455 lines
19 KiB
ReStructuredText
455 lines
19 KiB
ReStructuredText
=====================
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DRM Memory Management
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=====================
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Modern Linux systems require large amount of graphics memory to store
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frame buffers, textures, vertices and other graphics-related data. Given
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the very dynamic nature of many of that data, managing graphics memory
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efficiently is thus crucial for the graphics stack and plays a central
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role in the DRM infrastructure.
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The DRM core includes two memory managers, namely Translation Table Maps
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(TTM) and Graphics Execution Manager (GEM). TTM was the first DRM memory
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manager to be developed and tried to be a one-size-fits-them all
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solution. It provides a single userspace API to accommodate the need of
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all hardware, supporting both Unified Memory Architecture (UMA) devices
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and devices with dedicated video RAM (i.e. most discrete video cards).
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This resulted in a large, complex piece of code that turned out to be
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hard to use for driver development.
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GEM started as an Intel-sponsored project in reaction to TTM's
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complexity. Its design philosophy is completely different: instead of
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providing a solution to every graphics memory-related problems, GEM
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identified common code between drivers and created a support library to
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share it. GEM has simpler initialization and execution requirements than
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TTM, but has no video RAM management capabilities and is thus limited to
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UMA devices.
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The Translation Table Manager (TTM)
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===================================
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TTM design background and information belongs here.
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TTM initialization
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------------------
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**Warning**
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This section is outdated.
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Drivers wishing to support TTM must fill out a drm_bo_driver
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structure. The structure contains several fields with function pointers
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for initializing the TTM, allocating and freeing memory, waiting for
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command completion and fence synchronization, and memory migration. See
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the radeon_ttm.c file for an example of usage.
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The ttm_global_reference structure is made up of several fields:
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::
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struct ttm_global_reference {
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enum ttm_global_types global_type;
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size_t size;
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void *object;
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int (*init) (struct ttm_global_reference *);
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void (*release) (struct ttm_global_reference *);
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};
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There should be one global reference structure for your memory manager
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as a whole, and there will be others for each object created by the
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memory manager at runtime. Your global TTM should have a type of
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TTM_GLOBAL_TTM_MEM. The size field for the global object should be
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sizeof(struct ttm_mem_global), and the init and release hooks should
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point at your driver-specific init and release routines, which probably
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eventually call ttm_mem_global_init and ttm_mem_global_release,
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respectively.
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Once your global TTM accounting structure is set up and initialized by
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calling ttm_global_item_ref() on it, you need to create a buffer
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object TTM to provide a pool for buffer object allocation by clients and
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the kernel itself. The type of this object should be
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TTM_GLOBAL_TTM_BO, and its size should be sizeof(struct
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ttm_bo_global). Again, driver-specific init and release functions may
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be provided, likely eventually calling ttm_bo_global_init() and
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ttm_bo_global_release(), respectively. Also, like the previous
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object, ttm_global_item_ref() is used to create an initial reference
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count for the TTM, which will call your initialization function.
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The Graphics Execution Manager (GEM)
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====================================
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The GEM design approach has resulted in a memory manager that doesn't
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provide full coverage of all (or even all common) use cases in its
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userspace or kernel API. GEM exposes a set of standard memory-related
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operations to userspace and a set of helper functions to drivers, and
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let drivers implement hardware-specific operations with their own
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private API.
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The GEM userspace API is described in the `GEM - the Graphics Execution
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Manager <http://lwn.net/Articles/283798/>`__ article on LWN. While
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slightly outdated, the document provides a good overview of the GEM API
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principles. Buffer allocation and read and write operations, described
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as part of the common GEM API, are currently implemented using
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driver-specific ioctls.
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GEM is data-agnostic. It manages abstract buffer objects without knowing
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what individual buffers contain. APIs that require knowledge of buffer
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contents or purpose, such as buffer allocation or synchronization
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primitives, are thus outside of the scope of GEM and must be implemented
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using driver-specific ioctls.
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On a fundamental level, GEM involves several operations:
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- Memory allocation and freeing
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- Command execution
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- Aperture management at command execution time
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Buffer object allocation is relatively straightforward and largely
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provided by Linux's shmem layer, which provides memory to back each
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object.
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Device-specific operations, such as command execution, pinning, buffer
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read & write, mapping, and domain ownership transfers are left to
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driver-specific ioctls.
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GEM Initialization
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------------------
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Drivers that use GEM must set the DRIVER_GEM bit in the struct
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:c:type:`struct drm_driver <drm_driver>` driver_features
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field. The DRM core will then automatically initialize the GEM core
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before calling the load operation. Behind the scene, this will create a
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DRM Memory Manager object which provides an address space pool for
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object allocation.
