+when a page fault occurs as a result of a userspace memory access or
+when the driver needs to start a DMA transfer involving the memory).
+
+Anonymous pageable memory allocation is not always desired, for instance
+when the hardware requires physically contiguous system memory as is
+often the case in embedded devices. Drivers can create GEM objects with
+no shmfs backing (called private GEM objects) by initializing them with
+a call to :c:func:`drm_gem_private_object_init()` instead of
+:c:func:`drm_gem_object_init()`. Storage for private GEM objects
+must be managed by drivers.
+
+GEM Objects Lifetime
+^^^^^^^^^^^^^^^^^^^^
+
+All GEM objects are reference-counted by the GEM core. References can be
+acquired and release by :c:func:`calling
+drm_gem_object_reference()` and
+:c:func:`drm_gem_object_unreference()` respectively. The caller
+must hold the :c:type:`struct drm_device <drm_device>`
+struct_mutex lock when calling
+:c:func:`drm_gem_object_reference()`. As a convenience, GEM
+provides :c:func:`drm_gem_object_unreference_unlocked()`
+functions that can be called without holding the lock.
+
+When the last reference to a GEM object is released the GEM core calls
+the :c:type:`struct drm_driver <drm_driver>` gem_free_object
+operation. That operation is mandatory for GEM-enabled drivers and must
+free the GEM object and all associated resources.
+
+void (\*gem_free_object) (struct drm_gem_object \*obj); Drivers are
+responsible for freeing all GEM object resources. This includes the
+resources created by the GEM core, which need to be released with
+:c:func:`drm_gem_object_release()`.
+
+GEM Objects Naming
+^^^^^^^^^^^^^^^^^^
+
+Communication between userspace and the kernel refers to GEM objects
+using local handles, global names or, more recently, file descriptors.
+All of those are 32-bit integer values; the usual Linux kernel limits
+apply to the file descriptors.
+
+GEM handles are local to a DRM file. Applications get a handle to a GEM
+object through a driver-specific ioctl, and can use that handle to refer
+to the GEM object in other standard or driver-specific ioctls. Closing a
+DRM file handle frees all its GEM handles and dereferences the
+associated GEM objects.
+
+To create a handle for a GEM object drivers call
+:c:func:`drm_gem_handle_create()`. The function takes a pointer
+to the DRM file and the GEM object and returns a locally unique handle.
+When the handle is no longer needed drivers delete it with a call to
+:c:func:`drm_gem_handle_delete()`. Finally the GEM object
+associated with a handle can be retrieved by a call to
+:c:func:`drm_gem_object_lookup()`.
+
+Handles don't take ownership of GEM objects, they only take a reference
+to the object that will be dropped when the handle is destroyed. To
+avoid leaking GEM objects, drivers must make sure they drop the
+reference(s) they own (such as the initial reference taken at object
+creation time) as appropriate, without any special consideration for the
+handle. For example, in the particular case of combined GEM object and
+handle creation in the implementation of the dumb_create operation,
+drivers must drop the initial reference to the GEM object before
+returning the handle.
+
+GEM names are similar in purpose to handles but are not local to DRM
+files. They can be passed between processes to reference a GEM object
+globally. Names can't be used directly to refer to objects in the DRM
+API, applications must convert handles to names and names to handles
+using the DRM_IOCTL_GEM_FLINK and DRM_IOCTL_GEM_OPEN ioctls
+respectively. The conversion is handled by the DRM core without any
+driver-specific support.
+
+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 ?. 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.
+
+GEM Objects Mapping
+^^^^^^^^^^^^^^^^^^^
+
+Because mapping operations are fairly heavyweight GEM favours
+read/write-like access to buffers, implemented through driver-specific
+ioctls, over mapping buffers to userspace. However, when random access
+to the buffer is needed (to perform software rendering for instance),
+direct access to the object can be more efficient.
+
+The mmap system call can't be used directly to map GEM objects, as they
+don't have their own file handle. Two alternative methods currently
+co-exist to map GEM objects to userspace. The first method uses a
+driver-specific ioctl to perform the mapping operation, calling
+:c:func:`do_mmap()` under the hood. This is often considered
+dubious, seems to be discouraged for new GEM-enabled drivers, and will
+thus not be described here.
+
+The second method uses the mmap system call on the DRM file handle. void
+\*mmap(void \*addr, size_t length, int prot, int flags, int fd, off_t
+offset); DRM identifies the GEM object to be mapped by a fake offset
+passed through the mmap offset argument. Prior to being mapped, a GEM
+object must thus be associated with a fake offset. To do so, drivers
+must call :c:func:`drm_gem_create_mmap_offset()` on the object.
