xref: /openbmc/linux/Documentation/gpu/drm-kms.rst (revision bc5aa3a0)

=========================
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 <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
<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
   <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 <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 <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 <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_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 <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 <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 <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
<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 <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 <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 <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 <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: