Documentation/gpu/drm-uapi.rst

   1 ===================
   2 Userland interfaces
   3 ===================
   4
   5 The DRM core exports several interfaces to applications, generally
   6 intended to be used through corresponding libdrm wrapper functions. In
   7 addition, drivers export device-specific interfaces for use by userspace
   8 drivers & device-aware applications through ioctls and sysfs files.
   9
  10 External interfaces include: memory mapping, context management, DMA
  11 operations, AGP management, vblank control, fence management, memory
  12 management, and output management.
  13
  14 Cover generic ioctls and sysfs layout here. We only need high-level
  15 info, since man pages should cover the rest.
  16
  17 libdrm Device Lookup
  18 ====================
  19
  20 .. kernel-doc:: drivers/gpu/drm/drm_ioctl.c
  21    :doc: getunique and setversion story
  22
  23
  24 .. _drm_primary_node:
  25
  26 Primary Nodes, DRM Master and Authentication
  27 ============================================
  28
  29 .. kernel-doc:: drivers/gpu/drm/drm_auth.c
  30    :doc: master and authentication
  31
  32 .. kernel-doc:: drivers/gpu/drm/drm_auth.c
  33    :export:
  34
  35 .. kernel-doc:: include/drm/drm_auth.h
  36    :internal:
  37
  38 Open-Source Userspace Requirements
  39 ==================================
  40
  41 The DRM subsystem has stricter requirements than most other kernel subsystems on
  42 what the userspace side for new uAPI needs to look like. This section here
  43 explains what exactly those requirements are, and why they exist.
  44
  45 The short summary is that any addition of DRM uAPI requires corresponding
  46 open-sourced userspace patches, and those patches must be reviewed and ready for
  47 merging into a suitable and canonical upstream project.
  48
  49 GFX devices (both display and render/GPU side) are really complex bits of
  50 hardware, with userspace and kernel by necessity having to work together really
  51 closely.  The interfaces, for rendering and modesetting, must be extremely wide
  52 and flexible, and therefore it is almost always impossible to precisely define
  53 them for every possible corner case. This in turn makes it really practically
  54 infeasible to differentiate between behaviour that's required by userspace, and
  55 which must not be changed to avoid regressions, and behaviour which is only an
  56 accidental artifact of the current implementation.
  57
  58 Without access to the full source code of all userspace users that means it
  59 becomes impossible to change the implementation details, since userspace could
  60 depend upon the accidental behaviour of the current implementation in minute
  61 details. And debugging such regressions without access to source code is pretty
  62 much impossible. As a consequence this means:
  63
  64 - The Linux kernel's "no regression" policy holds in practice only for
  65   open-source userspace of the DRM subsystem. DRM developers are perfectly fine
  66   if closed-source blob drivers in userspace use the same uAPI as the open
  67   drivers, but they must do so in the exact same way as the open drivers.
  68   Creative (ab)use of the interfaces will, and in the past routinely has, lead
  69   to breakage.
  70
  71 - Any new userspace interface must have an open-source implementation as
  72   demonstration vehicle.
  73
  74 The other reason for requiring open-source userspace is uAPI review. Since the
  75 kernel and userspace parts of a GFX stack must work together so closely, code
  76 review can only assess whether a new interface achieves its goals by looking at
  77 both sides. Making sure that the interface indeed covers the use-case fully
  78 leads to a few additional requirements:
  79
  80 - The open-source userspace must not be a toy/test application, but the real
  81   thing. Specifically it needs to handle all the usual error and corner cases.
  82   These are often the places where new uAPI falls apart and hence essential to
  83   assess the fitness of a proposed interface.
  84
  85 - The userspace side must be fully reviewed and tested to the standards of that
  86   userspace project. For e.g. mesa this means piglit testcases and review on the
  87   mailing list. This is again to ensure that the new interface actually gets the
  88   job done.
  89
  90 - The userspace patches must be against the canonical upstream, not some vendor
  91   fork. This is to make sure that no one cheats on the review and testing
  92   requirements by doing a quick fork.
  93
  94 - The kernel patch can only be merged after all the above requirements are met,
  95   but it **must** be merged **before** the userspace patches land. uAPI always flows
  96   from the kernel, doing things the other way round risks divergence of the uAPI
  97   definitions and header files.
  98
  99 These are fairly steep requirements, but have grown out from years of shared
 100 pain and experience with uAPI added hastily, and almost always regretted about
 101 just as fast. GFX devices change really fast, requiring a paradigm shift and
 102 entire new set of uAPI interfaces every few years at least. Together with the
 103 Linux kernel's guarantee to keep existing userspace running for 10+ years this
 104 is already rather painful for the DRM subsystem, with multiple different uAPIs
 105 for the same thing co-existing. If we add a few more complete mistakes into the
 106 mix every year it would be entirely unmanageable.
 107
 108 .. _drm_render_node:
 109
 110 Render nodes
 111 ============
 112
 113 DRM core provides multiple character-devices for user-space to use.
 114 Depending on which device is opened, user-space can perform a different
 115 set of operations (mainly ioctls). The primary node is always created
 116 and called card<num>. Additionally, a currently unused control node,
 117 called controlD<num> is also created. The primary node provides all
 118 legacy operations and historically was the only interface used by
 119 userspace. With KMS, the control node was introduced. However, the
 120 planned KMS control interface has never been written and so the control
 121 node stays unused to date.
 122
 123 With the increased use of offscreen renderers and GPGPU applications,
 124 clients no longer require running compositors or graphics servers to
 125 make use of a GPU. But the DRM API required unprivileged clients to
 126 authenticate to a DRM-Master prior to getting GPU access. To avoid this
 127 step and to grant clients GPU access without authenticating, render
 128 nodes were introduced. Render nodes solely serve render clients, that
 129 is, no modesetting or privileged ioctls can be issued on render nodes.
 130 Only non-global rendering commands are allowed. If a driver supports
 131 render nodes, it must advertise it via the DRIVER_RENDER DRM driver
 132 capability. If not supported, the primary node must be used for render
 133 clients together with the legacy drmAuth authentication procedure.
 134
 135 If a driver advertises render node support, DRM core will create a
 136 separate render node called renderD<num>. There will be one render node
 137 per device. No ioctls except PRIME-related ioctls will be allowed on
 138 this node. Especially GEM_OPEN will be explicitly prohibited. Render
 139 nodes are designed to avoid the buffer-leaks, which occur if clients
 140 guess the flink names or mmap offsets on the legacy interface.
 141 Additionally to this basic interface, drivers must mark their
 142 driver-dependent render-only ioctls as DRM_RENDER_ALLOW so render
 143 clients can use them. Driver authors must be careful not to allow any
 144 privileged ioctls on render nodes.
 145
 146 With render nodes, user-space can now control access to the render node
 147 via basic file-system access-modes. A running graphics server which
 148 authenticates clients on the privileged primary/legacy node is no longer
 149 required. Instead, a client can open the render node and is immediately
 150 granted GPU access. Communication between clients (or servers) is done
 151 via PRIME. FLINK from render node to legacy node is not supported. New
 152 clients must not use the insecure FLINK interface.
 153
 154 Besides dropping all modeset/global ioctls, render nodes also drop the
 155 DRM-Master concept. There is no reason to associate render clients with
 156 a DRM-Master as they are independent of any graphics server. Besides,
 157 they must work without any running master, anyway. Drivers must be able
 158 to run without a master object if they support render nodes. If, on the
 159 other hand, a driver requires shared state between clients which is
 160 visible to user-space and accessible beyond open-file boundaries, they
 161 cannot support render nodes.
 162
 163 .. _drm_driver_ioctl:
 164
 165 IOCTL Support on Device Nodes
 166 =============================
 167
 168 .. kernel-doc:: drivers/gpu/drm/drm_ioctl.c
 169    :doc: driver specific ioctls
 170
 171 Recommended IOCTL Return Values
 172 -------------------------------
 173
 174 In theory a driver's IOCTL callback is only allowed to return very few error
 175 codes. In practice it's good to abuse a few more. This section documents common
 176 practice within the DRM subsystem:
 177
 178 ENOENT:
 179         Strictly this should only be used when a file doesn't exist e.g. when
 180         calling the open() syscall. We reuse that to signal any kind of object
 181         lookup failure, e.g. for unknown GEM buffer object handles, unknown KMS
 182         object handles and similar cases.
 183
 184 ENOSPC:
 185         Some drivers use this to differentiate "out of kernel memory" from "out
 186         of VRAM". Sometimes also applies to other limited gpu resources used for
 187         rendering (e.g. when you have a special limited compression buffer).
 188         Sometimes resource allocation/reservation issues in command submission
 189         IOCTLs are also signalled through EDEADLK.
 190
 191         Simply running out of kernel/system memory is signalled through ENOMEM.
 192
 193 EPERM/EACCES:
 194         Returned for an operation that is valid, but needs more privileges.
 195         E.g. root-only or much more common, DRM master-only operations return
 196         this when when called by unpriviledged clients. There's no clear
 197         difference between EACCES and EPERM.
