262 lines
10 KiB
ReStructuredText
262 lines
10 KiB
ReStructuredText
===================
|
|
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
|
|
|
|
|
|
.. _drm_primary_node:
|
|
|
|
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.
|
|
|
|
.. _drm_render_node:
|
|
|
|
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.
|
|
|
|
IOCTL Support on Device Nodes
|
|
=============================
|
|
|
|
.. kernel-doc:: drivers/gpu/drm/drm_ioctl.c
|
|
:doc: driver specific ioctls
|
|
|
|
.. kernel-doc:: include/drm/drm_ioctl.h
|
|
:internal:
|
|
|
|
.. kernel-doc:: drivers/gpu/drm/drm_ioctl.c
|
|
:export:
|
|
|
|
.. kernel-doc:: drivers/gpu/drm/drm_ioc32.c
|
|
:export:
|
|
|
|
Testing and validation
|
|
======================
|
|
|
|
Validating changes with IGT
|
|
---------------------------
|
|
|
|
There's a collection of tests that aims to cover the whole functionality of
|
|
DRM drivers and that can be used to check that changes to DRM drivers or the
|
|
core don't regress existing functionality. This test suite is called IGT and
|
|
its code can be found in https://cgit.freedesktop.org/drm/igt-gpu-tools/.
|
|
|
|
To build IGT, start by installing its build dependencies. In Debian-based
|
|
systems::
|
|
|
|
# apt-get build-dep intel-gpu-tools
|
|
|
|
And in Fedora-based systems::
|
|
|
|
# dnf builddep intel-gpu-tools
|
|
|
|
Then clone the repository::
|
|
|
|
$ git clone git://anongit.freedesktop.org/drm/igt-gpu-tools
|
|
|
|
Configure the build system and start the build::
|
|
|
|
$ cd igt-gpu-tools && ./autogen.sh && make -j6
|
|
|
|
Download the piglit dependency::
|
|
|
|
$ ./scripts/run-tests.sh -d
|
|
|
|
And run the tests::
|
|
|
|
$ ./scripts/run-tests.sh -t kms -t core -s
|
|
|
|
run-tests.sh is a wrapper around piglit that will execute the tests matching
|
|
the -t options. A report in HTML format will be available in
|
|
./results/html/index.html. Results can be compared with piglit.
|
|
|
|
Display CRC Support
|
|
-------------------
|
|
|
|
.. kernel-doc:: drivers/gpu/drm/drm_debugfs_crc.c
|
|
:doc: CRC ABI
|
|
|
|
.. kernel-doc:: drivers/gpu/drm/drm_debugfs_crc.c
|
|
:export:
|
|
|
|
Debugfs Support
|
|
---------------
|
|
|
|
.. kernel-doc:: include/drm/drm_debugfs.h
|
|
:internal:
|
|
|
|
.. kernel-doc:: drivers/gpu/drm/drm_debugfs.c
|
|
:export:
|
|
|
|
Sysfs Support
|
|
=============
|
|
|
|
.. kernel-doc:: drivers/gpu/drm/drm_sysfs.c
|
|
:doc: overview
|
|
|
|
.. kernel-doc:: drivers/gpu/drm/drm_sysfs.c
|
|
:export:
|
|
|
|
|
|
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.
|