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===================
Userland interfaces
===================
The DRM core exports several interfaces to applications, generally
intended to be used through corresponding libdrm wrapper functions. In
addition, drivers export device-specific interfaces for use by userspace
drivers & device-aware applications through ioctls and sysfs files.
External interfaces include: memory mapping, context management, DMA
operations, AGP management, vblank control, fence management, memory
management, and output management.
Cover generic ioctls and sysfs layout here. We only need high-level
info, since man pages should cover the rest.
libdrm Device Lookup
====================
.. kernel-doc:: drivers/gpu/drm/drm_ioctl.c
:doc: getunique and setversion story
Primary Nodes, DRM Master and Authentication
============================================
.. kernel-doc:: drivers/gpu/drm/drm_auth.c
:doc: master and authentication
.. kernel-doc:: drivers/gpu/drm/drm_auth.c
:export:
.. kernel-doc:: include/drm/drm_auth.h
:internal:
Open-Source Userspace Requirements
==================================
The DRM subsystem has stricter requirements than most other kernel subsystems on
what the userspace side for new uAPI needs to look like. This section here
explains what exactly those requirements are, and why they exist.
The short summary is that any addition of DRM uAPI requires corresponding
open-sourced userspace patches, and those patches must be reviewed and ready for
merging into a suitable and canonical upstream project.
GFX devices (both display and render/GPU side) are really complex bits of
hardware, with userspace and kernel by necessity having to work together really
closely. The interfaces, for rendering and modesetting, must be extremely wide
and flexible, and therefore it is almost always impossible to precisely define
them for every possible corner case. This in turn makes it really practically
infeasible to differentiate between behaviour that's required by userspace, and
which must not be changed to avoid regressions, and behaviour which is only an
accidental artifact of the current implementation.
Without access to the full source code of all userspace users that means it
becomes impossible to change the implementation details, since userspace could
depend upon the accidental behaviour of the current implementation in minute
details. And debugging such regressions without access to source code is pretty
much impossible. As a consequence this means:
- The Linux kernel's "no regression" policy holds in practice only for
open-source userspace of the DRM subsystem. DRM developers are perfectly fine
if closed-source blob drivers in userspace use the same uAPI as the open
drivers, but they must do so in the exact same way as the open drivers.
Creative (ab)use of the interfaces will, and in the past routinely has, lead
to breakage.
- Any new userspace interface must have an open-source implementation as
demonstration vehicle.
The other reason for requiring open-source userspace is uAPI review. Since the
kernel and userspace parts of a GFX stack must work together so closely, code
review can only assess whether a new interface achieves its goals by looking at
both sides. Making sure that the interface indeed covers the use-case fully
leads to a few additional requirements:
- The open-source userspace must not be a toy/test application, but the real
thing. Specifically it needs to handle all the usual error and corner cases.
These are often the places where new uAPI falls apart and hence essential to
assess the fitness of a proposed interface.
- The userspace side must be fully reviewed and tested to the standards of that
userspace project. For e.g. mesa this means piglit testcases and review on the
mailing list. This is again to ensure that the new interface actually gets the
job done.
- The userspace patches must be against the canonical upstream, not some vendor
fork. This is to make sure that no one cheats on the review and testing
requirements by doing a quick fork.
- The kernel patch can only be merged after all the above requirements are met,
but it **must** be merged **before** the userspace patches land. uAPI always flows
from the kernel, doing things the other way round risks divergence of the uAPI
definitions and header files.
These are fairly steep requirements, but have grown out from years of shared
pain and experience with uAPI added hastily, and almost always regretted about
just as fast. GFX devices change really fast, requiring a paradigm shift and
entire new set of uAPI interfaces every few years at least. Together with the
Linux kernel's guarantee to keep existing userspace running for 10+ years this
is already rather painful for the DRM subsystem, with multiple different uAPIs
for the same thing co-existing. If we add a few more complete mistakes into the
mix every year it would be entirely unmanageable.
Render nodes
============
DRM core provides multiple character-devices for user-space to use.
Depending on which device is opened, user-space can perform a different
set of operations (mainly ioctls). The primary node is always created
and called card<num>. Additionally, a currently unused control node,
called controlD<num> is also created. The primary node provides all
legacy operations and historically was the only interface used by
userspace. With KMS, the control node was introduced. However, the
planned KMS control interface has never been written and so the control
node stays unused to date.
With the increased use of offscreen renderers and GPGPU applications,
clients no longer require running compositors or graphics servers to
make use of a GPU. But the DRM API required unprivileged clients to
authenticate to a DRM-Master prior to getting GPU access. To avoid this
step and to grant clients GPU access without authenticating, render
nodes were introduced. Render nodes solely serve render clients, that
is, no modesetting or privileged ioctls can be issued on render nodes.
Only non-global rendering commands are allowed. If a driver supports
render nodes, it must advertise it via the DRIVER_RENDER DRM driver
capability. If not supported, the primary node must be used for render
clients together with the legacy drmAuth authentication procedure.
If a driver advertises render node support, DRM core will create a
separate render node called renderD<num>. There will be one render node
per device. No ioctls except PRIME-related ioctls will be allowed on
this node. Especially GEM_OPEN will be explicitly prohibited. Render
nodes are designed to avoid the buffer-leaks, which occur if clients
guess the flink names or mmap offsets on the legacy interface.
Additionally to this basic interface, drivers must mark their
driver-dependent render-only ioctls as DRM_RENDER_ALLOW so render
clients can use them. Driver authors must be careful not to allow any
privileged ioctls on render nodes.
With render nodes, user-space can now control access to the render node
via basic file-system access-modes. A running graphics server which
authenticates clients on the privileged primary/legacy node is no longer
required. Instead, a client can open the render node and is immediately
granted GPU access. Communication between clients (or servers) is done
via PRIME. FLINK from render node to legacy node is not supported. New
clients must not use the insecure FLINK interface.
Besides dropping all modeset/global ioctls, render nodes also drop the
DRM-Master concept. There is no reason to associate render clients with
a DRM-Master as they are independent of any graphics server. Besides,
they must work without any running master, anyway. Drivers must be able
to run without a master object if they support render nodes. If, on the
other hand, a driver requires shared state between clients which is
visible to user-space and accessible beyond open-file boundaries, they
cannot support render nodes.
VBlank event handling
=====================
The DRM core exposes two vertical blank related ioctls:
DRM_IOCTL_WAIT_VBLANK
This takes a struct drm_wait_vblank structure as its argument, and
it is used to block or request a signal when a specified vblank
event occurs.
DRM_IOCTL_MODESET_CTL
This was only used for user-mode-settind drivers around modesetting
changes to allow the kernel to update the vblank interrupt after
mode setting, since on many devices the vertical blank counter is
reset to 0 at some point during modeset. Modern drivers should not
call this any more since with kernel mode setting it is a no-op.
This second part of the GPU Driver Developer's Guide documents driver
code, implementation details and also all the driver-specific userspace
interfaces. Especially since all hardware-acceleration interfaces to
userspace are driver specific for efficiency and other reasons these
interfaces can be rather substantial. Hence every driver has its own
chapter.
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