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 .. Copyright 2020 DisplayLink (UK) Ltd.
 
 ===================
 Userland interfaces
 ===================
 
 The DRM core exports several interfaces to applications, generally
 intended to be used through corresponding libdrm wrapper functions. In
 addition, drivers export device-specific interfaces for use by userspace
 drivers & device-aware applications through ioctls and sysfs files.
 
 External interfaces include: memory mapping, context management, DMA
 operations, AGP management, vblank control, fence management, memory
 management, and output management.
 
 Cover generic ioctls and sysfs layout here. We only need high-level
 info, since man pages should cover the rest.
 
 libdrm Device Lookup
 ====================
 
 .. kernel-doc:: drivers/gpu/drm/drm_ioctl.c
    :doc: getunique and setversion story
 
 
 .. _drm_primary_node:
 
 Primary Nodes, DRM Master and Authentication
 ============================================
 
 .. kernel-doc:: drivers/gpu/drm/drm_auth.c
    :doc: master and authentication
 
 .. kernel-doc:: drivers/gpu/drm/drm_auth.c
    :export:
 
 .. kernel-doc:: include/drm/drm_auth.h
    :internal:
 
 
 .. _drm_leasing:
 
 DRM Display Resource Leasing
 ============================
 
 .. kernel-doc:: drivers/gpu/drm/drm_lease.c
    :doc: drm leasing
 
 Open-Source Userspace Requirements
 ==================================
 
 The DRM subsystem has stricter requirements than most other kernel subsystems on
 what the userspace side for new uAPI needs to look like. This section here
 explains what exactly those requirements are, and why they exist.
 
 The short summary is that any addition of DRM uAPI requires corresponding
 open-sourced userspace patches, and those patches must be reviewed and ready for
 merging into a suitable and canonical upstream project.
 
 GFX devices (both display and render/GPU side) are really complex bits of
 hardware, with userspace and kernel by necessity having to work together really
 closely.  The interfaces, for rendering and modesetting, must be extremely wide
 and flexible, and therefore it is almost always impossible to precisely define
 them for every possible corner case. This in turn makes it really practically
 infeasible to differentiate between behaviour that's required by userspace, and
 which must not be changed to avoid regressions, and behaviour which is only an
 accidental artifact of the current implementation.
 
 Without access to the full source code of all userspace users that means it
 becomes impossible to change the implementation details, since userspace could
 depend upon the accidental behaviour of the current implementation in minute
 details. And debugging such regressions without access to source code is pretty
 much impossible. As a consequence this means:
 
 - The Linux kernel's "no regression" policy holds in practice only for
   open-source userspace of the DRM subsystem. DRM developers are perfectly fine
   if closed-source blob drivers in userspace use the same uAPI as the open
   drivers, but they must do so in the exact same way as the open drivers.
   Creative (ab)use of the interfaces will, and in the past routinely has, lead
   to breakage.
 
 - Any new userspace interface must have an open-source implementation as
   demonstration vehicle.
 
 The other reason for requiring open-source userspace is uAPI review. Since the
 kernel and userspace parts of a GFX stack must work together so closely, code
 review can only assess whether a new interface achieves its goals by looking at
 both sides. Making sure that the interface indeed covers the use-case fully
 leads to a few additional requirements:
 
 - The open-source userspace must not be a toy/test application, but the real
   thing. Specifically it needs to handle all the usual error and corner cases.
   These are often the places where new uAPI falls apart and hence essential to
   assess the fitness of a proposed interface.
 
 - The userspace side must be fully reviewed and tested to the standards of that
   userspace project. For e.g. mesa this means piglit testcases and review on the
   mailing list. This is again to ensure that the new interface actually gets the
   job done.  The userspace-side reviewer should also provide an Acked-by on the
   kernel uAPI patch indicating that they believe the proposed uAPI is sound and
   sufficiently documented and validated for userspace's consumption.
 
 - The userspace patches must be against the canonical upstream, not some vendor
   fork. This is to make sure that no one cheats on the review and testing
   requirements by doing a quick fork.
 
 - The kernel patch can only be merged after all the above requirements are met,
   but it **must** be merged to either drm-next or drm-misc-next **before** the
   userspace patches land. uAPI always flows from the kernel, doing things the
   other way round risks divergence of the uAPI definitions and header files.
 
 These are fairly steep requirements, but have grown out from years of shared
 pain and experience with uAPI added hastily, and almost always regretted about
 just as fast. GFX devices change really fast, requiring a paradigm shift and
 entire new set of uAPI interfaces every few years at least. Together with the
 Linux kernel's guarantee to keep existing userspace running for 10+ years this
 is already rather painful for the DRM subsystem, with multiple different uAPIs
 for the same thing co-existing. If we add a few more complete mistakes into the
 mix every year it would be entirely unmanageable.
 