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In a KMS configuration, drivers need to allocate and initialize a
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command ring buffer following core GEM initialization if required by the
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hardware. UMA devices usually have what is called a "stolen" memory
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region, which provides space for the initial framebuffer and large,
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contiguous memory regions required by the device. This space is
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typically not managed by GEM, and must be initialized separately into
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its own DRM MM object.
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GEM Objects Creation
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--------------------
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GEM splits creation of GEM objects and allocation of the memory that
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backs them in two distinct operations.
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GEM objects are represented by an instance of struct :c:type:`struct
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drm_gem_object <drm_gem_object>`. Drivers usually need to
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extend GEM objects with private information and thus create a
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driver-specific GEM object structure type that embeds an instance of
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struct :c:type:`struct drm_gem_object <drm_gem_object>`.
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To create a GEM object, a driver allocates memory for an instance of its
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specific GEM object type and initializes the embedded struct
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:c:type:`struct drm_gem_object <drm_gem_object>` with a call
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to :c:func:`drm_gem_object_init()`. The function takes a pointer
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to the DRM device, a pointer to the GEM object and the buffer object
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size in bytes.
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GEM uses shmem to allocate anonymous pageable memory.
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:c:func:`drm_gem_object_init()` will create an shmfs file of the
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requested size and store it into the struct :c:type:`struct
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drm_gem_object <drm_gem_object>` filp field. The memory is
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used as either main storage for the object when the graphics hardware
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uses system memory directly or as a backing store otherwise.
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Drivers are responsible for the actual physical pages allocation by
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calling :c:func:`shmem_read_mapping_page_gfp()` for each page.
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Note that they can decide to allocate pages when initializing the GEM
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object, or to delay allocation until the memory is needed (for instance
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when a page fault occurs as a result of a userspace memory access or
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when the driver needs to start a DMA transfer involving the memory).
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Anonymous pageable memory allocation is not always desired, for instance
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when the hardware requires physically contiguous system memory as is
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often the case in embedded devices. Drivers can create GEM objects with
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no shmfs backing (called private GEM objects) by initializing them with
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a call to :c:func:`drm_gem_private_object_init()` instead of
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:c:func:`drm_gem_object_init()`. Storage for private GEM objects
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must be managed by drivers.
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GEM Objects Lifetime
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--------------------
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All GEM objects are reference-counted by the GEM core. References can be
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acquired and release by :c:func:`calling
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drm_gem_object_reference()` and
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:c:func:`drm_gem_object_unreference()` respectively. The caller
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must hold the :c:type:`struct drm_device <drm_device>`
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struct_mutex lock when calling
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:c:func:`drm_gem_object_reference()`. As a convenience, GEM
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provides :c:func:`drm_gem_object_unreference_unlocked()`
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functions that can be called without holding the lock.
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When the last reference to a GEM object is released the GEM core calls
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the :c:type:`struct drm_driver <drm_driver>` gem_free_object
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operation. That operation is mandatory for GEM-enabled drivers and must
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free the GEM object and all associated resources.
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void (\*gem_free_object) (struct drm_gem_object \*obj); Drivers are
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responsible for freeing all GEM object resources. This includes the
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resources created by the GEM core, which need to be released with
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:c:func:`drm_gem_object_release()`.
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GEM Objects Naming
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------------------
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Communication between userspace and the kernel refers to GEM objects
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using local handles, global names or, more recently, file descriptors.
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All of those are 32-bit integer values; the usual Linux kernel limits
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apply to the file descriptors.
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GEM handles are local to a DRM file. Applications get a handle to a GEM
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object through a driver-specific ioctl, and can use that handle to refer
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to the GEM object in other standard or driver-specific ioctls. Closing a
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DRM file handle frees all its GEM handles and dereferences the
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associated GEM objects.
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To create a handle for a GEM object drivers call
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:c:func:`drm_gem_handle_create()`. The function takes a pointer
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to the DRM file and the GEM object and returns a locally unique handle.
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When the handle is no longer needed drivers delete it with a call to
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:c:func:`drm_gem_handle_delete()`. Finally the GEM object
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associated with a handle can be retrieved by a call to
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:c:func:`drm_gem_object_lookup()`.
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Handles don't take ownership of GEM objects, they only take a reference
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to the object that will be dropped when the handle is destroyed. To
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avoid leaking GEM objects, drivers must make sure they drop the
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reference(s) they own (such as the initial reference taken at object
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creation time) as appropriate, without any special consideration for the
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handle. For example, in the particular case of combined GEM object and
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handle creation in the implementation of the dumb_create operation,
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drivers must drop the initial reference to the GEM object before
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returning the handle.
