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 24.. _drm_primary_node: 25 26Primary Nodes, DRM Master and Authentication 27============================================ 28 29.. kernel-doc:: drivers/gpu/drm/drm_auth.c 30 :doc: master and authentication 31 32.. kernel-doc:: drivers/gpu/drm/drm_auth.c 33 :export: 34 35.. kernel-doc:: include/drm/drm_auth.h 36 :internal: 37 38Open-Source Userspace Requirements 39================================== 40 41The DRM subsystem has stricter requirements than most other kernel subsystems on 42what the userspace side for new uAPI needs to look like. This section here 43explains what exactly those requirements are, and why they exist. 44 45The short summary is that any addition of DRM uAPI requires corresponding 46open-sourced userspace patches, and those patches must be reviewed and ready for 47merging into a suitable and canonical upstream project. 48 49GFX devices (both display and render/GPU side) are really complex bits of 50hardware, with userspace and kernel by necessity having to work together really 51closely. The interfaces, for rendering and modesetting, must be extremely wide 52and flexible, and therefore it is almost always impossible to precisely define 53them for every possible corner case. This in turn makes it really practically 54infeasible to differentiate between behaviour that's required by userspace, and 55which must not be changed to avoid regressions, and behaviour which is only an 56accidental artifact of the current implementation. 57 58Without access to the full source code of all userspace users that means it 59becomes impossible to change the implementation details, since userspace could 60depend upon the accidental behaviour of the current implementation in minute 61details. And debugging such regressions without access to source code is pretty 62much impossible. As a consequence this means: 63 64- The Linux kernel's "no regression" policy holds in practice only for 65 open-source userspace of the DRM subsystem. DRM developers are perfectly fine 66 if closed-source blob drivers in userspace use the same uAPI as the open 67 drivers, but they must do so in the exact same way as the open drivers. 68 Creative (ab)use of the interfaces will, and in the past routinely has, lead 69 to breakage. 70 71- Any new userspace interface must have an open-source implementation as 72 demonstration vehicle. 73 74The other reason for requiring open-source userspace is uAPI review. Since the 75kernel and userspace parts of a GFX stack must work together so closely, code 76review can only assess whether a new interface achieves its goals by looking at 77both sides. Making sure that the interface indeed covers the use-case fully 78leads to a few additional requirements: 79 80- The open-source userspace must not be a toy/test application, but the real 81 thing. Specifically it needs to handle all the usual error and corner cases. 82 These are often the places where new uAPI falls apart and hence essential to 83 assess the fitness of a proposed interface. 84 85- The userspace side must be fully reviewed and tested to the standards of that 86 userspace project. For e.g. mesa this means piglit testcases and review on the 87 mailing list. This is again to ensure that the new interface actually gets the 88 job done. 89 90- The userspace patches must be against the canonical upstream, not some vendor 91 fork. This is to make sure that no one cheats on the review and testing 92 requirements by doing a quick fork. 93 94- The kernel patch can only be merged after all the above requirements are met, 95 but it **must** be merged **before** the userspace patches land. uAPI always flows 96 from the kernel, doing things the other way round risks divergence of the uAPI 97 definitions and header files. 98 99These are fairly steep requirements, but have grown out from years of shared 100pain and experience with uAPI added hastily, and almost always regretted about 101just as fast. GFX devices change really fast, requiring a paradigm shift and 102entire new set of uAPI interfaces every few years at least. Together with the 103Linux kernel's guarantee to keep existing userspace running for 10+ years this 104is already rather painful for the DRM subsystem, with multiple different uAPIs 105for the same thing co-existing. If we add a few more complete mistakes into the 106mix every year it would be entirely unmanageable. 107 108.. _drm_render_node: 109 110Render nodes 111============ 112 113DRM core provides multiple character-devices for user-space to use. 114Depending on which device is opened, user-space can perform a different 115set of operations (mainly ioctls). The primary node is always created 116and called card<num>. Additionally, a currently unused control node, 117called controlD<num> is also created. The primary node provides all 118legacy operations and historically was the only interface used by 119userspace. With KMS, the control node was introduced. However, the 120planned KMS control interface has never been written and so the control 121node stays unused to date. 122 123With the increased use of offscreen renderers and GPGPU applications, 124clients no longer require running compositors or graphics servers to 125make use of a GPU. But the DRM API required unprivileged clients to 126authenticate to a DRM-Master prior to getting GPU access. To avoid this 127step and to grant clients GPU access without authenticating, render 128nodes were introduced. Render nodes solely serve render clients, that 129is, no modesetting or privileged ioctls can be issued on render nodes. 