1=================== 2Userland interfaces 3=================== 4 5The DRM core exports several interfaces to applications, generally 6intended to be used through corresponding libdrm wrapper functions. In 7addition, drivers export device-specific interfaces for use by userspace 8drivers & device-aware applications through ioctls and sysfs files. 9 10External interfaces include: memory mapping, context management, DMA 11operations, AGP management, vblank control, fence management, memory 12management, and output management. 13 14Cover generic ioctls and sysfs layout here. We only need high-level 15info, since man pages should cover the rest. 16 17libdrm 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 26Primary 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 38Open-Source Userspace Requirements 39================================== 40 41The DRM subsystem has stricter requirements than most other kernel subsystems on 42what the userspace side for new uAPI needs to look like. This section here 43explains what exactly those requirements are, and why they exist. 44 45The short summary is that any addition of DRM uAPI requires corresponding 46open-sourced userspace patches, and those patches must be reviewed and ready for 47merging into a suitable and canonical upstream project. 48 49GFX devices (both display and render/GPU side) are really complex bits of 50hardware, with userspace and kernel by necessity having to work together really 51closely. The interfaces, for rendering and modesetting, must be extremely wide 52and flexible, and therefore it is almost always impossible to precisely define 53them for every possible corner case. This in turn makes it really practically 54infeasible to differentiate between behaviour that's required by userspace, and 55which must not be changed to avoid regressions, and behaviour which is only an 56accidental artifact of the current implementation. 57 58Without access to the full source code of all userspace users that means it 59becomes impossible to change the implementation details, since userspace could 60depend upon the accidental behaviour of the current implementation in minute 61details. And debugging such regressions without access to source code is pretty 62much 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 74The other reason for requiring open-source userspace is uAPI review. Since the 75kernel and userspace parts of a GFX stack must work together so closely, code 76review can only assess whether a new interface achieves its goals by looking at 77both sides. Making sure that the interface indeed covers the use-case fully 78leads 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. The userspace-side reviewer should also provide an Acked-by on the 89 kernel uAPI patch indicating that they believe the proposed uAPI is sound and 90 sufficiently documented and validated for userspace's consumption. 91 92- The userspace patches must be against the canonical upstream, not some vendor 93 fork. This is to make sure that no one cheats on the review and testing 94 requirements by doing a quick fork. 95 96- The kernel patch can only be merged after all the above requirements are met, 97 but it **must** be merged to either drm-next or drm-misc-next **before** the 98 userspace patches land. uAPI always flows from the kernel, doing things the 99 other way round risks divergence of the uAPI definitions and header files. 100 101These are fairly steep requirements, but have grown out from years of shared 102pain and experience with uAPI added hastily, and almost always regretted about 103just as fast. GFX devices change really fast, requiring a paradigm shift and 104entire new set of uAPI interfaces every few years at least. Together with the 105Linux kernel's guarantee to keep existing userspace running for 10+ years this 106is already rather painful for the DRM subsystem, with multiple different uAPIs 107for the same thing co-existing. If we add a few more complete mistakes into the 108mix every year it would be entirely unmanageable. 109 110.. _drm_render_node: 111 112Render nodes 113============ 114 115DRM core provides multiple character-devices for user-space to use. 116Depending on which device is opened, user-space can perform a different 117set of operations (mainly ioctls). The primary node is always created 118and called card<num>. Additionally, a currently unused control node, 119called controlD<num> is also created. The primary node provides all 120legacy operations and historically was the only interface used by 121userspace. With KMS, the control node was introduced. However, the 122planned KMS control interface has never been written and so the control 123node stays unused to date. 124 125With the increased use of offscreen renderers and GPGPU applications, 126clients no longer require running compositors or graphics servers to 127make use of a GPU. But the DRM API required unprivileged clients to 128authenticate to a DRM-Master prior to getting GPU access. To avoid this 129step and to grant clients GPU access without authenticating, render 130nodes were introduced. Render nodes solely serve render clients, that 131is, no modesetting or privileged ioctls can be issued on render nodes. 