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GEM names are similar in purpose to handles but are not local to DRM
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files. They can be passed between processes to reference a GEM object
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globally. Names can't be used directly to refer to objects in the DRM
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API, applications must convert handles to names and names to handles
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using the DRM_IOCTL_GEM_FLINK and DRM_IOCTL_GEM_OPEN ioctls
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respectively. The conversion is handled by the DRM core without any
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driver-specific support.
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GEM also supports buffer sharing with dma-buf file descriptors through
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PRIME. GEM-based drivers must use the provided helpers functions to
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implement the exporting and importing correctly. See ?. Since sharing
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file descriptors is inherently more secure than the easily guessable and
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global GEM names it is the preferred buffer sharing mechanism. Sharing
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buffers through GEM names is only supported for legacy userspace.
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Furthermore PRIME also allows cross-device buffer sharing since it is
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based on dma-bufs.
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GEM Objects Mapping
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-------------------
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Because mapping operations are fairly heavyweight GEM favours
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read/write-like access to buffers, implemented through driver-specific
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ioctls, over mapping buffers to userspace. However, when random access
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to the buffer is needed (to perform software rendering for instance),
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direct access to the object can be more efficient.
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The mmap system call can't be used directly to map GEM objects, as they
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don't have their own file handle. Two alternative methods currently
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co-exist to map GEM objects to userspace. The first method uses a
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driver-specific ioctl to perform the mapping operation, calling
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:c:func:`do_mmap()` under the hood. This is often considered
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dubious, seems to be discouraged for new GEM-enabled drivers, and will
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thus not be described here.
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The second method uses the mmap system call on the DRM file handle. void
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\*mmap(void \*addr, size_t length, int prot, int flags, int fd, off_t
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offset); DRM identifies the GEM object to be mapped by a fake offset
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passed through the mmap offset argument. Prior to being mapped, a GEM
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object must thus be associated with a fake offset. To do so, drivers
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must call :c:func:`drm_gem_create_mmap_offset()` on the object.
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Once allocated, the fake offset value must be passed to the application
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in a driver-specific way and can then be used as the mmap offset
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argument.
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The GEM core provides a helper method :c:func:`drm_gem_mmap()` to
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handle object mapping. The method can be set directly as the mmap file
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operation handler. It will look up the GEM object based on the offset
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value and set the VMA operations to the :c:type:`struct drm_driver
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<drm_driver>` gem_vm_ops field. Note that
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:c:func:`drm_gem_mmap()` doesn't map memory to userspace, but
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relies on the driver-provided fault handler to map pages individually.
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To use :c:func:`drm_gem_mmap()`, drivers must fill the struct
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:c:type:`struct drm_driver <drm_driver>` gem_vm_ops field
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with a pointer to VM operations.
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struct vm_operations_struct \*gem_vm_ops struct
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vm_operations_struct { void (\*open)(struct vm_area_struct \* area);
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void (\*close)(struct vm_area_struct \* area); int (\*fault)(struct
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vm_area_struct \*vma, struct vm_fault \*vmf); };
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The open and close operations must update the GEM object reference
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count. Drivers can use the :c:func:`drm_gem_vm_open()` and
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:c:func:`drm_gem_vm_close()` helper functions directly as open
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and close handlers.
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The fault operation handler is responsible for mapping individual pages
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to userspace when a page fault occurs. Depending on the memory
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allocation scheme, drivers can allocate pages at fault time, or can
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decide to allocate memory for the GEM object at the time the object is
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created.
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Drivers that want to map the GEM object upfront instead of handling page
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faults can implement their own mmap file operation handler.
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Memory Coherency
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----------------
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When mapped to the device or used in a command buffer, backing pages for
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an object are flushed to memory and marked write combined so as to be
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coherent with the GPU. Likewise, if the CPU accesses an object after the
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GPU has finished rendering to the object, then the object must be made
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coherent with the CPU's view of memory, usually involving GPU cache
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flushing of various kinds. This core CPU<->GPU coherency management is
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provided by a device-specific ioctl, which evaluates an object's current
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domain and performs any necessary flushing or synchronization to put the
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object into the desired coherency domain (note that the object may be
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busy, i.e. an active render target; in that case, setting the domain
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blocks the client and waits for rendering to complete before performing
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any necessary flushing operations).
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Command Execution
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-----------------
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Perhaps the most important GEM function for GPU devices is providing a
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command execution interface to clients. Client programs construct
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command buffers containing references to previously allocated memory
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objects, and then submit them to GEM. At that point, GEM takes care to
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bind all the objects into the GTT, execute the buffer, and provide
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necessary synchronization between clients accessing the same buffers.
