1.. Copyright 2020 DisplayLink (UK) Ltd. 2 3=================== 4Userland interfaces 5=================== 6 7The DRM core exports several interfaces to applications, generally 8intended to be used through corresponding libdrm wrapper functions. In 9addition, drivers export device-specific interfaces for use by userspace 10drivers & device-aware applications through ioctls and sysfs files. 11 12External interfaces include: memory mapping, context management, DMA 13operations, AGP management, vblank control, fence management, memory 14management, and output management. 15 16Cover generic ioctls and sysfs layout here. We only need high-level 17info, since man pages should cover the rest. 18 19libdrm Device Lookup 20==================== 21 22.. kernel-doc:: drivers/gpu/drm/drm_ioctl.c 23 :doc: getunique and setversion story 24 25 26.. _drm_primary_node: 27 28Primary Nodes, DRM Master and Authentication 29============================================ 30 31.. kernel-doc:: drivers/gpu/drm/drm_auth.c 32 :doc: master and authentication 33 34.. kernel-doc:: drivers/gpu/drm/drm_auth.c 35 :export: 36 37.. kernel-doc:: include/drm/drm_auth.h 38 :internal: 39 40 41.. _drm_leasing: 42 43DRM Display Resource Leasing 44============================ 45 46.. kernel-doc:: drivers/gpu/drm/drm_lease.c 47 :doc: drm leasing 48 49Open-Source Userspace Requirements 50================================== 51 52The DRM subsystem has stricter requirements than most other kernel subsystems on 53what the userspace side for new uAPI needs to look like. This section here 54explains what exactly those requirements are, and why they exist. 55 56The short summary is that any addition of DRM uAPI requires corresponding 57open-sourced userspace patches, and those patches must be reviewed and ready for 58merging into a suitable and canonical upstream project. 59 60GFX devices (both display and render/GPU side) are really complex bits of 61hardware, with userspace and kernel by necessity having to work together really 62closely. The interfaces, for rendering and modesetting, must be extremely wide 63and flexible, and therefore it is almost always impossible to precisely define 64them for every possible corner case. This in turn makes it really practically 65infeasible to differentiate between behaviour that's required by userspace, and 66which must not be changed to avoid regressions, and behaviour which is only an 67accidental artifact of the current implementation. 68 69Without access to the full source code of all userspace users that means it 70becomes impossible to change the implementation details, since userspace could 71depend upon the accidental behaviour of the current implementation in minute 72details. And debugging such regressions without access to source code is pretty 73much impossible. As a consequence this means: 74 75- The Linux kernel's "no regression" policy holds in practice only for 76 open-source userspace of the DRM subsystem. DRM developers are perfectly fine 77 if closed-source blob drivers in userspace use the same uAPI as the open 78 drivers, but they must do so in the exact same way as the open drivers. 79 Creative (ab)use of the interfaces will, and in the past routinely has, lead 80 to breakage. 81 82- Any new userspace interface must have an open-source implementation as 83 demonstration vehicle. 84 85The other reason for requiring open-source userspace is uAPI review. Since the 86kernel and userspace parts of a GFX stack must work together so closely, code 87review can only assess whether a new interface achieves its goals by looking at 88both sides. Making sure that the interface indeed covers the use-case fully 89leads to a few additional requirements: 90 91- The open-source userspace must not be a toy/test application, but the real 92 thing. Specifically it needs to handle all the usual error and corner cases. 93 These are often the places where new uAPI falls apart and hence essential to 94 assess the fitness of a proposed interface. 95 96- The userspace side must be fully reviewed and tested to the standards of that 97 userspace project. For e.g. mesa this means piglit testcases and review on the 98 mailing list. This is again to ensure that the new interface actually gets the 99 job done. The userspace-side reviewer should also provide an Acked-by on the 100 kernel uAPI patch indicating that they believe the proposed uAPI is sound and 101 sufficiently documented and validated for userspace's consumption. 102 103- The userspace patches must be against the canonical upstream, not some vendor 104 fork. This is to make sure that no one cheats on the review and testing 105 requirements by doing a quick fork. 106 107- The kernel patch can only be merged after all the above requirements are met, 108 but it **must** be merged to either drm-next or drm-misc-next **before** the 109 userspace patches land. uAPI always flows from the kernel, doing things the 110 other way round risks divergence of the uAPI definitions and header files. 111 112These are fairly steep requirements, but have grown out from years of shared 113pain and experience with uAPI added hastily, and almost always regretted about 114just as fast. GFX devices change really fast, requiring a paradigm shift and 115entire new set of uAPI interfaces every few years at least. Together with the 116Linux kernel's guarantee to keep existing userspace running for 10+ years this 117is already rather painful for the DRM subsystem, with multiple different uAPIs 118for the same thing co-existing. If we add a few more complete mistakes into the 119mix every year it would be entirely unmanageable. 120 121.. _drm_render_node: 122 123Render nodes 124============ 125 126DRM core provides multiple character-devices for user-space to use. 127Depending on which device is opened, user-space can perform a different 128set of operations (mainly ioctls). The primary node is always created 129and called card<num>. Additionally, a currently unused control node, 130called controlD<num> is also created. The primary node provides all 131legacy operations and historically was the only interface used by 132userspace. With KMS, the control node was introduced. However, the 133planned KMS control interface has never been written and so the control 134node stays unused to date. 135 136With the increased use of offscreen renderers and GPGPU applications, 137clients no longer require running compositors or graphics servers to 138make use of a GPU. But the DRM API required unprivileged clients to 139authenticate to a DRM-Master prior to getting GPU access. To avoid this 140step and to grant clients GPU access without authenticating, render 141nodes were introduced. Render nodes solely serve render clients, that 142is, no modesetting or privileged ioctls can be issued on render nodes. 143Only non-global rendering commands are allowed. If a driver supports 144render nodes, it must advertise it via the DRIVER_RENDER DRM driver 145capability. If not supported, the primary node must be used for render 146clients together with the legacy drmAuth authentication procedure. 147 148If a driver advertises render node support, DRM core will create a 149separate render node called renderD<num>. There will be one render node 150per device. No ioctls except PRIME-related ioctls will be allowed on 151this node. Especially GEM_OPEN will be explicitly prohibited. For a 152complete list of driver-independent ioctls that can be used on render 153nodes, see the ioctls marked DRM_RENDER_ALLOW in drm_ioctl.c Render 154nodes are designed to avoid the buffer-leaks, which occur if clients 155guess the flink names or mmap offsets on the legacy interface. 156Additionally to this basic interface, drivers must mark their 157driver-dependent render-only ioctls as DRM_RENDER_ALLOW so render 158clients can use them. Driver authors must be careful not to allow any 159privileged ioctls on render nodes. 160 161With render nodes, user-space can now control access to the render node 162via basic file-system access-modes. A running graphics server which 163authenticates clients on the privileged primary/legacy node is no longer 164required. Instead, a client can open the render node and is immediately 165granted GPU access. Communication between clients (or servers) is done 166via PRIME. FLINK from render node to legacy node is not supported. New 167clients must not use the insecure FLINK interface. 