 198
 199 ENODEV:
 200         The device is not (yet) present or fully initialized.
 201
 202 EOPNOTSUPP:
 203         Feature (like PRIME, modesetting, GEM) is not supported by the driver.
 204
 205 ENXIO:
 206         Remote failure, either a hardware transaction (like i2c), but also used
 207         when the exporting driver of a shared dma-buf or fence doesn't support a
 208         feature needed.
 209
 210 EINTR:
 211         DRM drivers assume that userspace restarts all IOCTLs. Any DRM IOCTL can
 212         return EINTR and in such a case should be restarted with the IOCTL
 213         parameters left unchanged.
 214
 215 EIO:
 216         The GPU died and couldn't be resurrected through a reset. Modesetting
 217         hardware failures are signalled through the "link status" connector
 218         property.
 219
 220 EINVAL:
 221         Catch-all for anything that is an invalid argument combination which
 222         cannot work.
 223
 224 IOCTL also use other error codes like ETIME, EFAULT, EBUSY, ENOTTY but their
 225 usage is in line with the common meanings. The above list tries to just document
 226 DRM specific patterns. Note that ENOTTY has the slightly unintuitive meaning of
 227 "this IOCTL does not exist", and is used exactly as such in DRM.
 228
 229 .. kernel-doc:: include/drm/drm_ioctl.h
 230    :internal:
 231
 232 .. kernel-doc:: drivers/gpu/drm/drm_ioctl.c
 233    :export:
 234
 235 .. kernel-doc:: drivers/gpu/drm/drm_ioc32.c
 236    :export:
 237
 238 Testing and validation
 239 ======================
 240
 241 Testing Requirements for userspace API
 242 --------------------------------------
 243
 244 New cross-driver userspace interface extensions, like new IOCTL, new KMS
 245 properties, new files in sysfs or anything else that constitutes an API change
 246 should have driver-agnostic testcases in IGT for that feature, if such a test
 247 can be reasonably made using IGT for the target hardware.
 248
 249 Validating changes with IGT
 250 ---------------------------
 251
 252 There's a collection of tests that aims to cover the whole functionality of
 253 DRM drivers and that can be used to check that changes to DRM drivers or the
 254 core don't regress existing functionality. This test suite is called IGT and
 255 its code can be found in https://cgit.freedesktop.org/drm/igt-gpu-tools/.
 256
 257 To build IGT, start by installing its build dependencies. In Debian-based
 258 systems::
 259
 260         # apt-get build-dep intel-gpu-tools
 261
 262 And in Fedora-based systems::
 263
 264         # dnf builddep intel-gpu-tools
 265
 266 Then clone the repository::
 267
 268         $ git clone git://anongit.freedesktop.org/drm/igt-gpu-tools
 269
 270 Configure the build system and start the build::
 271
 272         $ cd igt-gpu-tools && ./autogen.sh && make -j6
 273
 274 Download the piglit dependency::
 275
 276         $ ./scripts/run-tests.sh -d
 277
 278 And run the tests::
 279
 280         $ ./scripts/run-tests.sh -t kms -t core -s
 281
 282 run-tests.sh is a wrapper around piglit that will execute the tests matching
 283 the -t options. A report in HTML format will be available in
 284 ./results/html/index.html. Results can be compared with piglit.
 285
 286 Display CRC Support
 287 -------------------
 288
 289 .. kernel-doc:: drivers/gpu/drm/drm_debugfs_crc.c
 290    :doc: CRC ABI
 291
 292 .. kernel-doc:: drivers/gpu/drm/drm_debugfs_crc.c
 293    :export:
 294
 295 Debugfs Support
 296 ---------------
 297
 298 .. kernel-doc:: include/drm/drm_debugfs.h
 299    :internal:
 300
 301 .. kernel-doc:: drivers/gpu/drm/drm_debugfs.c
 302    :export:
 303
 304 Sysfs Support
 305 =============
 306
 307 .. kernel-doc:: drivers/gpu/drm/drm_sysfs.c
 308    :doc: overview
 309
 310 .. kernel-doc:: drivers/gpu/drm/drm_sysfs.c
 311    :export:
 312
 313
 314 VBlank event handling
 315 =====================
 316
 317 The DRM core exposes two vertical blank related ioctls:
 318
 319 DRM_IOCTL_WAIT_VBLANK
 320     This takes a struct drm_wait_vblank structure as its argument, and
 321     it is used to block or request a signal when a specified vblank
 322     event occurs.
 323
 324 DRM_IOCTL_MODESET_CTL
 325     This was only used for user-mode-settind drivers around modesetting
 326     changes to allow the kernel to update the vblank interrupt after
 327     mode setting, since on many devices the vertical blank counter is
 328     reset to 0 at some point during modeset. Modern drivers should not
 329     call this any more since with kernel mode setting it is a no-op.