The core idea of GEM is to identify graphic buffer objects with 32bit ids. The reason being "X runs out of open fds" (KDE easily reaches a few thousand).
The core design principle behind GEM is that the kernel is in full control of the allocation of these buffer objects and is free to move the around in any way it sees fit. This is to make concurrent rendering by multiple processes possible while userspace can still assume that it is in sole possession of the gpu - GEM means "graphics execution manager".
Below some more details on what GEM is and does, what it does _not_ do and how it relates to other graphic subsystems.
GEM does ...
- lifecycle management. Userspace references are associated with the drm fd and get reaped on close (in case userspace forgets about them).
- per-device global names to exchange buffers between processes (eg dri2). These names are again 32bit ids. These global ids do not count as userspace references and don't prevent a buffer from being reaped.
- it implements very few generic ioctls:
- flink for creating a global name for a buffer object
- open for getting a per-fd handle to a buffer object with a global name
- close for dropping a per-fd handle.
- a little bit of kernel-internal helpers to facilitate mmap (by blending multiple buffer objects into the single drm device address space) and a few other things.
That's it, i.e. GEM is very much meant to be as simple as possible.
Driver-specific GEM ioctls
The generic GEM stuff is obviously not very useful. So drivers implement
quite a bit driver-specific ioctls, like:
- buffer creation. In recent kernels there is some support to create dumb scanout objects for KMS. But they're only really useful for boot-splashs and unaccelerated dumb KMS drivers. Creating buffers usable for rendering is only possible with driver specific ioctls.
- command submission. An important part is mapping abstract buffer ids to actual gpu address (and rewriting batchbuffers with these). In the future, with support for virtual gpu address spaces this might change.
- tiling management. The kernel needs to know this to correctly tile/detile buffers when moving them around (e.g. evicting from vram).
- command completion signalling and gpu/cpu synchronization.
There are currently two approaches for implementing a GEM driver:
- roll-your-own, used by drm/i915 (and sometimes getting flaked for NIH).
- ttm-base: radeon & nouveau.
GEM does not ...
This still leaves out a few things that I've seen mentioned as
ideas/requirements here and elsewhere:
- cross-device buffer sharing and namespaces (see below) and
- buffer format handling and mediation between different users (except
tiling as mentioned above). The reason here is that gpus are a mess
and one of the worst parts is format handling. Better keep that out
of the kernel ...
KMS (kernel mode setting)
KMS is essentially just a port of the xrandr api to the kernel as an ioctl
- crtcs feed (possible multiple) outputs and get their data from a framebuffer object. A major part of KMS is also the support for vsynced-pageflipping of framebuffers.
- Internally there's some support infrastructure to simplify drivers (all the drm_*_helper.c code).
- framebuffers are created from a opaque driver-specific 32bit id and a format description. For GEM drivers these ids name GEM objects, but that need not be: The recently merged qemu kms driver does not implement gem and has one unique buffer object with id 0.
- as mentioned above there newly is a generic ioctl to create an object suitable as a dumb scanout (plus some support to mmap it).
- currently KMS has no generic support for overlays (there are driver-specific ioctls in i915 and vmgfx, though). Jesse Barnes has posted an RFC to remedy this: http://email@example.com/msg10415.html
GEM and PRIME
PRIME is a proof-of-concept implementation from Dave Airlie for sharing
GEM objects between drivers/devices: Buffer sharing is done with a list of
struct page pointers. While being shared, buffers can't be moved anymore.
No further buffer description is passed along in the kernel, format/layout
mediation is to be handled in userspace.
Blog-post describing the initial design for sharing buffers between an
integrated Intel igd and a discrete ATI gpu: http://airlied.livejournal.com/71734.html
Other code using the same framework to render on an Intel igd and display
the framebuffer on an usb-connected displayport: http://git.kernel.org/?p=linux/kernel/git/airlied/drm-testing.git;a=shortlog;h=refs/heads/udl-v2
GEM/KMS and fbdev
There's some minimal support to emulate an fbdev with a gem/kms driver.
Resolution can't be changed and it's unaccelerated. There's been some
muttering once in a while to better integrate this with either a kms
kernel console driver or by routing fbdev resolution changes to kms.
But the main use case is to display a kernel oops, which works. For
everything else there's X (or an EGL client that understands kms).