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 288Device reset 289============ 290 291The GPU stack is really complex and is prone to errors, from hardware bugs, 292faulty applications and everything in between the many layers. Some errors 293require resetting the device in order to make the device usable again. This 294section describes the expectations for DRM and usermode drivers when a 295device resets and how to propagate the reset status. 296 297Device resets can not be disabled without tainting the kernel, which can lead to 298hanging the entire kernel through shrinkers/mmu_notifiers. Userspace role in 299device resets is to propagate the message to the application and apply any 300special policy for blocking guilty applications, if any. Corollary is that 301debugging a hung GPU context require hardware support to be able to preempt such 302a GPU context while it's stopped. 303 304Kernel Mode Driver 305------------------ 306 307The KMD is responsible for checking if the device needs a reset, and to perform 308it as needed. Usually a hang is detected when a job gets stuck executing. 309 310Propagation of errors to userspace has proven to be tricky since it goes in 311the opposite direction of the usual flow of commands. Because of this vendor 312independent error handling was added to the &dma_fence object, this way drivers 313can add an error code to their fences before signaling them. See function 314dma_fence_set_error() on how to do this and for examples of error codes to use. 315 316The DRM scheduler also allows setting error codes on all pending fences when 317hardware submissions are restarted after an reset. Error codes are also 318forwarded from the hardware fence to the scheduler fence to bubble up errors 319to the higher levels of the stack and eventually userspace. 320 321Fence errors can be queried by userspace through the generic SYNC_IOC_FILE_INFO 322IOCTL as well as through driver specific interfaces. 323 324Additional to setting fence errors drivers should also keep track of resets per 325context, the DRM scheduler provides the drm_sched_entity_error() function as 326helper for this use case. After a reset, KMD should reject new command 327submissions for affected contexts. 328 329User Mode Driver 330---------------- 331 332After command submission, UMD should check if the submission was accepted or 333rejected. After a reset, KMD should reject submissions, and UMD can issue an 334ioctl to the KMD to check the reset status, and this can be checked more often 335if the UMD requires it. After detecting a reset, UMD will then proceed to report 336it to the application using the appropriate API error code, as explained in the 337section below about robustness. 338 339Robustness 340---------- 341 342The only way to try to keep a graphical API context working after a reset is if 343it complies with the robustness aspects of the graphical API that it is using. 344 345Graphical APIs provide ways to applications to deal with device resets. However, 346there is no guarantee that the app will use such features correctly, and a 347userspace that doesn't support robust interfaces (like a non-robust 348OpenGL context or API without any robustness support like libva) leave the 349robustness handling entirely to the userspace driver. There is no strong 350community consensus on what the userspace driver should do in that case, 351since all reasonable approaches have some clear downsides. 352 353OpenGL 354~~~~~~ 355 356Apps using OpenGL should use the available robust interfaces, like the 357extension ``GL_ARB_robustness`` (or ``GL_EXT_robustness`` for OpenGL ES). This 358interface tells if a reset has happened, and if so, all the context state is 359considered lost and the app proceeds by creating new ones. There's no consensus 360on what to do to if robustness is not in use. 361 362Vulkan 363~~~~~~ 364 365Apps using Vulkan should check for ``VK_ERROR_DEVICE_LOST`` for submissions. 366This error code means, among other things, that a device reset has happened and 367it needs to recreate the contexts to keep going. 368 369Reporting causes of resets 370-------------------------- 371 372Apart from propagating the reset through the stack so apps can recover, it's 373really useful for driver developers to learn more about what caused the reset in 374the first place. For this, drivers can make use of devcoredump to store relevant 375information about the reset and send device wedged event with ``none`` recovery 376method (as explained in "Device Wedging" chapter) to notify userspace, so this 377information can be collected and added to user bug reports. 378 379Device Wedging 380============== 381 382Drivers can optionally make use of device wedged event (implemented as 383drm_dev_wedged_event() in DRM subsystem), which notifies userspace of 'wedged' 384(hanged/unusable) state of the DRM device through a uevent. This is useful 385especially in cases where the device is no longer operating as expected and has 386become unrecoverable from driver context. Purpose of this implementation is to 387provide drivers a generic way to recover the device with the help of userspace 388intervention, without taking any drastic measures (like resetting or 389re-enumerating the full bus, on which the underlying physical device is sitting) 390in the driver. 