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diff --git a/Documentation/gpu/drm-uapi.rst b/Documentation/gpu/drm-uapi.rst new file mode 100644 index 000000000..a2214cc1f --- /dev/null +++ b/Documentation/gpu/drm-uapi.rst @@ -0,0 +1,318 @@ +=================== +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. + +.. _drm_driver_ioctl: + +IOCTL Support on Device Nodes +============================= + +.. kernel-doc:: drivers/gpu/drm/drm_ioctl.c + :doc: driver specific ioctls + +Recommended IOCTL Return Values +------------------------------- + +In theory a driver's IOCTL callback is only allowed to return very few error +codes. In practice it's good to abuse a few more. This section documents common +practice within the DRM subsystem: + +ENOENT: + Strictly this should only be used when a file doesn't exist e.g. when + calling the open() syscall. We reuse that to signal any kind of object + lookup failure, e.g. for unknown GEM buffer object handles, unknown KMS + object handles and similar cases. + +ENOSPC: + Some drivers use this to differentiate "out of kernel memory" from "out + of VRAM". Sometimes also applies to other limited gpu resources used for + rendering (e.g. when you have a special limited compression buffer). + Sometimes resource allocation/reservation issues in command submission + IOCTLs are also signalled through EDEADLK. + + Simply running out of kernel/system memory is signalled through ENOMEM. + +EPERM/EACCESS: + Returned for an operation that is valid, but needs more privileges. + E.g. root-only or much more common, DRM master-only operations return + this when when called by unpriviledged clients. There's no clear + difference between EACCESS and EPERM. + +ENODEV: + Feature (like PRIME, modesetting, GEM) is not supported by the driver. + +ENXIO: + Remote failure, either a hardware transaction (like i2c), but also used + when the exporting driver of a shared dma-buf or fence doesn't support a + feature needed. + +EINTR: + DRM drivers assume that userspace restarts all IOCTLs. Any DRM IOCTL can + return EINTR and in such a case should be restarted with the IOCTL + parameters left unchanged. + +EIO: + The GPU died and couldn't be resurrected through a reset. Modesetting + hardware failures are signalled through the "link status" connector + property. + +EINVAL: + Catch-all for anything that is an invalid argument combination which + cannot work. + +IOCTL also use other error codes like ETIME, EFAULT, EBUSY, ENOTTY but their +usage is in line with the common meanings. The above list tries to just document +DRM specific patterns. Note that ENOTTY has the slightly unintuitive meaning of +"this IOCTL does not exist", and is used exactly as such in DRM. + +.. 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. |