========================= Kernel Mode Setting (KMS) ========================= Mode Setting ============ Drivers must initialize the mode setting core by calling :c:func:`drm_mode_config_init()` on the DRM device. The function initializes the :c:type:`struct drm_device ` mode_config field and never fails. Once done, mode configuration must be setup by initializing the following fields. - int min_width, min_height; int max_width, max_height; Minimum and maximum width and height of the frame buffers in pixel units. - struct drm_mode_config_funcs \*funcs; Mode setting functions. Display Modes Function Reference -------------------------------- .. kernel-doc:: include/drm/drm_modes.h :internal: .. kernel-doc:: drivers/gpu/drm/drm_modes.c :export: Atomic Mode Setting Function Reference -------------------------------------- .. kernel-doc:: drivers/gpu/drm/drm_atomic.c :export: .. kernel-doc:: drivers/gpu/drm/drm_atomic.c :internal: Frame Buffer Abstraction ------------------------ Frame buffers are abstract memory objects that provide a source of pixels to scanout to a CRTC. Applications explicitly request the creation of frame buffers through the DRM_IOCTL_MODE_ADDFB(2) ioctls and receive an opaque handle that can be passed to the KMS CRTC control, plane configuration and page flip functions. Frame buffers rely on the underneath memory manager for low-level memory operations. When creating a frame buffer applications pass a memory handle (or a list of memory handles for multi-planar formats) through the ``drm_mode_fb_cmd2`` argument. For drivers using GEM as their userspace buffer management interface this would be a GEM handle. Drivers are however free to use their own backing storage object handles, e.g. vmwgfx directly exposes special TTM handles to userspace and so expects TTM handles in the create ioctl and not GEM handles. The lifetime of a drm framebuffer is controlled with a reference count, drivers can grab additional references with :c:func:`drm_framebuffer_reference()`and drop them again with :c:func:`drm_framebuffer_unreference()`. For driver-private framebuffers for which the last reference is never dropped (e.g. for the fbdev framebuffer when the struct :c:type:`struct drm_framebuffer ` is embedded into the fbdev helper struct) drivers can manually clean up a framebuffer at module unload time with :c:func:`drm_framebuffer_unregister_private()`. DRM Format Handling ------------------- .. kernel-doc:: drivers/gpu/drm/drm_fourcc.c :export: Dumb Buffer Objects ------------------- The KMS API doesn't standardize backing storage object creation and leaves it to driver-specific ioctls. Furthermore actually creating a buffer object even for GEM-based drivers is done through a driver-specific ioctl - GEM only has a common userspace interface for sharing and destroying objects. While not an issue for full-fledged graphics stacks that include device-specific userspace components (in libdrm for instance), this limit makes DRM-based early boot graphics unnecessarily complex. Dumb objects partly alleviate the problem by providing a standard API to create dumb buffers suitable for scanout, which can then be used to create KMS frame buffers. To support dumb objects drivers must implement the dumb_create, dumb_destroy and dumb_map_offset operations. - int (\*dumb_create)(struct drm_file \*file_priv, struct drm_device \*dev, struct drm_mode_create_dumb \*args); The dumb_create operation creates a driver object (GEM or TTM handle) suitable for scanout based on the width, height and depth from the struct :c:type:`struct drm_mode_create_dumb ` argument. It fills the argument's handle, pitch and size fields with a handle for the newly created object and its line pitch and size in bytes. - int (\*dumb_destroy)(struct drm_file \*file_priv, struct drm_device \*dev, uint32_t handle); The dumb_destroy operation destroys a dumb object created by dumb_create. - int (\*dumb_map_offset)(struct drm_file \*file_priv, struct drm_device \*dev, uint32_t handle, uint64_t \*offset); The dumb_map_offset operation associates an mmap fake offset with the object given by the handle and returns it. Drivers must use the :c:func:`drm_gem_create_mmap_offset()` function to associate the fake offset as described in ?. Note that dumb objects may not be used for gpu acceleration, as has been attempted on some ARM embedded platforms. Such drivers really must have a hardware-specific ioctl to allocate suitable buffer objects. Output Polling -------------- void (\*output_poll_changed)(struct drm_device \*dev); This operation notifies the driver that the status of one or more connectors has changed. Drivers that use the fb helper can just call the :c:func:`drm_fb_helper_hotplug_event()` function to handle this operation. KMS Initialization and Cleanup ============================== A KMS device is abstracted and exposed as a set of planes, CRTCs, encoders and connectors. KMS drivers must thus create and initialize all those objects at load time after initializing mode setting. CRTCs (:c:type:`struct drm_crtc `) -------------------------------------------- A CRTC is an abstraction representing a part of the chip that contains a pointer to a scanout buffer. Therefore, the number of CRTCs available determines how many independent scanout buffers can be active at any given time. The CRTC structure contains several fields to support this: a pointer to some video memory (abstracted as a frame buffer object), a display mode, and an (x, y) offset into the video memory to support panning or configurations where one piece of video memory spans multiple CRTCs. CRTC Initialization ~~~~~~~~~~~~~~~~~~~ A KMS device must create and register at least one struct :c:type:`struct drm_crtc ` instance. The instance is allocated and zeroed by the driver, possibly as part of a larger structure, and registered with a call to :c:func:`drm_crtc_init()` with a pointer to CRTC functions. Planes (:c:type:`struct drm_plane `) ----------------------------------------------- A plane represents an image source that can be blended with or overlayed on top of a CRTC during the scanout process. Planes are associated with a frame buffer to crop a portion of the image memory (source) and optionally scale it to a destination size. The result is then blended with or overlayed on top of a CRTC. The DRM core recognizes three types of planes: - DRM_PLANE_TYPE_PRIMARY represents a "main" plane for a CRTC. Primary planes are the planes operated upon by CRTC modesetting and flipping operations described in the page_flip hook in :c:type:`struct drm_crtc_funcs `. - DRM_PLANE_TYPE_CURSOR represents a "cursor" plane for a CRTC. Cursor planes are the planes operated upon by the DRM_IOCTL_MODE_CURSOR and DRM_IOCTL_MODE_CURSOR2 ioctls. - DRM_PLANE_TYPE_OVERLAY represents all non-primary, non-cursor planes. Some drivers refer to these types of planes as "sprites" internally. For compatibility with legacy userspace, only overlay planes are made available to userspace by default. Userspace clients may set the DRM_CLIENT_CAP_UNIVERSAL_PLANES client capability bit to indicate that they wish to receive a universal plane list containing all plane types. Plane Initialization ~~~~~~~~~~~~~~~~~~~~ To create a plane, a KMS drivers allocates and zeroes an instances of :c:type:`struct drm_plane ` (possibly as part of a larger structure) and registers it with a call to :c:func:`drm_universal_plane_init()`. The function takes a bitmask of the CRTCs that can be associated with the plane, a pointer to the plane functions, a list of format supported formats, and the type of plane (primary, cursor, or overlay) being initialized. Cursor and overlay planes are optional. All drivers should provide one primary plane per CRTC (although this requirement may change in the future); drivers that do not wish to provide special handling for primary planes may make use of the helper functions described in ? to create and register a primary plane with standard capabilities. Encoders (:c:type:`struct drm_encoder `) ----------------------------------------------------- An encoder takes pixel data from a CRTC and converts it to a format suitable for any attached connectors. On some devices, it may be possible to have a CRTC send data to more than one encoder. In that case, both encoders would receive data from the same scanout buffer, resulting in a "cloned" display configuration across the connectors attached to each encoder. Encoder Initialization ~~~~~~~~~~~~~~~~~~~~~~ As for CRTCs, a KMS driver must create, initialize and register at least one :c:type:`struct drm_encoder ` instance. The instance is allocated and zeroed by the driver, possibly as part of a larger structure. Drivers must initialize the :c:type:`struct drm_encoder ` possible_crtcs and possible_clones fields before registering the encoder. Both fields are bitmasks of respectively the CRTCs that the encoder can be connected to, and sibling encoders candidate for cloning. After being initialized, the encoder must be registered with a call to :c:func:`drm_encoder_init()`. The function takes a pointer to the encoder functions and an encoder type. Supported types are - DRM_MODE_ENCODER_DAC for VGA and analog on DVI-I/DVI-A - DRM_MODE_ENCODER_TMDS for DVI, HDMI and (embedded) DisplayPort - DRM_MODE_ENCODER_LVDS for display panels - DRM_MODE_ENCODER_TVDAC for TV output (Composite, S-Video, Component, SCART) - DRM_MODE_ENCODER_VIRTUAL for virtual machine displays Encoders must be attached to a CRTC to be used. DRM drivers leave encoders unattached at initialization time. Applications (or the fbdev compatibility layer when implemented) are responsible for attaching the encoders they want to use to a CRTC. Connectors (:c:type:`struct drm_connector `) ----------------------------------------------------------- A connector is the final destination for pixel data on a device, and usually connects directly to an external display device like a monitor or laptop panel. A connector can only be attached to one encoder at a time. The connector is also the structure where information about the attached display