391 392A 'wedged' device is basically a device that is declared dead by the driver 393after exhausting all possible attempts to recover it from driver context. The 394uevent is the notification that is sent to userspace along with a hint about 395what could possibly be attempted to recover the device from userspace and bring 396it back to usable state. Different drivers may have different ideas of a 397'wedged' device depending on hardware implementation of the underlying physical 398device, and hence the vendor agnostic nature of the event. It is up to the 399drivers to decide when they see the need for device recovery and how they want 400to recover from the available methods. 401 402Driver prerequisites 403-------------------- 404 405The driver, before opting for recovery, needs to make sure that the 'wedged' 406device doesn't harm the system as a whole by taking care of the prerequisites. 407Necessary actions must include disabling DMA to system memory as well as any 408communication channels with other devices. Further, the driver must ensure 409that all dma_fences are signalled and any device state that the core kernel 410might depend on is cleaned up. All existing mmaps should be invalidated and 411page faults should be redirected to a dummy page. Once the event is sent, the 412device must be kept in 'wedged' state until the recovery is performed. New 413accesses to the device (IOCTLs) should be rejected, preferably with an error 414code that resembles the type of failure the device has encountered. This will 415signify the reason for wedging, which can be reported to the application if 416needed. 417 418Recovery 419-------- 420 421Current implementation defines four recovery methods, out of which, drivers 422can use any one, multiple or none. Method(s) of choice will be sent in the 423uevent environment as ``WEDGED=<method1>[,..,<methodN>]`` in order of less to 424more side-effects. See the section `Vendor Specific Recovery`_ 425for ``WEDGED=vendor-specific``. If driver is unsure about recovery or 426method is unknown, ``WEDGED=unknown`` will be sent instead. 427 428Userspace consumers can parse this event and attempt recovery as per the 429following expectations. 430 431 =============== ======================================== 432 Recovery method Consumer expectations 433 =============== ======================================== 434 none optional telemetry collection 435 rebind unbind + bind driver 436 bus-reset unbind + bus reset/re-enumeration + bind 437 vendor-specific vendor specific recovery method 438 unknown consumer policy 439 =============== ======================================== 440 441The only exception to this is ``WEDGED=none``, which signifies that the device 442was temporarily 'wedged' at some point but was recovered from driver context 443using device specific methods like reset. No explicit recovery is expected from 444the consumer in this case, but it can still take additional steps like gathering 445telemetry information (devcoredump, syslog). This is useful because the first 446hang is usually the most critical one which can result in consequential hangs or 447complete wedging. 448 449 450Vendor Specific Recovery 451------------------------ 452 453When ``WEDGED=vendor-specific`` is sent, it indicates that the device requires 454a recovery procedure specific to the hardware vendor and is not one of the 455standardized approaches. 456 457``WEDGED=vendor-specific`` may be used to indicate different cases within a 458single vendor driver, each requiring a distinct recovery procedure. 459In such scenarios, the vendor driver must provide comprehensive documentation 460that describes each case, include additional hints to identify specific case and 461outline the corresponding recovery procedure. The documentation includes: 462 463Case - A list of all cases that sends the ``WEDGED=vendor-specific`` recovery method. 464 465Hints - Additional Information to assist the userspace consumer in identifying and 466differentiating between different cases. This can be exposed through sysfs, debugfs, 467traces, dmesg etc. 468 469Recovery Procedure - Clear instructions and guidance for recovering each case. 470This may include userspace scripts, tools needed for the recovery procedure. 471 472It is the responsibility of the admin/userspace consumer to identify the case and 473verify additional identification hints before attempting a recovery procedure. 474 475Example: If the device uses the Xe driver, then userspace consumer should refer to 476:ref:`Xe Device Wedging <xe-device-wedging>` for the detailed documentation. 477 478Task information 479---------------- 480 481The information about which application (if any) was involved in the device 482wedging is useful for userspace if they want to notify the user about what 483happened (e.g. the compositor display a message to the user "The <task name> 484caused a graphical error and the system recovered") or to implement policies 485(e.g. the daemon may "ban" an task that keeps resetting the device). If the task 486information is available, the uevent will display as ``PID=<pid>`` and 487``TASK=<task name>``. Otherwise, ``PID`` and ``TASK`` will not appear in the 488event string. 