132Only non-global rendering commands are allowed. If a driver supports 133render nodes, it must advertise it via the DRIVER_RENDER DRM driver 134capability. If not supported, the primary node must be used for render 135clients together with the legacy drmAuth authentication procedure. 136 137If a driver advertises render node support, DRM core will create a 138separate render node called renderD<num>. There will be one render node 139per device. No ioctls except PRIME-related ioctls will be allowed on 140this node. Especially GEM_OPEN will be explicitly prohibited. Render 141nodes are designed to avoid the buffer-leaks, which occur if clients 142guess the flink names or mmap offsets on the legacy interface. 143Additionally to this basic interface, drivers must mark their 144driver-dependent render-only ioctls as DRM_RENDER_ALLOW so render 145clients can use them. Driver authors must be careful not to allow any 146privileged ioctls on render nodes. 147 148With render nodes, user-space can now control access to the render node 149via basic file-system access-modes. A running graphics server which 150authenticates clients on the privileged primary/legacy node is no longer 151required. Instead, a client can open the render node and is immediately 152granted GPU access. Communication between clients (or servers) is done 153via PRIME. FLINK from render node to legacy node is not supported. New 154clients must not use the insecure FLINK interface. 155 156Besides dropping all modeset/global ioctls, render nodes also drop the 157DRM-Master concept. There is no reason to associate render clients with 158a DRM-Master as they are independent of any graphics server. Besides, 159they must work without any running master, anyway. Drivers must be able 160to run without a master object if they support render nodes. If, on the 161other hand, a driver requires shared state between clients which is 162visible to user-space and accessible beyond open-file boundaries, they 163cannot support render nodes. 164 165.. _drm_driver_ioctl: 166 167IOCTL Support on Device Nodes 168============================= 169 170.. kernel-doc:: drivers/gpu/drm/drm_ioctl.c 171 :doc: driver specific ioctls 172 173Recommended IOCTL Return Values 174------------------------------- 175 176In theory a driver's IOCTL callback is only allowed to return very few error 177codes. In practice it's good to abuse a few more. This section documents common 178practice within the DRM subsystem: 179 180ENOENT: 181 Strictly this should only be used when a file doesn't exist e.g. when 182 calling the open() syscall. We reuse that to signal any kind of object 183 lookup failure, e.g. for unknown GEM buffer object handles, unknown KMS 184 object handles and similar cases. 185 186ENOSPC: 187 Some drivers use this to differentiate "out of kernel memory" from "out 188 of VRAM". Sometimes also applies to other limited gpu resources used for 189 rendering (e.g. when you have a special limited compression buffer). 190 Sometimes resource allocation/reservation issues in command submission 191 IOCTLs are also signalled through EDEADLK. 192 193 Simply running out of kernel/system memory is signalled through ENOMEM. 194 195EPERM/EACCES: 196 Returned for an operation that is valid, but needs more privileges. 197 E.g. root-only or much more common, DRM master-only operations return 198 this when when called by unpriviledged clients. There's no clear 199 difference between EACCES and EPERM. 200 201ENODEV: 202 The device is not (yet) present or fully initialized. 203 204EOPNOTSUPP: 205 Feature (like PRIME, modesetting, GEM) is not supported by the driver. 206 207ENXIO: 208 Remote failure, either a hardware transaction (like i2c), but also used 209 when the exporting driver of a shared dma-buf or fence doesn't support a 210 feature needed. 211 212EINTR: 213 DRM drivers assume that userspace restarts all IOCTLs. Any DRM IOCTL can 214 return EINTR and in such a case should be restarted with the IOCTL 215 parameters left unchanged. 216 217EIO: 218 The GPU died and couldn't be resurrected through a reset. Modesetting 219 hardware failures are signalled through the "link status" connector 220 property. 221 222EINVAL: 223 Catch-all for anything that is an invalid argument combination which 224 cannot work. 225 226IOCTL also use other error codes like ETIME, EFAULT, EBUSY, ENOTTY but their 227usage is in line with the common meanings. The above list tries to just document 228DRM specific patterns. Note that ENOTTY has the slightly unintuitive meaning of 229"this IOCTL does not exist", and is used exactly as such in DRM. 230 231.. kernel-doc:: include/drm/drm_ioctl.h 232 :internal: 233 234.. kernel-doc:: drivers/gpu/drm/drm_ioctl.c 235 :export: 236 237.. kernel-doc:: drivers/gpu/drm/drm_ioc32.c 238 :export: 239 240Testing and validation 241====================== 242 243Testing Requirements for userspace API 244-------------------------------------- 245 246New cross-driver userspace interface extensions, like new IOCTL, new KMS 247properties, new files in sysfs or anything else that constitutes an API change 248should have driver-agnostic testcases in IGT for that feature, if such a test 249can be reasonably made using IGT for the target hardware. 250 251Validating changes with IGT 252--------------------------- 253 254There's a collection of tests that aims to cover the whole functionality of 255DRM drivers and that can be used to check that changes to DRM drivers or the 256core don't regress existing functionality. This test suite is called IGT and 257its code and instructions to build and run can be found in 258https://gitlab.freedesktop.org/drm/igt-gpu-tools/. 259 260Using VKMS to test DRM API 261-------------------------- 262 263VKMS is a software-only model of a KMS driver that is useful for testing 264and for running compositors. VKMS aims to enable a virtual display without 265the need for a hardware display capability. These characteristics made VKMS 266a perfect tool for validating the DRM core behavior and also support the 267compositor developer. VKMS makes it possible to test DRM functions in a 268virtual machine without display, simplifying the validation of some of the 269core changes. 270 271To Validate changes in DRM API with VKMS, start setting the kernel: make 272sure to enable VKMS module; compile the kernel with the VKMS enabled and 273install it in the target machine. VKMS can be run in a Virtual Machine 274(QEMU, virtme or similar). It's recommended the use of KVM with the minimum 275of 1GB of RAM and four cores. 276 277It's possible to run the IGT-tests in a VM in two ways: 278 279 1. Use IGT inside a VM 280 2. Use IGT from the host machine and write the results in a shared directory. 281 282As follow, there is an example of using a VM with a shared directory with 283the host machine to run igt-tests. As an example it's used virtme:: 284 285 $ virtme-run --rwdir /path/for/shared_dir --kdir=path/for/kernel/directory --mods=auto 286 287Run the igt-tests in the guest machine, as example it's ran the 'kms_flip' 288tests:: 289 290 $ /path/for/igt-gpu-tools/scripts/run-tests.sh -p -s -t "kms_flip.*" -v 291 292In this example, instead of build the igt_runner, Piglit is used 293(-p option); it's created html summary of the tests results and it's saved 294in the folder "igt-gpu-tools/results"; it's executed only the igt-tests 295matching the -t option. 296 297Display CRC Support 298------------------- 299 300.. kernel-doc:: drivers/gpu/drm/drm_debugfs_crc.c 301 :doc: CRC ABI 302 303.. kernel-doc:: drivers/gpu/drm/drm_debugfs_crc.c 304 :export: 305 306Debugfs Support 307--------------- 308 309.. kernel-doc:: include/drm/drm_debugfs.h 310 :internal: 311 312.. kernel-doc:: drivers/gpu/drm/drm_debugfs.c 313 :export: 314 315Sysfs Support 316============= 317 318.. kernel-doc:: drivers/gpu/drm/drm_sysfs.c 319 :doc: overview 320 321.. kernel-doc:: drivers/gpu/drm/drm_sysfs.c 322 :export: 323 324 325VBlank event handling 326===================== 327 328The DRM core exposes two vertical blank related ioctls: 329 330DRM_IOCTL_WAIT_VBLANK 331 This takes a struct drm_wait_vblank structure as its argument, and 332 it is used to block or request a signal when a specified vblank 333 event occurs. 334 335DRM_IOCTL_MODESET_CTL 336 This was only used for user-mode-settind drivers around modesetting 337 changes to allow the kernel to update the vblank interrupt after 338 mode setting, since on many devices the vertical blank counter is 339 reset to 0 at some point during modeset. Modern drivers should not 340 call this any more since with kernel mode setting it is a no-op. 341 342Userspace API Structures 343======================== 344 345.. kernel-doc:: include/uapi/drm/drm_mode.h 346 :doc: overview 347 348.. kernel-doc:: include/uapi/drm/drm_mode.h 349 :internal: 350