is kept, so it contains fields for display data, EDID data, DPMS & connection status, and information about modes supported on the attached displays. Connector Initialization ~~~~~~~~~~~~~~~~~~~~~~~~ Finally a KMS driver must create, initialize, register and attach at least one :c:type:`struct drm_connector ` instance. The instance is created as other KMS objects and initialized by setting the following fields. interlace_allowed Whether the connector can handle interlaced modes. doublescan_allowed Whether the connector can handle doublescan. display_info Display information is filled from EDID information when a display is detected. For non hot-pluggable displays such as flat panels in embedded systems, the driver should initialize the display_info.width_mm and display_info.height_mm fields with the physical size of the display. polled Connector polling mode, a combination of DRM_CONNECTOR_POLL_HPD The connector generates hotplug events and doesn't need to be periodically polled. The CONNECT and DISCONNECT flags must not be set together with the HPD flag. DRM_CONNECTOR_POLL_CONNECT Periodically poll the connector for connection. DRM_CONNECTOR_POLL_DISCONNECT Periodically poll the connector for disconnection. Set to 0 for connectors that don't support connection status discovery. The connector is then registered with a call to :c:func:`drm_connector_init()` with a pointer to the connector functions and a connector type, and exposed through sysfs with a call to :c:func:`drm_connector_register()`. Supported connector types are - DRM_MODE_CONNECTOR_VGA - DRM_MODE_CONNECTOR_DVII - DRM_MODE_CONNECTOR_DVID - DRM_MODE_CONNECTOR_DVIA - DRM_MODE_CONNECTOR_Composite - DRM_MODE_CONNECTOR_SVIDEO - DRM_MODE_CONNECTOR_LVDS - DRM_MODE_CONNECTOR_Component - DRM_MODE_CONNECTOR_9PinDIN - DRM_MODE_CONNECTOR_DisplayPort - DRM_MODE_CONNECTOR_HDMIA - DRM_MODE_CONNECTOR_HDMIB - DRM_MODE_CONNECTOR_TV - DRM_MODE_CONNECTOR_eDP - DRM_MODE_CONNECTOR_VIRTUAL Connectors must be attached to an encoder to be used. For devices that map connectors to encoders 1:1, the connector should be attached at initialization time with a call to :c:func:`drm_mode_connector_attach_encoder()`. The driver must also set the :c:type:`struct drm_connector ` encoder field to point to the attached encoder. Finally, drivers must initialize the connectors state change detection with a call to :c:func:`drm_kms_helper_poll_init()`. If at least one connector is pollable but can't generate hotplug interrupts (indicated by the DRM_CONNECTOR_POLL_CONNECT and DRM_CONNECTOR_POLL_DISCONNECT connector flags), a delayed work will automatically be queued to periodically poll for changes. Connectors that can generate hotplug interrupts must be marked with the DRM_CONNECTOR_POLL_HPD flag instead, and their interrupt handler must call :c:func:`drm_helper_hpd_irq_event()`. The function will queue a delayed work to check the state of all connectors, but no periodic polling will be done. Connector Operations ~~~~~~~~~~~~~~~~~~~~ **Note** Unless otherwise state, all operations are mandatory. DPMS '''' void (\*dpms)(struct drm_connector \*connector, int mode); The DPMS operation sets the power state of a connector. The mode argument is one of - DRM_MODE_DPMS_ON - DRM_MODE_DPMS_STANDBY - DRM_MODE_DPMS_SUSPEND - DRM_MODE_DPMS_OFF In all but DPMS_ON mode the encoder to which the connector is attached should put the display in low-power mode by driving its signals appropriately. If more than one connector is attached to the encoder care should be taken not to change the power state of other displays as a side effect. Low-power mode should be propagated to the encoders and CRTCs when all related connectors are put in low-power mode. Modes ''''' int (\*fill_modes)(struct drm_connector \*connector, uint32_t max_width, uint32_t max_height); Fill the mode list with all supported modes for the connector. If the ``max_width`` and ``max_height`` arguments are non-zero, the implementation must ignore all modes wider than ``max_width`` or higher than ``max_height``. The connector must also fill in this operation its display_info width_mm and height_mm fields with the connected display physical size in millimeters. The fields should be set to 0 if the value isn't known or is not applicable (for instance for projector devices). Connection Status ''''''''''''''''' The connection status is updated through polling or hotplug events when supported (see ?). The status value is reported to userspace through ioctls and must not be used inside the driver, as it only gets initialized by a call to :c:func:`drm_mode_getconnector()` from userspace. enum drm_connector_status (\*detect)(struct drm_connector \*connector, bool force); Check to see if anything is attached to the connector. The ``force`` parameter is set to false whilst polling or to true when checking the connector due to user request. ``force`` can be used by the driver to avoid expensive, destructive operations during automated probing. Return connector_status_connected if something is connected to