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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
TTM device object reference
.. kernel-doc:: include/drm/ttm/ttm_device.h
.. kernel-doc:: drivers/gpu/drm/ttm/ttm_device.c
TTM resource placement reference
.. kernel-doc:: include/drm/ttm/ttm_placement.h
TTM resource object reference
.. kernel-doc:: include/drm/ttm/ttm_resource.h
.. kernel-doc:: drivers/gpu/drm/ttm/ttm_resource.c
TTM TT object reference
.. kernel-doc:: include/drm/ttm/ttm_tt.h
.. kernel-doc:: drivers/gpu/drm/ttm/ttm_tt.c
TTM page pool reference
.. kernel-doc:: include/drm/ttm/ttm_pool.h
.. kernel-doc:: drivers/gpu/drm/ttm/ttm_pool.c
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 <>`__ 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
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()
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
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
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
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
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
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_dma_get_unmapped_area(). The mmap() routines will call this to get a
proposed address for the mapping.
To use drm_gem_dma_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_dma_get_unmapped_area().
More detailed information about get_unmapped_area can be found in
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
.. kernel-doc:: drivers/gpu/drm/drm_gem.c
GEM DMA Helper Functions Reference
.. kernel-doc:: drivers/gpu/drm/drm_gem_dma_helper.c
:doc: dma helpers
.. kernel-doc:: include/drm/drm_gem_dma_helper.h
.. kernel-doc:: drivers/gpu/drm/drm_gem_dma_helper.c
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
.. kernel-doc:: drivers/gpu/drm/drm_gem_shmem_helper.c
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
.. kernel-doc:: drivers/gpu/drm/drm_gem_vram_helper.c
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
VMA Offset Manager
.. kernel-doc:: drivers/gpu/drm/drm_vma_manager.c
:doc: vma offset manager
.. kernel-doc:: include/drm/drm_vma_manager.h
.. kernel-doc:: drivers/gpu/drm/drm_vma_manager.c
.. _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
.. kernel-doc:: drivers/gpu/drm/drm_prime.c
DRM MM Range Allocator
.. 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
.. kernel-doc:: drivers/gpu/drm/drm_mm.c
.. _drm_gpuvm:
.. kernel-doc:: drivers/gpu/drm/drm_gpuvm.c
:doc: Overview
Split and Merge
.. kernel-doc:: drivers/gpu/drm/drm_gpuvm.c
:doc: Split and Merge
.. _drm_gpuvm_locking:
.. kernel-doc:: drivers/gpu/drm/drm_gpuvm.c
:doc: Locking
.. kernel-doc:: drivers/gpu/drm/drm_gpuvm.c
:doc: Examples
DRM GPUVM Function References
.. kernel-doc:: include/drm/drm_gpuvm.h
.. kernel-doc:: drivers/gpu/drm/drm_gpuvm.c
DRM Buddy Allocator
DRM Buddy Function References
.. kernel-doc:: drivers/gpu/drm/drm_buddy.c
DRM Cache Handling and Fast WC memcpy()
.. kernel-doc:: drivers/gpu/drm/drm_cache.c
.. _drm_sync_objects:
DRM Sync Objects
.. kernel-doc:: drivers/gpu/drm/drm_syncobj.c
:doc: Overview
.. kernel-doc:: include/drm/drm_syncobj.h
.. kernel-doc:: drivers/gpu/drm/drm_syncobj.c
DRM Execution context
.. kernel-doc:: drivers/gpu/drm/drm_exec.c
:doc: Overview
.. kernel-doc:: include/drm/drm_exec.h
.. kernel-doc:: drivers/gpu/drm/drm_exec.c
GPU Scheduler
.. kernel-doc:: drivers/gpu/drm/scheduler/sched_main.c
:doc: Overview
Flow Control
.. kernel-doc:: drivers/gpu/drm/scheduler/sched_main.c
:doc: Flow Control
Scheduler Function References
.. kernel-doc:: include/drm/gpu_scheduler.h
.. kernel-doc:: drivers/gpu/drm/scheduler/sched_main.c
.. kernel-doc:: drivers/gpu/drm/scheduler/sched_entity.c