============= DRM Internals ============= This chapter documents DRM internals relevant to driver authors and developers working to add support for the latest features to existing drivers. First, we go over some typical driver initialization requirements, like setting up command buffers, creating an initial output configuration, and initializing core services. Subsequent sections cover core internals in more detail, providing implementation notes and examples. The DRM layer provides several services to graphics drivers, many of them driven by the application interfaces it provides through libdrm, the library that wraps most of the DRM ioctls. These include vblank event handling, memory management, output management, framebuffer management, command submission & fencing, suspend/resume support, and DMA services. Driver Initialization ===================== At the core of every DRM driver is a :c:type:`struct drm_driver ` structure. Drivers typically statically initialize a drm_driver structure, and then pass it to :c:func:`drm_dev_alloc()` to allocate a device instance. After the device instance is fully initialized it can be registered (which makes it accessible from userspace) using :c:func:`drm_dev_register()`. The :c:type:`struct drm_driver ` structure contains static information that describes the driver and features it supports, and pointers to methods that the DRM core will call to implement the DRM API. We will first go through the :c:type:`struct drm_driver ` static information fields, and will then describe individual operations in details as they get used in later sections. Driver Information ------------------ Driver Features ~~~~~~~~~~~~~~~ Drivers inform the DRM core about their requirements and supported features by setting appropriate flags in the driver_features field. Since those flags influence the DRM core behaviour since registration time, most of them must be set to registering the :c:type:`struct drm_driver ` instance. u32 driver_features; DRIVER_USE_AGP Driver uses AGP interface, the DRM core will manage AGP resources. DRIVER_LEGACY Denote a legacy driver using shadow attach. Don't use. DRIVER_KMS_LEGACY_CONTEXT Used only by nouveau for backwards compatibility with existing userspace. Don't use. DRIVER_PCI_DMA Driver is capable of PCI DMA, mapping of PCI DMA buffers to userspace will be enabled. Deprecated. DRIVER_SG Driver can perform scatter/gather DMA, allocation and mapping of scatter/gather buffers will be enabled. Deprecated. DRIVER_HAVE_DMA Driver supports DMA, the userspace DMA API will be supported. Deprecated. DRIVER_HAVE_IRQ; DRIVER_IRQ_SHARED DRIVER_HAVE_IRQ indicates whether the driver has an IRQ handler managed by the DRM Core. The core will support simple IRQ handler installation when the flag is set. The installation process is described in ?. DRIVER_IRQ_SHARED indicates whether the device & handler support shared IRQs (note that this is required of PCI drivers). DRIVER_GEM Driver use the GEM memory manager. DRIVER_MODESET Driver supports mode setting interfaces (KMS). DRIVER_PRIME Driver implements DRM PRIME buffer sharing. DRIVER_RENDER Driver supports dedicated render nodes. DRIVER_ATOMIC Driver supports atomic properties. In this case the driver must implement appropriate obj->atomic_get_property() vfuncs for any modeset objects with driver specific properties. Major, Minor and Patchlevel ~~~~~~~~~~~~~~~~~~~~~~~~~~~ int major; int minor; int patchlevel; The DRM core identifies driver versions by a major, minor and patch level triplet. The information is printed to the kernel log at initialization time and passed to userspace through the DRM_IOCTL_VERSION ioctl. The major and minor numbers are also used to verify the requested driver API version passed to DRM_IOCTL_SET_VERSION. When the driver API changes between minor versions, applications can call DRM_IOCTL_SET_VERSION to select a specific version of the API. If the requested major isn't equal to the driver major, or the requested minor is larger than the driver minor, the DRM_IOCTL_SET_VERSION call will return an error. Otherwise the driver's set_version() method will be called with the requested version. Name, Description and Date ~~~~~~~~~~~~~~~~~~~~~~~~~~ char \*name; char \*desc; char \*date; The driver name is printed to the kernel log at initialization time, used for IRQ registration and passed to userspace through DRM_IOCTL_VERSION. The driver description is a purely informative string passed to userspace through the DRM_IOCTL_VERSION ioctl and otherwise unused by the kernel. The driver date, formatted as YYYYMMDD, is meant to identify the date of the latest modification to the driver. However, as most drivers fail to update it, its value is mostly useless. The DRM core prints it to the kernel log at initialization time and passes it to userspace through the DRM_IOCTL_VERSION ioctl. Device Instance and Driver Handling ----------------------------------- .. kernel-doc:: drivers/gpu/drm/drm_drv.c :doc: driver instance overview .. kernel-doc:: drivers/gpu/drm/drm_drv.c :export: .. kernel-doc:: include/drm/drm_drv.h :internal: Driver Load ----------- IRQ Registration ~~~~~~~~~~~~~~~~ The DRM core tries to facilitate IRQ handler registration and unregistration by providing :c:func:`drm_irq_install()` and :c:func:`drm_irq_uninstall()` functions. Those functions only support a single interrupt per device, devices that use more than one IRQs need to be handled manually. Managed IRQ Registration '''''''''''''''''''''''' :c:func:`drm_irq_install()` starts by calling the irq_preinstall driver operation. The operation is optional and must make sure that the interrupt will not get fired by clearing all pending interrupt flags or disabling the interrupt. The passed-in IRQ will then be requested by a call to :c:func:`request_irq()`. If the DRIVER_IRQ_SHARED driver feature flag is set, a shared (IRQF_SHARED) IRQ handler will be requested. The IRQ handler function must be provided as the mandatory irq_handler driver operation. It will get passed directly to :c:func:`request_irq()` and thus has the same prototype as all IRQ handlers. It will get called with a pointer to the DRM device as the second argument. Finally the function calls the optional irq_postinstall driver operation. The operation usually enables interrupts (excluding the vblank interrupt, which is enabled separately), but drivers may choose to enable/disable interrupts at a different time. :c:func:`drm_irq_uninstall()` is similarly used to uninstall an IRQ handler. It starts by waking up all processes waiting on a vblank interrupt to make sure they don't hang, and then calls the optional irq_uninstall driver operation. The operation must disable all hardware interrupts. Finally the function frees the IRQ by calling :c:func:`free_irq()`. Manual IRQ Registration ''''''''''''''''''''''' Drivers that require multiple interrupt handlers can't use the managed IRQ registration functions. In that case IRQs must be registered and unregistered manually (usually with the :c:func:`request_irq()` and :c:func:`free_irq()` functions, or their :c:func:`devm_request_irq()` and :c:func:`devm_free_irq()` equivalents). When manually registering IRQs, drivers must not set the DRIVER_HAVE_IRQ driver feature flag, and must not provide the irq_handler driver operation. They must set the :c:type:`struct drm_device ` irq_enabled field to 1 upon registration of the IRQs, and clear it to 0 after unregistering the IRQs. Memory Manager Initialization ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Every DRM driver requires a memory manager which must be initialized at load time. DRM currently contains two memory managers, the Translation Table Manager (TTM) and the Graphics Execution Manager (GEM). This document describes the use of the GEM memory manager only. See ? for details. Miscellaneous Device Configuration ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Another task that may be necessary for PCI devices during configuration is mapping the video BIOS. On many devices, the VBIOS describes device configuration, LCD panel timings (if any), and contains flags indicating device state. Mapping the BIOS can be done using the pci_map_rom() call, a convenience function that takes care of mapping the actual ROM, whether it has been shadowed into memory (typically at address 0xc0000) or exists on the PCI device in the ROM BAR. Note that after the ROM has been mapped and any necessary information has been extracted, it should be unmapped; on many devices, the ROM address decoder is shared with other BARs, so leaving it mapped could cause undesired behaviour like hangs or memory corruption. Bus-specific Device Registration and PCI Support ------------------------------------------------ A number of functions are provided to help with device registration. The functions deal with PCI and platform devices respectively and are only provided for historical reasons. These are all deprecated and shouldn't be used in new drivers. Besides that there's a few helpers for pci drivers. .. kernel-doc:: drivers/gpu/drm/drm_pci.c :export: .. kernel-doc:: drivers/gpu/drm/drm_platform.c :export: Open/Close, File Operations and IOCTLs ====================================== Open and Close -------------- Open and close handlers. None of those methods are mandatory:: int (*firstopen) (struct drm_device *); void (*lastclose) (struct drm_device *); int (*open) (struct drm_device *, struct drm_file *); void (*preclose) (struct drm_device *, struct drm_file *); void (*postclose) (struct drm_device *, struct drm_file *); The firstopen method is called by the DRM core for legacy UMS (User Mode Setting) drivers only when an application opens a device that has no other opened file handle. UMS drivers can implement it to acquire device resources. KMS drivers can't use the method and must acquire resources in the load method instead. Similarly the lastclose method is called when the last application holding a file handle opened on the device closes it, for both UMS and KMS drivers. Additionally, the method is also called at module unload time or, for hot-pluggable devices, when the device is unplugged. The firstopen and lastclose calls can thus be unbalanced. The open method is called every time the device is opened by an application. Drivers can allocate per-file private data in this method and store them in the struct :c:type:`struct drm_file ` driver_priv field. Note that the open method is called before firstopen. The close operation is split into preclose and postclose methods. Drivers must stop and cleanup all per-file operations in the preclose method. For instance pending vertical blanking and page flip events must be cancelled. No per-file operation is allowed on the file handle after returning from the preclose method. Finally the postclose method is called as the last step of the close operation, right before calling the lastclose method if no other open file handle exists for the device. Drivers that have allocated per-file private data in the open method should free it here. The lastclose method should restore CRTC and plane properties to default value, so that a subsequent open of the device will not inherit state from the previous user. It can also be used to execute delayed power switching state changes, e.g. in conjunction with the :ref:`vga_switcheroo` infrastructure. Beyond that KMS drivers should not do any further cleanup. Only legacy UMS drivers might need to clean up device state so that the vga console or an independent fbdev driver could take over. File Operations --------------- .. kernel-doc:: drivers/gpu/drm/drm_fops.c :doc: file operations .. kernel-doc:: drivers/gpu/drm/drm_fops.c :export: IOCTLs ------ struct drm_ioctl_desc \*ioctls; int num_ioctls; Driver-specific ioctls descriptors table. Driver-specific ioctls numbers start at DRM_COMMAND_BASE. The ioctls descriptors table is indexed by the ioctl number offset from the base value. Drivers can use the DRM_IOCTL_DEF_DRV() macro to initialize the table entries. :: DRM_IOCTL_DEF_DRV(ioctl, func, flags) ``ioctl`` is the ioctl name. Drivers must define the DRM_##ioctl and DRM_IOCTL_##ioctl macros to the ioctl number offset from DRM_COMMAND_BASE and the ioctl number respectively. The first macro is private to the device while the second must be exposed to userspace in a public header. ``func`` is a pointer to the ioctl handler function compatible with the ``drm_ioctl_t`` type. :: typedef int drm_ioctl_t(struct drm_device *dev, void *data, struct drm_file *file_priv); ``flags`` is a bitmask combination of the following values. It restricts how the ioctl is allowed to be called. - DRM_AUTH - Only authenticated callers allowed - DRM_MASTER - The ioctl can only be called on the master file handle - DRM_ROOT_ONLY - Only callers with the SYSADMIN capability allowed - DRM_CONTROL_ALLOW - The ioctl can only be called on a control device - DRM_UNLOCKED - The ioctl handler will be called without locking the DRM global mutex. This is the enforced default for kms drivers (i.e. using the DRIVER_MODESET flag) and hence shouldn't be used any more for new drivers. .. kernel-doc:: drivers/gpu/drm/drm_ioctl.c :export: Misc Utilities ============== Printer ------- .. kernel-doc:: include/drm/drm_print.h :doc: print .. kernel-doc:: include/drm/drm_print.h :internal: .. kernel-doc:: drivers/gpu/drm/drm_print.c :export: Legacy Support Code =================== The section very briefly covers some of the old legacy support code which is only used by old DRM drivers which have done a so-called shadow-attach to the underlying device instead of registering as a real driver. This also includes some of the old generic buffer management and command submission code. Do not use any of this in new and modern drivers. Legacy Suspend/Resume --------------------- The DRM core provides some suspend/resume code, but drivers wanting full suspend/resume support should provide save() and restore() functions. These are called at suspend, hibernate, or resume time, and should perform any state save or restore required by your device across suspend or hibernate states. int (\*suspend) (struct drm_device \*, pm_message_t state); int (\*resume) (struct drm_device \*); Those are legacy suspend and resume methods which *only* work with the legacy shadow-attach driver registration functions. New driver should use the power management interface provided by their bus type (usually through the :c:type:`struct device_driver ` dev_pm_ops) and set these methods to NULL. Legacy DMA Services ------------------- This should cover how DMA mapping etc. is supported by the core. These functions are deprecated and should not be used.