the connector, connector_status_disconnected if nothing is connected and connector_status_unknown if the connection state isn't known. Drivers should only return connector_status_connected if the connection status has really been probed as connected. Connectors that can't detect the connection status, or failed connection status probes, should return connector_status_unknown. Cleanup ------- The DRM core manages its objects' lifetime. When an object is not needed anymore the core calls its destroy function, which must clean up and free every resource allocated for the object. Every :c:func:`drm_\*_init()` call must be matched with a corresponding :c:func:`drm_\*_cleanup()` call to cleanup CRTCs (:c:func:`drm_crtc_cleanup()`), planes (:c:func:`drm_plane_cleanup()`), encoders (:c:func:`drm_encoder_cleanup()`) and connectors (:c:func:`drm_connector_cleanup()`). Furthermore, connectors that have been added to sysfs must be removed by a call to :c:func:`drm_connector_unregister()` before calling :c:func:`drm_connector_cleanup()`. Connectors state change detection must be cleanup up with a call to :c:func:`drm_kms_helper_poll_fini()`. Output discovery and initialization example ------------------------------------------- :: void intel_crt_init(struct drm_device *dev) { struct drm_connector *connector; struct intel_output *intel_output; intel_output = kzalloc(sizeof(struct intel_output), GFP_KERNEL); if (!intel_output) return; connector = &intel_output->base; drm_connector_init(dev, &intel_output->base, &intel_crt_connector_funcs, DRM_MODE_CONNECTOR_VGA); drm_encoder_init(dev, &intel_output->enc, &intel_crt_enc_funcs, DRM_MODE_ENCODER_DAC); drm_mode_connector_attach_encoder(&intel_output->base, &intel_output->enc); /* Set up the DDC bus. */ intel_output->ddc_bus = intel_i2c_create(dev, GPIOA, "CRTDDC_A"); if (!intel_output->ddc_bus) { dev_printk(KERN_ERR, &dev->pdev->dev, "DDC bus registration " "failed.\n"); return; } intel_output->type = INTEL_OUTPUT_ANALOG; connector->interlace_allowed = 0; connector->doublescan_allowed = 0; drm_encoder_helper_add(&intel_output->enc, &intel_crt_helper_funcs); drm_connector_helper_add(connector, &intel_crt_connector_helper_funcs); drm_connector_register(connector); } In the example above (taken from the i915 driver), a CRTC, connector and encoder combination is created. A device-specific i2c bus is also created for fetching EDID data and performing monitor detection. Once the process is complete, the new connector is registered with sysfs to make its properties available to applications. KMS API Functions ----------------- .. kernel-doc:: drivers/gpu/drm/drm_crtc.c :export: KMS Data Structures ------------------- .. kernel-doc:: include/drm/drm_crtc.h :internal: KMS Locking ----------- .. kernel-doc:: drivers/gpu/drm/drm_modeset_lock.c :doc: kms locking .. kernel-doc:: include/drm/drm_modeset_lock.h :internal: .. kernel-doc:: drivers/gpu/drm/drm_modeset_lock.c :export: KMS Properties ============== Drivers may need to expose additional parameters to applications than those described in the previous sections. KMS supports attaching properties to CRTCs, connectors and planes and offers a userspace API to list, get and set the property values. Properties are identified by a name that uniquely defines the property purpose, and store an associated value. For all property types except blob properties the value is a 64-bit unsigned integer. KMS differentiates between properties and property instances. Drivers first create properties and then create and associate individual instances of those properties to objects. A property can be instantiated multiple times and associated with different objects. Values are stored in property instances, and all other property information are stored in the property and shared between all instances of the property. Every property is created with a type that influences how the KMS core handles the property. Supported property types are DRM_MODE_PROP_RANGE Range properties report their minimum and maximum admissible values. The KMS core verifies that values set by application fit in that range. DRM_MODE_PROP_ENUM Enumerated properties take a numerical value that ranges from 0 to the number of enumerated values defined by the property minus one, and associate a free-formed string name to each value. Applications can retrieve the list of defined value-name pairs and use the numerical value to get and set property instance values. DRM_MODE_PROP_BITMASK Bitmask properties are enumeration properties that additionally restrict all enumerated values to the 0..63 range. Bitmask property instance values combine one or more of the enumerated bits defined by the property. DRM_MODE_PROP_BLOB Blob properties store a binary blob without any format restriction. The binary blobs are created as KMS standalone objects, and blob property instance values store the ID of their associated blob object. Blob properties are only used for the connector EDID property and cannot be created by drivers. To create a property drivers call one