274 lines
8.6 KiB
ReStructuredText
274 lines
8.6 KiB
ReStructuredText
=============
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DRM Internals
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=============
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This chapter documents DRM internals relevant to driver authors and
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developers working to add support for the latest features to existing
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drivers.
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First, we go over some typical driver initialization requirements, like
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setting up command buffers, creating an initial output configuration,
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and initializing core services. Subsequent sections cover core internals
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in more detail, providing implementation notes and examples.
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The DRM layer provides several services to graphics drivers, many of
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them driven by the application interfaces it provides through libdrm,
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the library that wraps most of the DRM ioctls. These include vblank
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event handling, memory management, output management, framebuffer
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management, command submission & fencing, suspend/resume support, and
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DMA services.
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Driver Initialization
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=====================
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At the core of every DRM driver is a :c:type:`struct drm_driver
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<drm_driver>` structure. Drivers typically statically initialize
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a drm_driver structure, and then pass it to
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drm_dev_alloc() to allocate a device instance. After the
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device instance is fully initialized it can be registered (which makes
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it accessible from userspace) using drm_dev_register().
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The :c:type:`struct drm_driver <drm_driver>` structure
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contains static information that describes the driver and features it
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supports, and pointers to methods that the DRM core will call to
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implement the DRM API. We will first go through the :c:type:`struct
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drm_driver <drm_driver>` static information fields, and will
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then describe individual operations in details as they get used in later
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sections.
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Driver Information
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------------------
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Major, Minor and Patchlevel
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~~~~~~~~~~~~~~~~~~~~~~~~~~~
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int major; int minor; int patchlevel;
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The DRM core identifies driver versions by a major, minor and patch
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level triplet. The information is printed to the kernel log at
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initialization time and passed to userspace through the
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DRM_IOCTL_VERSION ioctl.
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The major and minor numbers are also used to verify the requested driver
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API version passed to DRM_IOCTL_SET_VERSION. When the driver API
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changes between minor versions, applications can call
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DRM_IOCTL_SET_VERSION to select a specific version of the API. If the
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requested major isn't equal to the driver major, or the requested minor
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is larger than the driver minor, the DRM_IOCTL_SET_VERSION call will
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return an error. Otherwise the driver's set_version() method will be
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called with the requested version.
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Name, Description and Date
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~~~~~~~~~~~~~~~~~~~~~~~~~~
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char \*name; char \*desc; char \*date;
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The driver name is printed to the kernel log at initialization time,
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used for IRQ registration and passed to userspace through
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DRM_IOCTL_VERSION.
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The driver description is a purely informative string passed to
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userspace through the DRM_IOCTL_VERSION ioctl and otherwise unused by
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the kernel.
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The driver date, formatted as YYYYMMDD, is meant to identify the date of
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the latest modification to the driver. However, as most drivers fail to
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update it, its value is mostly useless. The DRM core prints it to the
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kernel log at initialization time and passes it to userspace through the
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DRM_IOCTL_VERSION ioctl.
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Module Initialization
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---------------------
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.. kernel-doc:: include/drm/drm_module.h
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:doc: overview
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Managing Ownership of the Framebuffer Aperture
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----------------------------------------------
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.. kernel-doc:: drivers/gpu/drm/drm_aperture.c
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:doc: overview
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.. kernel-doc:: include/drm/drm_aperture.h
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:internal:
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.. kernel-doc:: drivers/gpu/drm/drm_aperture.c
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:export:
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Device Instance and Driver Handling
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-----------------------------------
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.. kernel-doc:: drivers/gpu/drm/drm_drv.c
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:doc: driver instance overview
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.. kernel-doc:: include/drm/drm_device.h
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:internal:
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.. kernel-doc:: include/drm/drm_drv.h
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:internal:
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.. kernel-doc:: drivers/gpu/drm/drm_drv.c
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:export:
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Driver Load
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-----------
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Component Helper Usage
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~~~~~~~~~~~~~~~~~~~~~~
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.. kernel-doc:: drivers/gpu/drm/drm_drv.c
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:doc: component helper usage recommendations
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Memory Manager Initialization
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Every DRM driver requires a memory manager which must be initialized at
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load time. DRM currently contains two memory managers, the Translation
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Table Manager (TTM) and the Graphics Execution Manager (GEM). This
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document describes the use of the GEM memory manager only. See ? for
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details.
