1=================== 2Userland interfaces 3=================== 4 5The DRM core exports several interfaces to applications, generally 6intended to be used through corresponding libdrm wrapper functions. In 7addition, drivers export device-specific interfaces for use by userspace 8drivers & device-aware applications through ioctls and sysfs files. 9 10External interfaces include: memory mapping, context management, DMA 11operations, AGP management, vblank control, fence management, memory 12management, and output management. 13 14Cover generic ioctls and sysfs layout here. We only need high-level 15info, since man pages should cover the rest. 16 17libdrm Device Lookup 18==================== 19 20.. kernel-doc:: drivers/gpu/drm/drm_ioctl.c 21 :doc: getunique and setversion story 22 23 24Primary Nodes, DRM Master and Authentication 25============================================ 26 27.. kernel-doc:: drivers/gpu/drm/drm_auth.c 28 :doc: master and authentication 29 30.. kernel-doc:: drivers/gpu/drm/drm_auth.c 31 :export: 32 33.. kernel-doc:: include/drm/drm_auth.h 34 :internal: 35 36Open-Source Userspace Requirements 37================================== 38 39The DRM subsystem has stricter requirements than most other kernel subsystems on 40what the userspace side for new uAPI needs to look like. This section here 41explains what exactly those requirements are, and why they exist. 42 43The short summary is that any addition of DRM uAPI requires corresponding 44open-sourced userspace patches, and those patches must be reviewed and ready for 45merging into a suitable and canonical upstream project. 46 47GFX devices (both display and render/GPU side) are really complex bits of 48hardware, with userspace and kernel by necessity having to work together really 49closely. The interfaces, for rendering and modesetting, must be extremely wide 50and flexible, and therefore it is almost always impossible to precisely define 51them for every possible corner case. This in turn makes it really practically 52infeasible to differentiate between behaviour that's required by userspace, and 53which must not be changed to avoid regressions, and behaviour which is only an 54accidental artifact of the current implementation. 55 56Without access to the full source code of all userspace users that means it 57becomes impossible to change the implementation details, since userspace could 58depend upon the accidental behaviour of the current implementation in minute 59details. And debugging such regressions without access to source code is pretty 60much impossible. As a consequence this means: 61 62- The Linux kernel's "no regression" policy holds in practice only for 63 open-source userspace of the DRM subsystem. DRM developers are perfectly fine 64 if closed-source blob drivers in userspace use the same uAPI as the open 65 drivers, but they must do so in the exact same way as the open drivers. 66 Creative (ab)use of the interfaces will, and in the past routinely has, lead 67 to breakage. 68 69- Any new userspace interface must have an open-source implementation as 70 demonstration vehicle. 71 72The other reason for requiring open-source userspace is uAPI review. Since the 73kernel and userspace parts of a GFX stack must work together so closely, code 74review can only assess whether a new interface achieves its goals by looking at 75both sides. Making sure that the interface indeed covers the use-case fully 76leads to a few additional requirements: 77 78- The open-source userspace must not be a toy/test application, but the real 79 thing. Specifically it needs to handle all the usual error and corner cases. 80 These are often the places where new uAPI falls apart and hence essential to 81 assess the fitness of a proposed interface. 82 83- The userspace side must be fully reviewed and tested to the standards of that 84 userspace project. For e.g. mesa this means piglit testcases and review on the 85 mailing list. This is again to ensure that the new interface actually gets the 86 job done. 87 88- The userspace patches must be against the canonical upstream, not some vendor 89 fork. This is to make sure that no one cheats on the review and testing 90 requirements by doing a quick fork. 91 92- The kernel patch can only be merged after all the above requirements are met, 93 but it **must** be merged **before** the userspace patches land. uAPI always flows 94 from the kernel, doing things the other way round risks divergence of the uAPI 95 definitions and header files. 96 97These are fairly steep requirements, but have grown out from years of shared 98pain and experience with uAPI added hastily, and almost always regretted about 99just as fast. GFX devices change really fast, requiring a paradigm shift and 100entire new set of uAPI interfaces every few years at least. Together with the 101Linux kernel's guarantee to keep existing userspace running for 10+ years this 102is already rather painful for the DRM subsystem, with multiple different uAPIs 103for the same thing co-existing. If we add a few more complete mistakes into the 104mix every year it would be entirely unmanageable. 