226 lines
9.7 KiB
ReStructuredText
226 lines
9.7 KiB
ReStructuredText
===================
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Userland interfaces
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===================
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The DRM core exports several interfaces to applications, generally
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intended to be used through corresponding libdrm wrapper functions. In
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addition, drivers export device-specific interfaces for use by userspace
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drivers & device-aware applications through ioctls and sysfs files.
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External interfaces include: memory mapping, context management, DMA
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operations, AGP management, vblank control, fence management, memory
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management, and output management.
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Cover generic ioctls and sysfs layout here. We only need high-level
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info, since man pages should cover the rest.
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libdrm Device Lookup
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====================
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.. kernel-doc:: drivers/gpu/drm/drm_ioctl.c
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:doc: getunique and setversion story
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.. _drm_primary_node:
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Primary Nodes, DRM Master and Authentication
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============================================
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.. kernel-doc:: drivers/gpu/drm/drm_auth.c
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:doc: master and authentication
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.. kernel-doc:: drivers/gpu/drm/drm_auth.c
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:export:
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.. kernel-doc:: include/drm/drm_auth.h
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:internal:
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Open-Source Userspace Requirements
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==================================
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The DRM subsystem has stricter requirements than most other kernel subsystems on
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what the userspace side for new uAPI needs to look like. This section here
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explains what exactly those requirements are, and why they exist.
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The short summary is that any addition of DRM uAPI requires corresponding
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open-sourced userspace patches, and those patches must be reviewed and ready for
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merging into a suitable and canonical upstream project.
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GFX devices (both display and render/GPU side) are really complex bits of
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hardware, with userspace and kernel by necessity having to work together really
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closely. The interfaces, for rendering and modesetting, must be extremely wide
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and flexible, and therefore it is almost always impossible to precisely define
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them for every possible corner case. This in turn makes it really practically
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infeasible to differentiate between behaviour that's required by userspace, and
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which must not be changed to avoid regressions, and behaviour which is only an
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accidental artifact of the current implementation.
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Without access to the full source code of all userspace users that means it
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becomes impossible to change the implementation details, since userspace could
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depend upon the accidental behaviour of the current implementation in minute
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details. And debugging such regressions without access to source code is pretty
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much impossible. As a consequence this means:
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- The Linux kernel's "no regression" policy holds in practice only for
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open-source userspace of the DRM subsystem. DRM developers are perfectly fine
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if closed-source blob drivers in userspace use the same uAPI as the open
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drivers, but they must do so in the exact same way as the open drivers.
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Creative (ab)use of the interfaces will, and in the past routinely has, lead
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to breakage.
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- Any new userspace interface must have an open-source implementation as
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demonstration vehicle.
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The other reason for requiring open-source userspace is uAPI review. Since the
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kernel and userspace parts of a GFX stack must work together so closely, code
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review can only assess whether a new interface achieves its goals by looking at
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both sides. Making sure that the interface indeed covers the use-case fully
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leads to a few additional requirements:
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- The open-source userspace must not be a toy/test application, but the real
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thing. Specifically it needs to handle all the usual error and corner cases.
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These are often the places where new uAPI falls apart and hence essential to
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assess the fitness of a proposed interface.
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- The userspace side must be fully reviewed and tested to the standards of that
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userspace project. For e.g. mesa this means piglit testcases and review on the
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mailing list. This is again to ensure that the new interface actually gets the
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job done.
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- The userspace patches must be against the canonical upstream, not some vendor
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fork. This is to make sure that no one cheats on the review and testing
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requirements by doing a quick fork.
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- The kernel patch can only be merged after all the above requirements are met,
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but it **must** be merged **before** the userspace patches land. uAPI always flows
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from the kernel, doing things the other way round risks divergence of the uAPI
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definitions and header files.
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These are fairly steep requirements, but have grown out from years of shared
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pain and experience with uAPI added hastily, and almost always regretted about
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just as fast. GFX devices change really fast, requiring a paradigm shift and
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entire new set of uAPI interfaces every few years at least. Together with the
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Linux kernel's guarantee to keep existing userspace running for 10+ years this
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is already rather painful for the DRM subsystem, with multiple different uAPIs
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for the same thing co-existing. If we add a few more complete mistakes into the
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mix every year it would be entirely unmanageable.
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.. _drm_render_node:
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Render nodes
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============
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DRM core provides multiple character-devices for user-space to use.