 .. _drm_render_node:
 
 Render nodes
 ============
 
 DRM core provides multiple character-devices for user-space to use.
 Depending on which device is opened, user-space can perform a different
 set of operations (mainly ioctls). The primary node is always created
 and called card<num>. Additionally, a currently unused control node,
 called controlD<num> is also created. The primary node provides all
 legacy operations and historically was the only interface used by
 userspace. With KMS, the control node was introduced. However, the
 planned KMS control interface has never been written and so the control
 node stays unused to date.
 
 With the increased use of offscreen renderers and GPGPU applications,
 clients no longer require running compositors or graphics servers to
 make use of a GPU. But the DRM API required unprivileged clients to
 authenticate to a DRM-Master prior to getting GPU access. To avoid this
 step and to grant clients GPU access without authenticating, render
 nodes were introduced. Render nodes solely serve render clients, that
 is, no modesetting or privileged ioctls can be issued on render nodes.
 Only non-global rendering commands are allowed. If a driver supports
 render nodes, it must advertise it via the DRIVER_RENDER DRM driver
 capability. If not supported, the primary node must be used for render
 clients together with the legacy drmAuth authentication procedure.
 
 If a driver advertises render node support, DRM core will create a
 separate render node called renderD<num>. There will be one render node
 per device. No ioctls except PRIME-related ioctls will be allowed on
 this node. Especially GEM_OPEN will be explicitly prohibited. For a
 complete list of driver-independent ioctls that can be used on render
 nodes, see the ioctls marked DRM_RENDER_ALLOW in drm_ioctl.c  Render
 nodes are designed to avoid the buffer-leaks, which occur if clients
 guess the flink names or mmap offsets on the legacy interface.
 Additionally to this basic interface, drivers must mark their
 driver-dependent render-only ioctls as DRM_RENDER_ALLOW so render
 clients can use them. Driver authors must be careful not to allow any
 privileged ioctls on render nodes.
 
 With render nodes, user-space can now control access to the render node
 via basic file-system access-modes. A running graphics server which
 authenticates clients on the privileged primary/legacy node is no longer
 required. Instead, a client can open the render node and is immediately
 granted GPU access. Communication between clients (or servers) is done
 via PRIME. FLINK from render node to legacy node is not supported. New
 clients must not use the insecure FLINK interface.
 
 Besides dropping all modeset/global ioctls, render nodes also drop the
 DRM-Master concept. There is no reason to associate render clients with
 a DRM-Master as they are independent of any graphics server. Besides,
 they must work without any running master, anyway. Drivers must be able
 to run without a master object if they support render nodes. If, on the
 other hand, a driver requires shared state between clients which is
 visible to user-space and accessible beyond open-file boundaries, they
 cannot support render nodes.
 
 Device Hot-Unplug
 =================
 
 .. note::
    The following is the plan. Implementation is not there yet
    (2020 May).
 
 Graphics devices (display and/or render) may be connected via USB (e.g.
 display adapters or docking stations) or Thunderbolt (e.g. eGPU). An end
 user is able to hot-unplug this kind of devices while they are being
 used, and expects that the very least the machine does not crash. Any
 damage from hot-unplugging a DRM device needs to be limited as much as
 possible and userspace must be given the chance to handle it if it wants
 to. Ideally, unplugging a DRM device still lets a desktop continue to
 run, but that is going to need explicit support throughout the whole
 graphics stack: from kernel and userspace drivers, through display
 servers, via window system protocols, and in applications and libraries.
 
 Other scenarios that should lead to the same are: unrecoverable GPU
 crash, PCI device disappearing off the bus, or forced unbind of a driver
 from the physical device.
 
 In other words, from userspace perspective everything needs to keep on
 working more or less, until userspace stops using the disappeared DRM
 device and closes it completely. Userspace will learn of the device
 disappearance from the device removed uevent, ioctls returning ENODEV
 (or driver-specific ioctls returning driver-specific things), or open()
 returning ENXIO.
 
 Only after userspace has closed all relevant DRM device and dmabuf file
 descriptors and removed all mmaps, the DRM driver can tear down its
 instance for the device that no longer exists. If the same physical
 device somehow comes back in the mean time, it shall be a new DRM
 device.
 
 Similar to PIDs, chardev minor numbers are not recycled immediately. A
 new DRM device always picks the next free minor number compared to the
 previous one allocated, and wraps around when minor numbers are
 exhausted.
 
 The goal raises at least the following requirements for the kernel and
 drivers.
 
 Requirements for KMS UAPI
 -------------------------
 
 - KMS connectors must change their status to disconnected.
 
 - Legacy modesets and pageflips, and atomic commits, both real and
   TEST_ONLY, and any other ioctls either fail with ENODEV or fake
   success.
 