+
+Once allocated, the fake offset value must be passed to the application
+in a driver-specific way and can then be used as the mmap offset
+argument.
+
+The GEM core provides a helper method :c:func:`drm_gem_mmap()` to
+handle object mapping. The method can be set directly as the mmap file
+operation handler. It will look up the GEM object based on the offset
+value and set the VMA operations to the :c:type:`struct drm_driver
+<drm_driver>` gem_vm_ops field. Note that
+:c:func:`drm_gem_mmap()` doesn't map memory to userspace, but
+relies on the driver-provided fault handler to map pages individually.
+
+To use :c:func:`drm_gem_mmap()`, drivers must fill the struct
+:c:type:`struct drm_driver <drm_driver>` gem_vm_ops field
+with a pointer to VM operations.
+
+struct vm_operations_struct \*gem_vm_ops struct
+vm_operations_struct { void (\*open)(struct vm_area_struct \* area);
+void (\*close)(struct vm_area_struct \* area); int (\*fault)(struct
+vm_area_struct \*vma, struct vm_fault \*vmf); };
+
+The open and close operations must update the GEM object reference
+count. Drivers can use the :c:func:`drm_gem_vm_open()` and
+:c:func:`drm_gem_vm_close()` helper functions directly as open
+and close handlers.
+
+The fault operation handler is responsible for mapping individual pages
+to userspace when a page fault occurs. Depending on the memory
+allocation scheme, drivers can allocate pages at fault time, or can
+decide to allocate memory for the GEM object at the time the object is
+created.
+
+Drivers that want to map the GEM object upfront instead of handling page
+faults can implement their own mmap file operation handler.
+
+Memory Coherency
+^^^^^^^^^^^^^^^^
+
+When mapped to the device or used in a command buffer, backing pages for
+an object are flushed to memory and marked write combined so as to be
+coherent with the GPU. Likewise, if the CPU accesses an object after the
+GPU has finished rendering to the object, then the object must be made
+coherent with the CPU's view of memory, usually involving GPU cache
+flushing of various kinds. This core CPU<->GPU coherency management is
+provided by a device-specific ioctl, 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, setting the domain
+blocks the client and waits for rendering to complete before performing
+any necessary flushing operations).
+
+Command Execution
+^^^^^^^^^^^^^^^^^
+
+Perhaps the most important GEM function for GPU devices is providing a
+command execution interface to clients. Client programs construct
+command buffers containing references to previously allocated memory
+objects, and then submit them to GEM. At that point, GEM takes 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; otherwise, 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.
+
+GEM Function Reference
+~~~~~~~~~~~~~~~~~~~~~~
+
+.. kernel-doc:: drivers/gpu/drm/drm_gem.c
+   :export:
+
+.. kernel-doc:: include/drm/drm_gem.h
+   :internal:
+
+VMA Offset Manager
+~~~~~~~~~~~~~~~~~~
+
+.. kernel-doc:: drivers/gpu/drm/drm_vma_manager.c
+   :doc: vma offset manager
+
+.. kernel-doc:: drivers/gpu/drm/drm_vma_manager.c
+   :export:
+
+.. kernel-doc:: include/drm/drm_vma_manager.h
+   :internal:
+
+PRIME Buffer Sharing
+~~~~~~~~~~~~~~~~~~~~
+
+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.
+
+Overview and Driver Interface
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+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.
+
+Drivers that support the PRIME API must set the DRIVER_PRIME bit in the
+struct :c:type:`struct drm_driver <drm_driver>`
+driver_features field, and implement the prime_handle_to_fd and
+prime_fd_to_handle operations.
+
+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); 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 unsigned
+integers.
+
+While non-GEM drivers must implement the operations themselves, GEM
+drivers must use the :c:func:`drm_gem_prime_handle_to_fd()` and
+:c:func:`drm_gem_prime_fd_to_handle()` helper functions. Those
+helpers rely on the driver gem_prime_export and gem_prime_import
+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).
+
+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); These two operations are mandatory for GEM drivers that
+support PRIME.