130Only non-global rendering commands are allowed. If a driver supports 131render nodes, it must advertise it via the DRIVER_RENDER DRM driver 132capability. If not supported, the primary node must be used for render 133clients together with the legacy drmAuth authentication procedure. 134 135If a driver advertises render node support, DRM core will create a 136separate render node called renderD<num>. There will be one render node 137per device. No ioctls except PRIME-related ioctls will be allowed on 138this node. Especially GEM_OPEN will be explicitly prohibited. Render 139nodes are designed to avoid the buffer-leaks, which occur if clients 140guess the flink names or mmap offsets on the legacy interface. 141Additionally to this basic interface, drivers must mark their 142driver-dependent render-only ioctls as DRM_RENDER_ALLOW so render 143clients can use them. Driver authors must be careful not to allow any 144privileged ioctls on render nodes. 145 146With render nodes, user-space can now control access to the render node 147via basic file-system access-modes. A running graphics server which 148authenticates clients on the privileged primary/legacy node is no longer 149required. Instead, a client can open the render node and is immediately 150granted GPU access. Communication between clients (or servers) is done 151via PRIME. FLINK from render node to legacy node is not supported. New 152clients must not use the insecure FLINK interface. 153 154Besides dropping all modeset/global ioctls, render nodes also drop the 155DRM-Master concept. There is no reason to associate render clients with 156a DRM-Master as they are independent of any graphics server. Besides, 157they must work without any running master, anyway. Drivers must be able 158to run without a master object if they support render nodes. If, on the 159other hand, a driver requires shared state between clients which is 160visible to user-space and accessible beyond open-file boundaries, they 161cannot support render nodes. 162 163IOCTL Support on Device Nodes 164============================= 165 166.. kernel-doc:: drivers/gpu/drm/drm_ioctl.c 167 :doc: driver specific ioctls 168 169.. kernel-doc:: include/drm/drm_ioctl.h 170 :internal: 171 172.. kernel-doc:: drivers/gpu/drm/drm_ioctl.c 173 :export: 174 175.. kernel-doc:: drivers/gpu/drm/drm_ioc32.c 176 :export: 177 178Testing and validation 179====================== 180 181Validating changes with IGT 182--------------------------- 183 184There's a collection of tests that aims to cover the whole functionality of 185DRM drivers and that can be used to check that changes to DRM drivers or the 186core don't regress existing functionality. This test suite is called IGT and 187its code can be found in https://cgit.freedesktop.org/drm/igt-gpu-tools/. 188 189To build IGT, start by installing its build dependencies. In Debian-based 190systems:: 191 192 # apt-get build-dep intel-gpu-tools 193 194And in Fedora-based systems:: 195 196 # dnf builddep intel-gpu-tools 197 198Then clone the repository:: 199 200 $ git clone git://anongit.freedesktop.org/drm/igt-gpu-tools 201 202Configure the build system and start the build:: 203 204 $ cd igt-gpu-tools && ./autogen.sh && make -j6 205 206Download the piglit dependency:: 207 208 $ ./scripts/run-tests.sh -d 209 210And run the tests:: 211 212 $ ./scripts/run-tests.sh -t kms -t core -s 213 214run-tests.sh is a wrapper around piglit that will execute the tests matching 215the -t options. A report in HTML format will be available in 216./results/html/index.html. Results can be compared with piglit. 217 218Display CRC Support 219------------------- 220 221.. kernel-doc:: drivers/gpu/drm/drm_debugfs_crc.c 222 :doc: CRC ABI 223 224.. kernel-doc:: drivers/gpu/drm/drm_debugfs_crc.c 225 :export: 226 227Debugfs Support 228--------------- 229 230.. kernel-doc:: include/drm/drm_debugfs.h 231 :internal: 232 233.. kernel-doc:: drivers/gpu/drm/drm_debugfs.c 234 :export: 235 236Sysfs Support 237============= 238 239.. kernel-doc:: drivers/gpu/drm/drm_sysfs.c 240 :doc: overview 241 242.. kernel-doc:: drivers/gpu/drm/drm_sysfs.c 243 :export: 244 245 246VBlank event handling 247===================== 248 249The DRM core exposes two vertical blank related ioctls: 250 251DRM_IOCTL_WAIT_VBLANK 252 This takes a struct drm_wait_vblank structure as its argument, and 253 it is used to block or request a signal when a specified vblank 254 event occurs. 255 256DRM_IOCTL_MODESET_CTL 257 This was only used for user-mode-settind drivers around modesetting 258 changes to allow the kernel to update the vblank interrupt after 259 mode setting, since on many devices the vertical blank counter is 260 reset to 0 at some point during modeset. Modern drivers should not 261 call this any more since with kernel mode setting it is a no-op. 262