168 169Besides dropping all modeset/global ioctls, render nodes also drop the 170DRM-Master concept. There is no reason to associate render clients with 171a DRM-Master as they are independent of any graphics server. Besides, 172they must work without any running master, anyway. Drivers must be able 173to run without a master object if they support render nodes. If, on the 174other hand, a driver requires shared state between clients which is 175visible to user-space and accessible beyond open-file boundaries, they 176cannot support render nodes. 177 178Device Hot-Unplug 179================= 180 181.. note:: 182 The following is the plan. Implementation is not there yet 183 (2020 May). 184 185Graphics devices (display and/or render) may be connected via USB (e.g. 186display adapters or docking stations) or Thunderbolt (e.g. eGPU). An end 187user is able to hot-unplug this kind of devices while they are being 188used, and expects that the very least the machine does not crash. Any 189damage from hot-unplugging a DRM device needs to be limited as much as 190possible and userspace must be given the chance to handle it if it wants 191to. Ideally, unplugging a DRM device still lets a desktop continue to 192run, but that is going to need explicit support throughout the whole 193graphics stack: from kernel and userspace drivers, through display 194servers, via window system protocols, and in applications and libraries. 195 196Other scenarios that should lead to the same are: unrecoverable GPU 197crash, PCI device disappearing off the bus, or forced unbind of a driver 198from the physical device. 199 200In other words, from userspace perspective everything needs to keep on 201working more or less, until userspace stops using the disappeared DRM 202device and closes it completely. Userspace will learn of the device 203disappearance from the device removed uevent, ioctls returning ENODEV 204(or driver-specific ioctls returning driver-specific things), or open() 205returning ENXIO. 206 207Only after userspace has closed all relevant DRM device and dmabuf file 208descriptors and removed all mmaps, the DRM driver can tear down its 209instance for the device that no longer exists. If the same physical 210device somehow comes back in the mean time, it shall be a new DRM 211device. 212 213Similar to PIDs, chardev minor numbers are not recycled immediately. A 214new DRM device always picks the next free minor number compared to the 215previous one allocated, and wraps around when minor numbers are 216exhausted. 217 218The goal raises at least the following requirements for the kernel and 219drivers. 220 221Requirements for KMS UAPI 222------------------------- 223 224- KMS connectors must change their status to disconnected. 225 226- Legacy modesets and pageflips, and atomic commits, both real and 227 TEST_ONLY, and any other ioctls either fail with ENODEV or fake 228 success. 229 230- Pending non-blocking KMS operations deliver the DRM events userspace 231 is expecting. This applies also to ioctls that faked success. 232 233- open() on a device node whose underlying device has disappeared will 234 fail with ENXIO. 235 236- Attempting to create a DRM lease on a disappeared DRM device will 237 fail with ENODEV. Existing DRM leases remain and work as listed 238 above. 239 240Requirements for Render and Cross-Device UAPI 241--------------------------------------------- 242 243- All GPU jobs that can no longer run must have their fences 244 force-signalled to avoid inflicting hangs on userspace. 245 The associated error code is ENODEV. 246 247- Some userspace APIs already define what should happen when the device 248 disappears (OpenGL, GL ES: `GL_KHR_robustness`_; `Vulkan`_: 249 VK_ERROR_DEVICE_LOST; etc.). DRM drivers are free to implement this 250 behaviour the way they see best, e.g. returning failures in 251 driver-specific ioctls and handling those in userspace drivers, or 252 rely on uevents, and so on. 253 254- dmabuf which point to memory that has disappeared will either fail to 255 import with ENODEV or continue to be successfully imported if it would 256 have succeeded before the disappearance. See also about memory maps 257 below for already imported dmabufs. 258 259- Attempting to import a dmabuf to a disappeared device will either fail 260 with ENODEV or succeed if it would have succeeded without the 261 disappearance. 262 263- open() on a device node whose underlying device has disappeared will 264 fail with ENXIO. 265 266.. _GL_KHR_robustness: https://www.khronos.org/registry/OpenGL/extensions/KHR/KHR_robustness.txt 267.. _Vulkan: https://www.khronos.org/vulkan/ 268 269Requirements for Memory Maps 270---------------------------- 271 272Memory maps have further requirements that apply to both existing maps 273and maps created after the device has disappeared. If the underlying 274memory disappears, the map is created or modified such that reads and 275writes will still complete successfully but the result is undefined. 276This applies to both userspace mmap()'d memory and memory pointed to by 277dmabuf which might be mapped to other devices (cross-device dmabuf 278imports). 279 280Raising SIGBUS is not an option, because userspace cannot realistically 281handle it. Signal handlers are global, which makes them extremely 282difficult to use correctly from libraries like those that Mesa produces. 283Signal handlers are not composable, you can't have different handlers 284for GPU1 and GPU2 from different vendors, and a third handler for 285mmapped regular files. Threads cause additional pain with signal 286handling as well. 287 288.. _drm_driver_ioctl: 289 290IOCTL Support on Device Nodes 291============================= 292 293.. kernel-doc:: drivers/gpu/drm/drm_ioctl.c 294 :doc: driver specific ioctls 295 296Recommended IOCTL Return Values 297------------------------------- 298 299In theory a driver's IOCTL callback is only allowed to return very few error 300codes. In practice it's good to abuse a few more. This section documents common 301practice within the DRM subsystem: 302 303ENOENT: 304 Strictly this should only be used when a file doesn't exist e.g. when 305 calling the open() syscall. We reuse that to signal any kind of object 306 lookup failure, e.g. for unknown GEM buffer object handles, unknown KMS 307 object handles and similar cases. 308 309ENOSPC: 310 Some drivers use this to differentiate "out of kernel memory" from "out 311 of VRAM". Sometimes also applies to other limited gpu resources used for 312 rendering (e.g. when you have a special limited compression buffer). 313 Sometimes resource allocation/reservation issues in command submission 314 IOCTLs are also signalled through EDEADLK. 315 316 Simply running out of kernel/system memory is signalled through ENOMEM. 317 318EPERM/EACCES: 319 Returned for an operation that is valid, but needs more privileges. 320 E.g. root-only or much more common, DRM master-only operations return 321 this when called by unpriviledged clients. There's no clear 322 difference between EACCES and EPERM. 323 324ENODEV: 325 The device is not present anymore or is not yet fully initialized. 326 327EOPNOTSUPP: 328 Feature (like PRIME, modesetting, GEM) is not supported by the driver. 329 330ENXIO: 331 Remote failure, either a hardware transaction (like i2c), but also used 332 when the exporting driver of a shared dma-buf or fence doesn't support a 333 feature needed. 334 335EINTR: 336 DRM drivers assume that userspace restarts all IOCTLs. Any DRM IOCTL can 337 return EINTR and in such a case should be restarted with the IOCTL 338 parameters left unchanged. 339 340EIO: 341 The GPU died and couldn't be resurrected through a reset. Modesetting 342 hardware failures are signalled through the "link status" connector 343 property. 344 345EINVAL: 346 Catch-all for anything that is an invalid argument combination which 347 cannot work. 348 349IOCTL also use other error codes like ETIME, EFAULT, EBUSY, ENOTTY but their 350usage is in line with the common meanings. The above list tries to just document 351DRM specific patterns. Note that ENOTTY has the slightly unintuitive meaning of 352"this IOCTL does not exist", and is used exactly as such in DRM. 353 354.. kernel-doc:: include/drm/drm_ioctl.h 355 :internal: 356 357.. kernel-doc:: drivers/gpu/drm/drm_ioctl.c 358 :export: 359 360.. kernel-doc:: drivers/gpu/drm/drm_ioc32.c 361 :export: 362 363Testing and validation 364====================== 365 366Testing Requirements for userspace API 367-------------------------------------- 368 369New cross-driver userspace interface extensions, like new IOCTL, new KMS 370properties, new files in sysfs or anything else that constitutes an API change 371should have driver-agnostic testcases in IGT for that feature, if such a test 372can be reasonably made using IGT for the target hardware. 373 374Validating changes with IGT 375--------------------------- 376 377There's a collection of tests that aims to cover the whole functionality of 378DRM drivers and that can be used to check that changes to DRM drivers or the 379core don't regress existing functionality. This test suite is called IGT and 380its code and instructions to build and run can be found in 381https://gitlab.freedesktop.org/drm/igt-gpu-tools/. 382 383Using VKMS to test DRM API 384-------------------------- 385 386VKMS is a software-only model of a KMS driver that is useful for testing 387and for running compositors. VKMS aims to enable a virtual display without 388the need for a hardware display capability. These characteristics made VKMS 389a perfect tool for validating the DRM core behavior and also support the 390compositor developer. VKMS makes it possible to test DRM functions in a 391virtual machine without display, simplifying the validation of some of the 392core changes. 393 394To Validate changes in DRM API with VKMS, start setting the kernel: make 395sure to enable VKMS module; compile the kernel with the VKMS enabled and 396install it in the target machine. VKMS can be run in a Virtual Machine 397(QEMU, virtme or similar). It's recommended the use of KVM with the minimum 398of 1GB of RAM and four cores. 399 400It's possible to run the IGT-tests in a VM in two ways: 401 402 1. Use IGT inside a VM 403 2. Use IGT from the host machine and write the results in a shared directory. 404 405Following is an example of using a VM with a shared directory with 406the host machine to run igt-tests. This example uses virtme:: 407 408 $ virtme-run --rwdir /path/for/shared_dir --kdir=path/for/kernel/directory --mods=auto 409 410Run the igt-tests in the guest machine. This example runs the 'kms_flip' 411tests:: 412 413 $ /path/for/igt-gpu-tools/scripts/run-tests.sh -p -s -t "kms_flip.*" -v 414 415In this example, instead of building the igt_runner, Piglit is used 416(-p option). It creates an HTML summary of the test results and saves 417them in the folder "igt-gpu-tools/results". It executes only the igt-tests 418matching the -t option. 419 420Display CRC Support 421------------------- 422 423.. kernel-doc:: drivers/gpu/drm/drm_debugfs_crc.c 424 :doc: CRC ABI 425 426.. kernel-doc:: drivers/gpu/drm/drm_debugfs_crc.c 427 :export: 428 429Debugfs Support 430--------------- 431 432.. kernel-doc:: include/drm/drm_debugfs.h 433 :internal: 434 435.. kernel-doc:: drivers/gpu/drm/drm_debugfs.c 436 :export: 437 438Sysfs Support 439============= 440 441.. kernel-doc:: drivers/gpu/drm/drm_sysfs.c 442 :doc: overview 443 444.. kernel-doc:: drivers/gpu/drm/drm_sysfs.c 445 :export: 446 447 448VBlank event handling 449===================== 450 451The DRM core exposes two vertical blank related ioctls: 452 453DRM_IOCTL_WAIT_VBLANK 454 This takes a struct drm_wait_vblank structure as its argument, and 455 it is used to block or request a signal when a specified vblank 456 event occurs. 457 458DRM_IOCTL_MODESET_CTL 459 This was only used for user-mode-settind drivers around modesetting 460 changes to allow the kernel to update the vblank interrupt after 461 mode setting, since on many devices the vertical blank counter is 462 reset to 0 at some point during modeset. Modern drivers should not 463 call this any more since with kernel mode setting it is a no-op. 464 465Userspace API Structures 466======================== 467 468.. kernel-doc:: include/uapi/drm/drm_mode.h 469 :doc: overview 470 471.. _crtc_index: 472 473CRTC index 474---------- 475 476CRTC's have both an object ID and an index, and they are not the same thing. 477The index is used in cases where a densely packed identifier for a CRTC is 478needed, for instance a bitmask of CRTC's. The member possible_crtcs of struct 479drm_mode_get_plane is an example. 480 481DRM_IOCTL_MODE_GETRESOURCES populates a structure with an array of CRTC ID's, 482and the CRTC index is its position in this array. 483 484.. kernel-doc:: include/uapi/drm/drm.h 485 :internal: 486 487.. kernel-doc:: include/uapi/drm/drm_mode.h 488 :internal: 489