105 106Render nodes 107============ 108 109DRM core provides multiple character-devices for user-space to use. 110Depending on which device is opened, user-space can perform a different 111set of operations (mainly ioctls). The primary node is always created 112and called card<num>. Additionally, a currently unused control node, 113called controlD<num> is also created. The primary node provides all 114legacy operations and historically was the only interface used by 115userspace. With KMS, the control node was introduced. However, the 116planned KMS control interface has never been written and so the control 117node stays unused to date. 118 119With the increased use of offscreen renderers and GPGPU applications, 120clients no longer require running compositors or graphics servers to 121make use of a GPU. But the DRM API required unprivileged clients to 122authenticate to a DRM-Master prior to getting GPU access. To avoid this 123step and to grant clients GPU access without authenticating, render 124nodes were introduced. Render nodes solely serve render clients, that 125is, no modesetting or privileged ioctls can be issued on render nodes. 126Only non-global rendering commands are allowed. If a driver supports 127render nodes, it must advertise it via the DRIVER_RENDER DRM driver 128capability. If not supported, the primary node must be used for render 129clients together with the legacy drmAuth authentication procedure. 130 131If a driver advertises render node support, DRM core will create a 132separate render node called renderD<num>. There will be one render node 133per device. No ioctls except PRIME-related ioctls will be allowed on 134this node. Especially GEM_OPEN will be explicitly prohibited. Render 135nodes are designed to avoid the buffer-leaks, which occur if clients 136guess the flink names or mmap offsets on the legacy interface. 137Additionally to this basic interface, drivers must mark their 138driver-dependent render-only ioctls as DRM_RENDER_ALLOW so render 139clients can use them. Driver authors must be careful not to allow any 140privileged ioctls on render nodes. 141 142With render nodes, user-space can now control access to the render node 143via basic file-system access-modes. A running graphics server which 144authenticates clients on the privileged primary/legacy node is no longer 145required. Instead, a client can open the render node and is immediately 146granted GPU access. Communication between clients (or servers) is done 147via PRIME. FLINK from render node to legacy node is not supported. New 148clients must not use the insecure FLINK interface. 149 150Besides dropping all modeset/global ioctls, render nodes also drop the 151DRM-Master concept. There is no reason to associate render clients with 152a DRM-Master as they are independent of any graphics server. Besides, 153they must work without any running master, anyway. Drivers must be able 154to run without a master object if they support render nodes. If, on the 155other hand, a driver requires shared state between clients which is 156visible to user-space and accessible beyond open-file boundaries, they 157cannot support render nodes. 158 159 160Testing and validation 161====================== 162 163Validating changes with IGT 164--------------------------- 165 166There's a collection of tests that aims to cover the whole functionality of 167DRM drivers and that can be used to check that changes to DRM drivers or the 168core don't regress existing functionality. This test suite is called IGT and 169its code can be found in https://cgit.freedesktop.org/drm/igt-gpu-tools/. 170 171To build IGT, start by installing its build dependencies. In Debian-based 172systems:: 173 174 # apt-get build-dep intel-gpu-tools 175 176And in Fedora-based systems:: 177 178 # dnf builddep intel-gpu-tools 179 180Then clone the repository:: 181 182 $ git clone git://anongit.freedesktop.org/drm/igt-gpu-tools 183 184Configure the build system and start the build:: 185 186 $ cd igt-gpu-tools && ./autogen.sh && make -j6 187 188Download the piglit dependency:: 189 190 $ ./scripts/run-tests.sh -d 191 192And run the tests:: 193 194 $ ./scripts/run-tests.sh -t kms -t core -s 195 196run-tests.sh is a wrapper around piglit that will execute the tests matching 197the -t options. A report in HTML format will be available in 198./results/html/index.html. Results can be compared with piglit. 199 200Display CRC Support 201------------------- 202 203.. kernel-doc:: drivers/gpu/drm/drm_debugfs_crc.c 204 :doc: CRC ABI 205 206VBlank event handling 207===================== 208 209The DRM core exposes two vertical blank related ioctls: 210 211DRM_IOCTL_WAIT_VBLANK 212 This takes a struct drm_wait_vblank structure as its argument, and 213 it is used to block or request a signal when a specified vblank 214 event occurs. 215 216DRM_IOCTL_MODESET_CTL 217 This was only used for user-mode-settind drivers around modesetting 218 changes to allow the kernel to update the vblank interrupt after 219 mode setting, since on many devices the vertical blank counter is 220 reset to 0 at some point during modeset. Modern drivers should not 221 call this any more since with kernel mode setting it is a no-op. 222