+
+PRIME Helper Functions
+^^^^^^^^^^^^^^^^^^^^^^
+
+.. kernel-doc:: drivers/gpu/drm/drm_prime.c
+   :doc: PRIME Helpers
+
+PRIME Function References
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. kernel-doc:: drivers/gpu/drm/drm_prime.c
+   :export:
+
+DRM MM Range Allocator
+~~~~~~~~~~~~~~~~~~~~~~
+
+Overview
+^^^^^^^^
+
+.. kernel-doc:: drivers/gpu/drm/drm_mm.c
+   :doc: Overview
+
+LRU Scan/Eviction Support
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+.. kernel-doc:: drivers/gpu/drm/drm_mm.c
+   :doc: lru scan roaster
+
+DRM MM Range Allocator Function References
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. kernel-doc:: drivers/gpu/drm/drm_mm.c
+   :export:
+
+.. kernel-doc:: include/drm/drm_mm.h
+   :internal:
+
+CMA Helper Functions Reference
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. kernel-doc:: drivers/gpu/drm/drm_gem_cma_helper.c
+   :doc: cma helpers
+
+.. kernel-doc:: drivers/gpu/drm/drm_gem_cma_helper.c
+   :export:
+
+.. kernel-doc:: include/drm/drm_gem_cma_helper.h
+   :internal:
+
+Mode Setting
+------------
+
+Drivers must initialize the mode setting core by calling
+:c:func:`drm_mode_config_init()` on the DRM device. The function
+initializes the :c:type:`struct drm_device <drm_device>`
+mode_config field and never fails. Once done, mode configuration must
+be setup by initializing the following fields.
+
+-  int min_width, min_height; int max_width, max_height;
+   Minimum and maximum width and height of the frame buffers in pixel
+   units.
+
+-  struct drm_mode_config_funcs \*funcs;
+   Mode setting functions.
+
+Display Modes Function Reference
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. kernel-doc:: include/drm/drm_modes.h
+   :internal:
+
+.. kernel-doc:: drivers/gpu/drm/drm_modes.c
+   :export:
+
+Atomic Mode Setting Function Reference
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. kernel-doc:: drivers/gpu/drm/drm_atomic.c
+   :export:
+
+.. kernel-doc:: drivers/gpu/drm/drm_atomic.c
+   :internal:
+
+Frame Buffer Abstraction
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+Frame buffers are abstract memory objects that provide a source of
+pixels to scanout to a CRTC. Applications explicitly request the
+creation of frame buffers through the DRM_IOCTL_MODE_ADDFB(2) ioctls
+and receive an opaque handle that can be passed to the KMS CRTC control,
+plane configuration and page flip functions.
+
+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 ``drm_mode_fb_cmd2`` 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.
+
+The lifetime of a drm framebuffer is controlled with a reference count,
+drivers can grab additional references with
+:c:func:`drm_framebuffer_reference()`and drop them again with
+:c:func:`drm_framebuffer_unreference()`. For driver-private
+framebuffers for which the last reference is never dropped (e.g. for the
+fbdev framebuffer when the struct :c:type:`struct drm_framebuffer
+<drm_framebuffer>` is embedded into the fbdev helper struct)
+drivers can manually clean up a framebuffer at module unload time with
+:c:func:`drm_framebuffer_unregister_private()`.
+
+DRM Format Handling
+~~~~~~~~~~~~~~~~~~~
+
+.. kernel-doc:: include/drm/drm_fourcc.h
+   :internal:
+
+.. kernel-doc:: drivers/gpu/drm/drm_fourcc.c
+   :export:
+
+Dumb Buffer Objects
+~~~~~~~~~~~~~~~~~~~
+
+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.
+
+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.
+
+To support dumb objects drivers must implement the dumb_create,
+dumb_destroy and dumb_map_offset operations.
+
+-  int (\*dumb_create)(struct drm_file \*file_priv, struct
+   drm_device \*dev, struct drm_mode_create_dumb \*args);
+   The dumb_create operation creates a driver object (GEM or TTM
+   handle) suitable for scanout based on the width, height and depth
+   from the struct :c:type:`struct drm_mode_create_dumb
+   <drm_mode_create_dumb>` argument. It fills the argument's
+   handle, pitch and size fields with a handle for the newly created
+   object and its line pitch and size in bytes.
+
+-  int (\*dumb_destroy)(struct drm_file \*file_priv, struct
+   drm_device \*dev, uint32_t handle);
+   The dumb_destroy operation destroys a dumb object created by
+   dumb_create.
+
+-  int (\*dumb_map_offset)(struct drm_file \*file_priv, struct
+   drm_device \*dev, uint32_t handle, uint64_t \*offset);
+   The dumb_map_offset operation associates an mmap fake offset with
+   the object given by the handle and returns it. Drivers must use the
+   :c:func:`drm_gem_create_mmap_offset()` function to associate
+   the fake offset as described in ?.
+
+Note that dumb objects may not be used for gpu acceleration, as has been
+attempted on some ARM embedded platforms. Such drivers really must have
+a hardware-specific ioctl t