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Miscellaneous Device Configuration
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Another task that may be necessary for PCI devices during configuration
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is mapping the video BIOS. On many devices, the VBIOS describes device
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configuration, LCD panel timings (if any), and contains flags indicating
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device state. Mapping the BIOS can be done using the pci_map_rom()
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call, a convenience function that takes care of mapping the actual ROM,
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whether it has been shadowed into memory (typically at address 0xc0000)
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or exists on the PCI device in the ROM BAR. Note that after the ROM has
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been mapped and any necessary information has been extracted, it should
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be unmapped; on many devices, the ROM address decoder is shared with
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other BARs, so leaving it mapped could cause undesired behaviour like
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hangs or memory corruption.
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Managed Resources
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-----------------
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.. kernel-doc:: drivers/gpu/drm/drm_managed.c
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:doc: managed resources
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.. kernel-doc:: drivers/gpu/drm/drm_managed.c
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:export:
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.. kernel-doc:: include/drm/drm_managed.h
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:internal:
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Bus-specific Device Registration and PCI Support
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------------------------------------------------
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A number of functions are provided to help with device registration. The
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functions deal with PCI and platform devices respectively and are only
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provided for historical reasons. These are all deprecated and shouldn't
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be used in new drivers. Besides that there's a few helpers for pci
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drivers.
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.. kernel-doc:: drivers/gpu/drm/drm_pci.c
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:export:
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Open/Close, File Operations and IOCTLs
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======================================
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.. _drm_driver_fops:
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File Operations
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---------------
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.. kernel-doc:: drivers/gpu/drm/drm_file.c
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:doc: file operations
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.. kernel-doc:: include/drm/drm_file.h
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:internal:
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.. kernel-doc:: drivers/gpu/drm/drm_file.c
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:export:
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Misc Utilities
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==============
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Printer
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-------
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.. kernel-doc:: include/drm/drm_print.h
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:doc: print
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.. kernel-doc:: include/drm/drm_print.h
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:internal:
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.. kernel-doc:: drivers/gpu/drm/drm_print.c
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:export:
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Utilities
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---------
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.. kernel-doc:: include/drm/drm_util.h
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:doc: drm utils
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.. kernel-doc:: include/drm/drm_util.h
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:internal:
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Unit testing
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============
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KUnit
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-----
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KUnit (Kernel unit testing framework) provides a common framework for unit tests
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within the Linux kernel.
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This section covers the specifics for the DRM subsystem. For general information
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about KUnit, please refer to Documentation/dev-tools/kunit/start.rst.
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How to run the tests?
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~~~~~~~~~~~~~~~~~~~~~
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In order to facilitate running the test suite, a configuration file is present
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in ``drivers/gpu/drm/tests/.kunitconfig``. It can be used by ``kunit.py`` as
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follows:
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.. code-block:: bash
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$ ./tools/testing/kunit/kunit.py run --kunitconfig=drivers/gpu/drm/tests \
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--kconfig_add CONFIG_VIRTIO_UML=y \
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--kconfig_add CONFIG_UML_PCI_OVER_VIRTIO=y
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.. note::
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The configuration included in ``.kunitconfig`` should be as generic as
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possible.
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``CONFIG_VIRTIO_UML`` and ``CONFIG_UML_PCI_OVER_VIRTIO`` are not
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included in it because they are only required for User Mode Linux.
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Legacy Support Code
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===================
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The section very briefly covers some of the old legacy support code
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which is only used by old DRM drivers which have done a so-called
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shadow-attach to the underlying device instead of registering as a real
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driver. This also includes some of the old generic buffer management and
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command submission code. Do not use any of this in new and modern
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drivers.
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Legacy Suspend/Resume
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---------------------
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The DRM core provides some suspend/resume code, but drivers wanting full
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suspend/resume support should provide save() and restore() functions.
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These are called at suspend, hibernate, or resume time, and should
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perform any state save or restore required by your device across suspend
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or hibernate states.
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int (\*suspend) (struct drm_device \*, pm_message_t state); int
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(\*resume) (struct drm_device \*);
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Those are legacy suspend and resume methods which *only* work with the
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legacy shadow-attach driver registration functions. New driver should
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use the power management interface provided by their bus type (usually
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through the :c:type:`struct device_driver <device_driver>`
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dev_pm_ops) and set these methods to NULL.
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Legacy DMA Services
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-------------------
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This should cover how DMA mapping etc. is supported by the core. These
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functions are deprecated and should not be used.
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