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This often involves evicting some objects from the GTT and re-binding
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others (a fairly expensive operation), and providing relocation support
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which hides fixed GTT offsets from clients. Clients must take care not
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to submit command buffers that reference more objects than can fit in
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the GTT; otherwise, GEM will reject them and no rendering will occur.
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Similarly, if several objects in the buffer require fence registers to
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be allocated for correct rendering (e.g. 2D blits on pre-965 chips),
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care must be taken not to require more fence registers than are
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available to the client. Such resource management should be abstracted
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from the client in libdrm.
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GEM Function Reference
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----------------------
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.. kernel-doc:: drivers/gpu/drm/drm_gem.c
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:export:
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.. kernel-doc:: include/drm/drm_gem.h
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:internal:
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GEM CMA Helper Functions Reference
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----------------------------------
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.. kernel-doc:: drivers/gpu/drm/drm_gem_cma_helper.c
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:doc: cma helpers
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.. kernel-doc:: drivers/gpu/drm/drm_gem_cma_helper.c
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:export:
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.. kernel-doc:: include/drm/drm_gem_cma_helper.h
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:internal:
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VMA Offset Manager
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==================
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.. kernel-doc:: drivers/gpu/drm/drm_vma_manager.c
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:doc: vma offset manager
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.. kernel-doc:: drivers/gpu/drm/drm_vma_manager.c
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:export:
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.. kernel-doc:: include/drm/drm_vma_manager.h
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:internal:
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PRIME Buffer Sharing
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====================
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PRIME is the cross device buffer sharing framework in drm, originally
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created for the OPTIMUS range of multi-gpu platforms. To userspace PRIME
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buffers are dma-buf based file descriptors.
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Overview and Driver Interface
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-----------------------------
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Similar to GEM global names, PRIME file descriptors are also used to
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share buffer objects across processes. They offer additional security:
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as file descriptors must be explicitly sent over UNIX domain sockets to
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be shared between applications, they can't be guessed like the globally
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unique GEM names.
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Drivers that support the PRIME API must set the DRIVER_PRIME bit in the
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struct :c:type:`struct drm_driver <drm_driver>`
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driver_features field, and implement the prime_handle_to_fd and
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prime_fd_to_handle operations.
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int (\*prime_handle_to_fd)(struct drm_device \*dev, struct drm_file
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\*file_priv, uint32_t handle, uint32_t flags, int \*prime_fd); int
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(\*prime_fd_to_handle)(struct drm_device \*dev, struct drm_file
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\*file_priv, int prime_fd, uint32_t \*handle); Those two operations
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convert a handle to a PRIME file descriptor and vice versa. Drivers must
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use the kernel dma-buf buffer sharing framework to manage the PRIME file
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descriptors. Similar to the mode setting API PRIME is agnostic to the
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underlying buffer object manager, as long as handles are 32bit unsigned
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integers.
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While non-GEM drivers must implement the operations themselves, GEM
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drivers must use the :c:func:`drm_gem_prime_handle_to_fd()` and
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:c:func:`drm_gem_prime_fd_to_handle()` helper functions. Those
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helpers rely on the driver gem_prime_export and gem_prime_import
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operations to create a dma-buf instance from a GEM object (dma-buf
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exporter role) and to create a GEM object from a dma-buf instance
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(dma-buf importer role).
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struct dma_buf \* (\*gem_prime_export)(struct drm_device \*dev,
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struct drm_gem_object \*obj, int flags); struct drm_gem_object \*
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(\*gem_prime_import)(struct drm_device \*dev, struct dma_buf
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\*dma_buf); These two operations are mandatory for GEM drivers that
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support PRIME.
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PRIME Helper Functions
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----------------------
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.. kernel-doc:: drivers/gpu/drm/drm_prime.c
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:doc: PRIME Helpers
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PRIME Function References
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-------------------------
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.. kernel-doc:: drivers/gpu/drm/drm_prime.c
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:export:
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DRM MM Range Allocator
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======================
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Overview
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--------
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.. kernel-doc:: drivers/gpu/drm/drm_mm.c
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:doc: Overview
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LRU Scan/Eviction Support
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-------------------------
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.. kernel-doc:: drivers/gpu/drm/drm_mm.c
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:doc: lru scan roaster
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DRM MM Range Allocator Function References
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------------------------------------------
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.. kernel-doc:: drivers/gpu/drm/drm_mm.c
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:export:
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.. kernel-doc:: include/drm/drm_mm.h
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:internal:
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