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 =====================
 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 Manager
 (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)
 ===================================
 
 .. kernel-doc:: drivers/gpu/drm/ttm/ttm_module.c
    :doc: TTM
 
 .. kernel-doc:: include/drm/ttm/ttm_caching.h
    :internal:
 
 TTM device object reference
 ---------------------------
 
 .. kernel-doc:: include/drm/ttm/ttm_device.h
    :internal:
 
 .. kernel-doc:: drivers/gpu/drm/ttm/ttm_device.c
    :export:
 
 TTM resource placement reference
 --------------------------------
 
 .. kernel-doc:: include/drm/ttm/ttm_placement.h
    :internal:
 
 TTM resource object reference
 -----------------------------
 
 .. kernel-doc:: include/drm/ttm/ttm_resource.h
    :internal:
 
 .. kernel-doc:: drivers/gpu/drm/ttm/ttm_resource.c
    :export:
 
 TTM TT object reference
 -----------------------
 
 .. kernel-doc:: include/drm/ttm/ttm_tt.h
    :internal:
 
 .. kernel-doc:: drivers/gpu/drm/ttm/ttm_tt.c
    :export:
 
 TTM page pool reference
 -----------------------
 
 .. kernel-doc:: include/drm/ttm/ttm_pool.h
    :internal:
 
 .. kernel-doc:: drivers/gpu/drm/ttm/ttm_pool.c
    :export:
 
 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_gem_object_funcs <gem_object_funcs>` free
 operation. That operation is mandatory for GEM-enabled drivers and must
 free the GEM object and all associated resources.
 
 void (\*free) (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 Buddy Allocator
 ===================
 
 DRM Buddy Function References
 -----------------------------
 
 .. kernel-doc:: drivers/gpu/drm/drm_buddy.c
    :export:
 
 DRM Cache Handling and Fast WC memcpy()
 =======================================
 
 .. 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:
 
 .. kernel-doc:: drivers/gpu/drm/scheduler/sched_entity.c
    :export:

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