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diff --git a/Documentation/gpu/drm-internals.rst b/Documentation/gpu/drm-internals.rst new file mode 100644 index 000000000..3bb261359 --- /dev/null +++ b/Documentation/gpu/drm-internals.rst @@ -0,0 +1,381 @@ +============= +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 +<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 <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 <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 <drm_driver>` instance. + +u32 driver_features; + +DRIVER_USE_AGP + Driver uses AGP interface, the DRM core will manage AGP resources. + +DRIVER_REQUIRE_AGP + Driver needs AGP interface to function. AGP initialization failure + will become a fatal error. + +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: + +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 <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 +<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: + +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 <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. |