489 490The reliability of this information is driver and hardware specific, and should 491be taken with a caution regarding it's precision. To have a big picture of what 492really happened, the devcoredump file provides much more detailed information 493about the device state and about the event. 494 495Consumer prerequisites 496---------------------- 497 498It is the responsibility of the consumer to make sure that the device or its 499resources are not in use by any process before attempting recovery. With IOCTLs 500erroring out, all device memory should be unmapped and file descriptors should 501be closed to prevent leaks or undefined behaviour. The idea here is to clear the 502device of all user context beforehand and set the stage for a clean recovery. 503 504For ``WEDGED=vendor-specific`` recovery method, it is the responsibility of the 505consumer to check the driver documentation and the usecase before attempting 506a recovery. 507 508Example - rebind 509---------------- 510 511Udev rule:: 512 513 SUBSYSTEM=="drm", ENV{WEDGED}=="rebind", DEVPATH=="*/drm/card[0-9]", 514 RUN+="/path/to/rebind.sh $env{DEVPATH}" 515 516Recovery script:: 517 518 #!/bin/sh 519 520 DEVPATH=$(readlink -f /sys/$1/device) 521 DEVICE=$(basename $DEVPATH) 522 DRIVER=$(readlink -f $DEVPATH/driver) 523 524 echo -n $DEVICE > $DRIVER/unbind 525 echo -n $DEVICE > $DRIVER/bind 526 527Customization 528------------- 529 530Although basic recovery is possible with a simple script, consumers can define 531custom policies around recovery. For example, if the driver supports multiple 532recovery methods, consumers can opt for the suitable one depending on scenarios 533like repeat offences or vendor specific failures. Consumers can also choose to 534have the device available for debugging or telemetry collection and base their 535recovery decision on the findings. This is useful especially when the driver is 536unsure about recovery or method is unknown. 537 538.. _drm_driver_ioctl: 539 540IOCTL Support on Device Nodes 541============================= 542 543.. kernel-doc:: drivers/gpu/drm/drm_ioctl.c 544 :doc: driver specific ioctls 545 546Recommended IOCTL Return Values 547------------------------------- 548 549In theory a driver's IOCTL callback is only allowed to return very few error 550codes. In practice it's good to abuse a few more. This section documents common 551practice within the DRM subsystem: 552 553ENOENT: 554 Strictly this should only be used when a file doesn't exist e.g. when 555 calling the open() syscall. We reuse that to signal any kind of object 556 lookup failure, e.g. for unknown GEM buffer object handles, unknown KMS 557 object handles and similar cases. 558 559ENOSPC: 560 Some drivers use this to differentiate "out of kernel memory" from "out 561 of VRAM". Sometimes also applies to other limited gpu resources used for 562 rendering (e.g. when you have a special limited compression buffer). 563 Sometimes resource allocation/reservation issues in command submission 564 IOCTLs are also signalled through EDEADLK. 565 566 Simply running out of kernel/system memory is signalled through ENOMEM. 567 568EPERM/EACCES: 569 Returned for an operation that is valid, but needs more privileges. 570 E.g. root-only or much more common, DRM master-only operations return 571 this when called by unpriviledged clients. There's no clear 572 difference between EACCES and EPERM. 573 574ENODEV: 575 The device is not present anymore or is not yet fully initialized. 576 577EOPNOTSUPP: 578 Feature (like PRIME, modesetting, GEM) is not supported by the driver. 579 580ENXIO: 581 Remote failure, either a hardware transaction (like i2c), but also used 582 when the exporting driver of a shared dma-buf or fence doesn't support a 583 feature needed. 584 585EINTR: 586 DRM drivers assume that userspace restarts all IOCTLs. Any DRM IOCTL can 587 return EINTR and in such a case should be restarted with the IOCTL 588 parameters left unchanged. 589 590EIO: 591 The GPU died and couldn't be resurrected through a reset. Modesetting 592 hardware failures are signalled through the "link status" connector 593 property. 594 595EINVAL: 596 Catch-all for anything that is an invalid argument combination which 597 cannot work. 598 599IOCTL also use other error codes like ETIME, EFAULT, EBUSY, ENOTTY but their 600usage is in line with the common meanings. The above list tries to just document 601DRM specific patterns. Note that ENOTTY has the slightly unintuitive meaning of 602"this IOCTL does not exist", and is used exactly as such in DRM. 603 604.. kernel-doc:: include/drm/drm_ioctl.h 605 :internal: 606 607.. kernel-doc:: drivers/gpu/drm/drm_ioctl.c 608 :export: 609 610.. kernel-doc:: drivers/gpu/drm/drm_ioc32.c 611 :export: 612 613Testing and validation 614====================== 615 616Testing Requirements for userspace API 617-------------------------------------- 618 619New cross-driver userspace interface extensions, like new IOCTL, new KMS 620properties, new files in sysfs or anything else that constitutes an API change 621should have driver-agnostic testcases in IGT for that feature, if such a test 622can be reasonably made using IGT for the target hardware. 623 624Validating changes with IGT 625--------------------------- 626 627There's a collection of tests that aims to cover the whole functionality of 628DRM drivers and that can be used to check that changes to DRM drivers or the 629core don't regress existing functionality. This test suite is called IGT and 630its code and instructions to build and run can be found in 631https://gitlab.freedesktop.org/drm/igt-gpu-tools/. 632 633Using VKMS to test DRM API 634-------------------------- 635 636VKMS is a software-only model of a KMS driver that is useful for testing 637and for running compositors. VKMS aims to enable a virtual display without 638the need for a hardware display capability. These characteristics made VKMS 639a perfect tool for validating the DRM core behavior and also support the 640compositor developer. VKMS makes it possible to test DRM functions in a 641virtual machine without display, simplifying the validation of some of the 642core changes. 643 644To Validate changes in DRM API with VKMS, start setting the kernel: make 645sure to enable VKMS module; compile the kernel with the VKMS enabled and 646install it in the target machine. VKMS can be run in a Virtual Machine 647(QEMU, virtme or similar). It's recommended the use of KVM with the minimum 648of 1GB of RAM and four cores. 649 650It's possible to run the IGT-tests in a VM in two ways: 651 652 1. Use IGT inside a VM 653 2. Use IGT from the host machine and write the results in a shared directory. 654 655Following is an example of using a VM with a shared directory with 656the host machine to run igt-tests. This example uses virtme:: 657 658 $ virtme-run --rwdir /path/for/shared_dir --kdir=path/for/kernel/directory --mods=auto 659 660Run the igt-tests in the guest machine. This example runs the 'kms_flip' 661tests:: 662 663 $ /path/for/igt-gpu-tools/scripts/run-tests.sh -p -s -t "kms_flip.*" -v 664 665In this example, instead of building the igt_runner, Piglit is used 666(-p option). It creates an HTML summary of the test results and saves 667them in the folder "igt-gpu-tools/results". It executes only the igt-tests 668matching the -t option. 669 670Display CRC Support 671------------------- 672 673.. kernel-doc:: drivers/gpu/drm/drm_debugfs_crc.c 674 :doc: CRC ABI 675 676.. kernel-doc:: drivers/gpu/drm/drm_debugfs_crc.c 677 :export: 678 679Debugfs Support 680--------------- 681 682.. kernel-doc:: include/drm/drm_debugfs.h 683 :internal: 684 685.. kernel-doc:: drivers/gpu/drm/drm_debugfs.c 686 :export: 687 688Sysfs Support 689============= 690 691.. kernel-doc:: drivers/gpu/drm/drm_sysfs.c 692 :doc: overview 693 694.. kernel-doc:: drivers/gpu/drm/drm_sysfs.c 695 :export: 696 697 698VBlank event handling 699===================== 700 701The DRM core exposes two vertical blank related ioctls: 702 703:c:macro:`DRM_IOCTL_WAIT_VBLANK` 704 This takes a struct drm_wait_vblank structure as its argument, and 705 it is used to block or request a signal when a specified vblank 706 event occurs. 707 708:c:macro:`DRM_IOCTL_MODESET_CTL` 709 This was only used for user-mode-settind drivers around modesetting 710 changes to allow the kernel to update the vblank interrupt after 711 mode setting, since on many devices the vertical blank counter is 712 reset to 0 at some point during modeset. Modern drivers should not 713 call this any more since with kernel mode setting it is a no-op. 714 715Userspace API Structures 716======================== 717 718.. kernel-doc:: include/uapi/drm/drm_mode.h 719 :doc: overview 720 721.. _crtc_index: 722 723CRTC index 724---------- 725 726CRTC's have both an object ID and an index, and they are not the same thing. 727The index is used in cases where a densely packed identifier for a CRTC is 728needed, for instance a bitmask of CRTC's. The member possible_crtcs of struct 729drm_mode_get_plane is an example. 730 731:c:macro:`DRM_IOCTL_MODE_GETRESOURCES` populates a structure with an array of 732CRTC ID's, and the CRTC index is its position in this array. 733 734.. kernel-doc:: include/uapi/drm/drm.h 735 :internal: 736 737.. kernel-doc:: include/uapi/drm/drm_mode.h 738 :internal: 739 740 741dma-buf interoperability 742======================== 743 744Please see Documentation/userspace-api/dma-buf-alloc-exchange.rst for 745information on how dma-buf is integrated and exposed within DRM. 746 747 748Trace events 749============ 750 751See Documentation/trace/tracepoints.rst for information about using 752Linux Kernel Tracepoints. 753In the DRM subsystem, some events are considered stable uAPI to avoid 754breaking tools (e.g.: GPUVis, umr) relying on them. Stable means that fields 755cannot be removed, nor their formatting updated. Adding new fields is 756possible, under the normal uAPI requirements. 757 758Stable uAPI events 759------------------ 760 761From ``drivers/gpu/drm/scheduler/gpu_scheduler_trace.h`` 762 763.. kernel-doc:: drivers/gpu/drm/scheduler/gpu_scheduler_trace.h 764 :doc: uAPI trace events