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Depending on which device is opened, user-space can perform a different
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set of operations (mainly ioctls). The primary node is always created
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and called card<num>. Additionally, a currently unused control node,
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called controlD<num> is also created. The primary node provides all
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legacy operations and historically was the only interface used by
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userspace. With KMS, the control node was introduced. However, the
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planned KMS control interface has never been written and so the control
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node stays unused to date.
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With the increased use of offscreen renderers and GPGPU applications,
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clients no longer require running compositors or graphics servers to
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make use of a GPU. But the DRM API required unprivileged clients to
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authenticate to a DRM-Master prior to getting GPU access. To avoid this
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step and to grant clients GPU access without authenticating, render
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nodes were introduced. Render nodes solely serve render clients, that
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is, no modesetting or privileged ioctls can be issued on render nodes.
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Only non-global rendering commands are allowed. If a driver supports
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render nodes, it must advertise it via the DRIVER_RENDER DRM driver
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capability. If not supported, the primary node must be used for render
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clients together with the legacy drmAuth authentication procedure.
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If a driver advertises render node support, DRM core will create a
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separate render node called renderD<num>. There will be one render node
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per device. No ioctls except PRIME-related ioctls will be allowed on
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this node. Especially GEM_OPEN will be explicitly prohibited. Render
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nodes are designed to avoid the buffer-leaks, which occur if clients
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guess the flink names or mmap offsets on the legacy interface.
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Additionally to this basic interface, drivers must mark their
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driver-dependent render-only ioctls as DRM_RENDER_ALLOW so render
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clients can use them. Driver authors must be careful not to allow any
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privileged ioctls on render nodes.
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With render nodes, user-space can now control access to the render node
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via basic file-system access-modes. A running graphics server which
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authenticates clients on the privileged primary/legacy node is no longer
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required. Instead, a client can open the render node and is immediately
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granted GPU access. Communication between clients (or servers) is done
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via PRIME. FLINK from render node to legacy node is not supported. New
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clients must not use the insecure FLINK interface.
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Besides dropping all modeset/global ioctls, render nodes also drop the
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DRM-Master concept. There is no reason to associate render clients with
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a DRM-Master as they are independent of any graphics server. Besides,
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they must work without any running master, anyway. Drivers must be able
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to run without a master object if they support render nodes. If, on the
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other hand, a driver requires shared state between clients which is
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visible to user-space and accessible beyond open-file boundaries, they
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cannot support render nodes.
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Testing and validation
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======================
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Validating changes with IGT
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---------------------------
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There's a collection of tests that aims to cover the whole functionality of
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DRM drivers and that can be used to check that changes to DRM drivers or the
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core don't regress existing functionality. This test suite is called IGT and
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its code can be found in https://cgit.freedesktop.org/drm/igt-gpu-tools/.
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To build IGT, start by installing its build dependencies. In Debian-based
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systems::
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# apt-get build-dep intel-gpu-tools
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And in Fedora-based systems::
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# dnf builddep intel-gpu-tools
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Then clone the repository::
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$ git clone git://anongit.freedesktop.org/drm/igt-gpu-tools
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Configure the build system and start the build::
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$ cd igt-gpu-tools && ./autogen.sh && make -j6
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Download the piglit dependency::
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$ ./scripts/run-tests.sh -d
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And run the tests::
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$ ./scripts/run-tests.sh -t kms -t core -s
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run-tests.sh is a wrapper around piglit that will execute the tests matching
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the -t options. A report in HTML format will be available in
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./results/html/index.html. Results can be compared with piglit.
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Display CRC Support
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-------------------
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.. kernel-doc:: drivers/gpu/drm/drm_debugfs_crc.c
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:doc: CRC ABI
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VBlank event handling
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=====================
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The DRM core exposes two vertical blank related ioctls:
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DRM_IOCTL_WAIT_VBLANK
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This takes a struct drm_wait_vblank structure as its argument, and
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it is used to block or request a signal when a specified vblank
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event occurs.
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DRM_IOCTL_MODESET_CTL
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This was only used for user-mode-settind drivers around modesetting
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changes to allow the kernel to update the vblank interrupt after
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mode setting, since on many devices the vertical blank counter is
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reset to 0 at some point during modeset. Modern drivers should not
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call this any more since with kernel mode setting it is a no-op.
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