 - Pending non-blocking KMS operations deliver the DRM events userspace
   is expecting. This applies also to ioctls that faked success.
 
 - open() on a device node whose underlying device has disappeared will
   fail with ENXIO.
 
 - Attempting to create a DRM lease on a disappeared DRM device will
   fail with ENODEV. Existing DRM leases remain and work as listed
   above.
 
 Requirements for Render and Cross-Device UAPI
 ---------------------------------------------
 
 - All GPU jobs that can no longer run must have their fences
   force-signalled to avoid inflicting hangs on userspace.
   The associated error code is ENODEV.
 
 - Some userspace APIs already define what should happen when the device
   disappears (OpenGL, GL ES: `GL_KHR_robustness`_; `Vulkan`_:
   VK_ERROR_DEVICE_LOST; etc.). DRM drivers are free to implement this
   behaviour the way they see best, e.g. returning failures in
   driver-specific ioctls and handling those in userspace drivers, or
   rely on uevents, and so on.
 
 - dmabuf which point to memory that has disappeared will either fail to
   import with ENODEV or continue to be successfully imported if it would
   have succeeded before the disappearance. See also about memory maps
   below for already imported dmabufs.
 
 - Attempting to import a dmabuf to a disappeared device will either fail
   with ENODEV or succeed if it would have succeeded without the
   disappearance.
 
 - open() on a device node whose underlying device has disappeared will
   fail with ENXIO.
 
 .. _GL_KHR_robustness: https://www.khronos.org/registry/OpenGL/extensions/KHR/KHR_robustness.txt
 .. _Vulkan: https://www.khronos.org/vulkan/
 
 Requirements for Memory Maps
 ----------------------------
 
 Memory maps have further requirements that apply to both existing maps
 and maps created after the device has disappeared. If the underlying
 memory disappears, the map is created or modified such that reads and
 writes will still complete successfully but the result is undefined.
 This applies to both userspace mmap()'d memory and memory pointed to by
 dmabuf which might be mapped to other devices (cross-device dmabuf
 imports).
 
 Raising SIGBUS is not an option, because userspace cannot realistically
 handle it. Signal handlers are global, which makes them extremely
 difficult to use correctly from libraries like those that Mesa produces.
 Signal handlers are not composable, you can't have different handlers
 for GPU1 and GPU2 from different vendors, and a third handler for
 mmapped regular files. Threads cause additional pain with signal
 handling as well.
 
 .. _drm_driver_ioctl:
 
 IOCTL Support on Device Nodes
 =============================
 
 .. kernel-doc:: drivers/gpu/drm/drm_ioctl.c
    :doc: driver specific ioctls
 
 Recommended IOCTL Return Values
 -------------------------------
 
 In theory a driver's IOCTL callback is only allowed to return very few error
 codes. In practice it's good to abuse a few more. This section documents common
 practice within the DRM subsystem:
 
 ENOENT:
         Strictly this should only be used when a file doesn't exist e.g. when
         calling the open() syscall. We reuse that to signal any kind of object
         lookup failure, e.g. for unknown GEM buffer object handles, unknown KMS
         object handles and similar cases.
 
 ENOSPC:
         Some drivers use this to differentiate "out of kernel memory" from "out
         of VRAM". Sometimes also applies to other limited gpu resources used for
         rendering (e.g. when you have a special limited compression buffer).
         Sometimes resource allocation/reservation issues in command submission
         IOCTLs are also signalled through EDEADLK.
 
         Simply running out of kernel/system memory is signalled through ENOMEM.
 
 EPERM/EACCES:
         Returned for an operation that is valid, but needs more privileges.
         E.g. root-only or much more common, DRM master-only operations return
         this when called by unpriviledged clients. There's no clear
         difference between EACCES and EPERM.
 
 ENODEV:
         The device is not present anymore or is not yet fully initialized.
 
 EOPNOTSUPP:
         Feature (like PRIME, modesetting, GEM) is not supported by the driver.
 
 ENXIO:
         Remote failure, either a hardware transaction (like i2c), but also used
         when the exporting driver of a shared dma-buf or fence doesn't support a
         feature needed.
 
 EINTR:
         DRM drivers assume that userspace restarts all IOCTLs. Any DRM IOCTL can
         return EINTR and in such a case should be restarted with the IOCTL
         parameters left unchanged.
 
 EIO:
         The GPU died and couldn't be resurrected through a reset. Modesetting
         hardware failures are signalled through the "link status" connector
         property.
 
 EINVAL:
         Catch-all for anything that is an invalid argument combination which
         cannot work.
 
 IOCTL also use other error codes like ETIME, EFAULT, EBUSY, ENOTTY but their
 usage is in line with the common meanings. The above list tries to just document
 DRM specific patterns. Note that ENOTTY has the slightly unintuitive meaning of
 "this IOCTL does not exist", and is used exactly as such in DRM.
 
 .. kernel-doc:: include/drm/drm_ioctl.h
    :internal:
 
 .. kernel-doc:: drivers/gpu/drm/drm_ioctl.c
    :export:
 
 .. kernel-doc:: drivers/gpu/drm/drm_ioc32.c
    :export:
 
 Testing and validation
 ======================
 
 Testing Requirements for userspace API
 --------------------------------------
 
 New cross-driver userspace interface extensions, like new IOCTL, new KMS
 properties, new files in sysfs or anything else that constitutes an API change
 should have driver-agnostic testcases in IGT for that feature, if such a test
 can be reasonably made using IGT for the target hardware.
 
 Validating changes with IGT
 ---------------------------
 
 There's a collection of tests that aims to cover the whole functionality of
 DRM drivers and that can be used to check that changes to DRM drivers or the
 core don't regress existing functionality. This test suite is called IGT and
 its code and instructions to build and run can be found in
 https://gitlab.freedesktop.org/drm/igt-gpu-tools/.
 
 Using VKMS to test DRM API
 --------------------------
 
 VKMS is a software-only model of a KMS driver that is useful for testing
 and for running compositors. VKMS aims to enable a virtual display without
 the need for a hardware display capability. These characteristics made VKMS
 a perfect tool for validating the DRM core behavior and also support the
 compositor developer. VKMS makes it possible to test DRM functions in a
 virtual machine without display, simplifying the validation of some of the
 core changes.
 
 To Validate changes in DRM API with VKMS, start setting the kernel: make
 sure to enable VKMS module; compile the kernel with the VKMS enabled and
 install it in the target machine. VKMS can be run in a Virtual Machine
 (QEMU, virtme or similar). It's recommended the use of KVM with the minimum
 of 1GB of RAM and four cores.
 
 It's possible to run the IGT-tests in a VM in two ways:
 
         1. Use IGT inside a VM
         2. Use IGT from the host machine and write the results in a shared directory.
 
 As follow, there is an example of using a VM with a shared directory with
 the host machine to run igt-tests. As an example it's used virtme::
 
         $ virtme-run --rwdir /path/for/shared_dir --kdir=path/for/kernel/directory --mods=auto
 
 Run the igt-tests in the guest machine, as example it's ran the 'kms_flip'
 tests::
 
         $ /path/for/igt-gpu-tools/scripts/run-tests.sh -p -s -t "kms_flip.*" -v
 
 In this example, instead of build the igt_runner, Piglit is used
 (-p option); it's created html summary of the tests results and it's saved
 in the folder "igt-gpu-tools/results"; it's executed only the igt-tests
 matching the -t option.
 
 Display CRC Support
 -------------------
 
 .. kernel-doc:: drivers/gpu/drm/drm_debugfs_crc.c
    :doc: CRC ABI
 
 .. kernel-doc:: drivers/gpu/drm/drm_debugfs_crc.c
    :export:
 
 Debugfs Support
 ---------------
 
 .. kernel-doc:: include/drm/drm_debugfs.h
    :internal:
 
 .. kernel-doc:: drivers/gpu/drm/drm_debugfs.c
    :export:
 
 Sysfs Support
 =============
 
 .. kernel-doc:: drivers/gpu/drm/drm_sysfs.c
    :doc: overview
 
 .. kernel-doc:: drivers/gpu/drm/drm_sysfs.c
    :export:
 
 
 VBlank event handling
 =====================
 
 The DRM core exposes two vertical blank related ioctls:
 
 DRM_IOCTL_WAIT_VBLANK
     This takes a struct drm_wait_vblank structure as its argument, and
     it is used to block or request a signal when a specified vblank
     event occurs.
 
 DRM_IOCTL_MODESET_CTL
     This was only used for user-mode-settind drivers around modesetting
     changes to allow the kernel to update the vblank interrupt after
     mode setting, since on many devices the vertical blank counter is
     reset to 0 at some point during modeset. Modern drivers should not
     call this any more since with kernel mode setting it is a no-op.
 
 Userspace API Structures
 ========================
 
 .. kernel-doc:: include/uapi/drm/drm_mode.h
    :doc: overview
 
 .. _crtc_index:
 
 CRTC index
 ----------
 
 CRTC's have both an object ID and an index, and they are not the same thing.
 The index is used in cases where a densely packed identifier for a CRTC is
 needed, for instance a bitmask of CRTC's. The member possible_crtcs of struct
 drm_mode_get_plane is an example.
 
 DRM_IOCTL_MODE_GETRESOURCES populates a structure with an array of CRTC ID's,
 and the CRTC index is its position in this array.
 
 .. kernel-doc:: include/uapi/drm/drm.h
    :internal:
 
 .. kernel-doc:: include/uapi/drm/drm_mode.h
    :internal:

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