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