Documentation/gpu/drm-mm: Add THP paragraph to GEM mapping section

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.

uses system memory directly or as a backing store otherwise. Drivers

can call drm_gem_huge_mnt_create() to create, mount and use a huge

shmem mountpoint instead of the default one ('shm_mnt'). For builds

with CONFIG_TRANSPARENT_HUGEPAGE enabled, further calls to

drm_gem_object_init() will let shmem allocate huge pages when

possible.

Drivers are responsible for the actual physical pages allocation by

calling shmem_read_mapping_page_gfp() for each page.

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.

The fault operation handler is responsible for mapping 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.

In order to reduce page table overhead, if the internal shmem mountpoint

"shm_mnt" is configured to use transparent huge pages (for builds with

CONFIG_TRANSPARENT_HUGEPAGE enabled) and if the shmem backing store

managed to allocate a huge page for a faulty address, the fault handler

will first attempt to insert that huge page into the VMA before falling

back to individual page insertion. mmap() user address alignment for GEM

objects is handled by providing a custom get_unmapped_area file

operation which forwards to the shmem backing store. For most drivers,

which don't create a huge mountpoint by default or through a module

parameter, transparent huge pages can be enabled by either setting the

"transparent_hugepage_shmem" kernel parameter or the

"/sys/kernel/mm/transparent_hugepage/shmem_enabled" sysfs knob.

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.