of the following functions depending on the property type. All property creation functions take property flags and name, as well as type-specific arguments. - struct drm_property \*drm_property_create_range(struct drm_device \*dev, int flags, const char \*name, uint64_t min, uint64_t max); Create a range property with the given minimum and maximum values. - struct drm_property \*drm_property_create_enum(struct drm_device \*dev, int flags, const char \*name, const struct drm_prop_enum_list \*props, int num_values); Create an enumerated property. The ``props`` argument points to an array of ``num_values`` value-name pairs. - struct drm_property \*drm_property_create_bitmask(struct drm_device \*dev, int flags, const char \*name, const struct drm_prop_enum_list \*props, int num_values); Create a bitmask property. The ``props`` argument points to an array of ``num_values`` value-name pairs. Properties can additionally be created as immutable, in which case they will be read-only for applications but can be modified by the driver. To create an immutable property drivers must set the DRM_MODE_PROP_IMMUTABLE flag at property creation time. When no array of value-name pairs is readily available at property creation time for enumerated or range properties, drivers can create the property using the :c:func:`drm_property_create()` function and manually add enumeration value-name pairs by calling the :c:func:`drm_property_add_enum()` function. Care must be taken to properly specify the property type through the ``flags`` argument. After creating properties drivers can attach property instances to CRTC, connector and plane objects by calling the :c:func:`drm_object_attach_property()`. The function takes a pointer to the target object, a pointer to the previously created property and an initial instance value. Existing KMS Properties ----------------------- The following table gives description of drm properties exposed by various modules/drivers. .. csv-table:: :header-rows: 1 :file: kms-properties.csv Vertical Blanking ================= Vertical blanking plays a major role in graphics rendering. To achieve tear-free display, users must synchronize page flips and/or rendering to vertical blanking. The DRM API offers ioctls to perform page flips synchronized to vertical blanking and wait for vertical blanking. The DRM core handles most of the vertical blanking management logic, which involves filtering out spurious interrupts, keeping race-free blanking counters, coping with counter wrap-around and resets and keeping use counts. It relies on the driver to generate vertical blanking interrupts and optionally provide a hardware vertical blanking counter. Drivers must implement the following operations. - int (\*enable_vblank) (struct drm_device \*dev, int crtc); void (\*disable_vblank) (struct drm_device \*dev, int crtc); Enable or disable vertical blanking interrupts for the given CRTC. - u32 (\*get_vblank_counter) (struct drm_device \*dev, int crtc); Retrieve the value of the vertical blanking counter for the given CRTC. If the hardware maintains a vertical blanking counter its value should be returned. Otherwise drivers can use the :c:func:`drm_vblank_count()` helper function to handle this operation. Drivers must initialize the vertical blanking handling core with a call to :c:func:`drm_vblank_init()` in their load operation. Vertical blanking interrupts can be enabled by the DRM core or by drivers themselves (for instance to handle page flipping operations). The DRM core maintains a vertical blanking use count to ensure that the interrupts are not disabled while a user still needs them. To increment the use count, drivers call :c:func:`drm_vblank_get()`. Upon return vertical blanking interrupts are guaranteed to be enabled. To decrement the use count drivers call :c:func:`drm_vblank_put()`. Only when the use count drops to zero will the DRM core disable the vertical blanking interrupts after a delay by scheduling a timer. The delay is accessible through the vblankoffdelay module parameter or the ``drm_vblank_offdelay`` global variable and expressed in milliseconds. Its default value is 5000 ms. Zero means never disable, and a negative value means disable immediately. Drivers may override the behaviour by setting the :c:type:`struct drm_device ` vblank_disable_immediate flag, which when set causes vblank interrupts to be disabled immediately regardless of the drm_vblank_offdelay value. The flag should only be set if there's a properly working hardware vblank counter present. When a vertical blanking interrupt occurs drivers only need to call the :c:func:`drm_handle_vblank()` function to account for the interrupt. Resources allocated by :c:func:`drm_vblank_init()` must be freed with a call to :c:func:`drm_vblank_cleanup()` in the driver unload operation handler. Vertical Blanking and Interrupt Handling Functions Reference ------------------------------------------------------------ .. kernel-doc:: drivers/gpu/drm/drm_irq.c :export: .. kernel-doc:: include/drm/drm_irq.h :internal: