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
405As follow, there is an example of using a VM with a shared directory with
406the host machine to run igt-tests. As an example it's used 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, as example it's ran 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 build the igt_runner, Piglit is used
416(-p option); it's created html summary of the tests results and it's saved
417in the folder "igt-gpu-tools/results"; it's executed 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