=====================
  DRM 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      This section is outdated.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 pass a filled :c:type:`ttm_bo_driver
  <ttm_bo_driver>` structure to ttm_bo_device_init, together with an
  initialized global reference to the memory manager.  The ttm_bo_driver
  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.

  The :c:type:`struct drm_global_reference <drm_global_reference>` is made
  up of several fields:

  .. code-block:: c

                struct drm_global_reference {

                        enum ttm_global_types global_type;
                        size_t size;
                        void *object;

                        int (*init) (struct drm_global_reference *);
                        void (*release) (struct drm_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_ref_init() and
  ttm_bo_global_ref_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.

  See the radeon_ttm.c file for an example of usage.

  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 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.

  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 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 drm_gem_private_object_init() instead of 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 calling drm_gem_object_get() and drm_gem_object_put()

  respectively.

  
  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_unlocked

  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

  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 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 drm_gem_handle_delete(). Finally the GEM object associated with a
  handle can be retrieved by a call to 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

  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 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 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 drm_gem_mmap() doesn't map memory to
  userspace, but relies on the driver-provided fault handler to map pages
  individually.

  To use 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.

  The VM operations is a :c:type:`struct vm_operations_struct <vm_operations_struct>`
  made up of several fields, the more interesting ones being:
  
  .. code-block:: c
  
  	struct vm_operations_struct {
  		void (*open)(struct vm_area_struct * area);
  		void (*close)(struct vm_area_struct * area);

  		vm_fault_t (*fault)(struct vm_fault *vmf);

  	};

  
  The open and close operations must update the GEM object reference

  count. Drivers can use the drm_gem_vm_open() and 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.

  For platforms without MMU the GEM core provides a helper method

  drm_gem_cma_get_unmapped_area(). The mmap() routines will call this to get a
  proposed address for the mapping.

  To use drm_gem_cma_get_unmapped_area(), drivers must fill the struct
  :c:type:`struct file_operations <file_operations>` get_unmapped_area field with
  a pointer on drm_gem_cma_get_unmapped_area().

  
  More detailed information about get_unmapped_area can be found in

  Documentation/admin-guide/mm/nommu-mmap.rst

  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:: include/drm/drm_gem.h
     :internal:

  .. kernel-doc:: drivers/gpu/drm/drm_gem.c
     :export:

  GEM CMA Helper Functions Reference
  ----------------------------------
  
  .. kernel-doc:: drivers/gpu/drm/drm_gem_cma_helper.c
     :doc: cma helpers

  .. kernel-doc:: include/drm/drm_gem_cma_helper.h
     :internal:

  .. kernel-doc:: drivers/gpu/drm/drm_gem_cma_helper.c
     :export:

  GEM SHMEM Helper Function Reference
  -----------------------------------
  
  .. kernel-doc:: drivers/gpu/drm/drm_gem_shmem_helper.c
     :doc: overview
  
  .. kernel-doc:: include/drm/drm_gem_shmem_helper.h
     :internal:
  
  .. kernel-doc:: drivers/gpu/drm/drm_gem_shmem_helper.c
     :export:

  GEM VRAM Helper Functions Reference
  -----------------------------------
  
  .. kernel-doc:: drivers/gpu/drm/drm_gem_vram_helper.c
     :doc: overview
  
  .. kernel-doc:: include/drm/drm_gem_vram_helper.h
     :internal:
  
  .. kernel-doc:: drivers/gpu/drm/drm_gem_vram_helper.c
     :export:

  GEM TTM Helper Functions Reference
  -----------------------------------
  
  .. kernel-doc:: drivers/gpu/drm/drm_gem_ttm_helper.c
     :doc: overview

  .. kernel-doc:: drivers/gpu/drm/drm_gem_ttm_helper.c
     :export:

  VMA Offset Manager

  ==================

  
  .. kernel-doc:: drivers/gpu/drm/drm_vma_manager.c
     :doc: vma offset manager

  .. kernel-doc:: include/drm/drm_vma_manager.h
     :internal:

  .. kernel-doc:: drivers/gpu/drm/drm_vma_manager.c
     :export:

  .. _prime_buffer_sharing:

  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 Lifetime Rules
  ---------------------------
  
  .. kernel-doc:: drivers/gpu/drm/drm_prime.c
     :doc: overview and lifetime rules

  
  PRIME Helper Functions

  ----------------------

  
  .. kernel-doc:: drivers/gpu/drm/drm_prime.c
     :doc: PRIME Helpers
  
  PRIME Function References
  -------------------------

  .. kernel-doc:: include/drm/drm_prime.h
     :internal:

  .. 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 roster

  
  DRM MM Range Allocator Function References
  ------------------------------------------

  .. kernel-doc:: include/drm/drm_mm.h
     :internal:

  .. kernel-doc:: drivers/gpu/drm/drm_mm.c
     :export:

  DRM Cache Handling
  ==================
  
  .. kernel-doc:: drivers/gpu/drm/drm_cache.c
     :export:

  
  DRM Sync Objects
  ===========================
  
  .. kernel-doc:: drivers/gpu/drm/drm_syncobj.c
     :doc: Overview
  
  .. kernel-doc:: include/drm/drm_syncobj.h

     :internal:

  
  .. kernel-doc:: drivers/gpu/drm/drm_syncobj.c
     :export:

  
  GPU Scheduler
  =============
  
  Overview
  --------

  .. kernel-doc:: drivers/gpu/drm/scheduler/sched_main.c

     :doc: Overview
  
  Scheduler Function References
  -----------------------------
  
  .. kernel-doc:: include/drm/gpu_scheduler.h
     :internal:

  .. kernel-doc:: drivers/gpu/drm/scheduler/sched_main.c

     :export: