source: rtems-docs/bsp-howto/frame_buffer.rst @ 362ae68

.. SPDX-License-Identifier: CC-BY-SA-4.0

.. Copyright (C) 1988, 2002 On-Line Applications Research Corporation (OAR)

Frame Buffer Driver
*******************

In this chapter, we present the basic functionality implemented by a frame
buffer driver:

- ``frame_buffer_initialize()``
- ``frame_buffer_open()``
- ``frame_buffer_close()``
- ``frame_buffer_read()``
- ``frame_buffer_write()``
- ``frame_buffer_control()``

Introduction
============

The purpose of the frame buffer driver is to provide an abstraction for
graphics hardware.  By using the frame buffer interface, an application can
display graphics without knowing anything about the low-level details of
interfacing to a particular graphics adapter. The parameters governing the
mapping of memory to displayed pixels (planar or linear, bit depth, etc) is
still implementation-specific, but device-independent methods are provided to
determine and potentially modify these parameters.

The frame buffer driver is commonly located in the ``console`` directory of the
BSP and registered by the name :file:`/dev/fb0`.  Additional frame buffers (if
available) are named :file:`/dev/fb1*,*/dev/fb2`, etc.

To work with the frame buffer, the following operation sequence is
used:``open()``, ``ioctls()`` to get the frame buffer info, ``read()``
and/or ``write()``, and ``close()``.

Driver Function Overview
========================

Initialization
--------------

The driver initialization is called once during the RTEMS initialization
process and returns ``RTEMS_SUCCESSFUL`` when the device driver is successfully
initialized. During the initialization, a name is assigned to the frame buffer
device.  If the graphics hardware supports console text output, as is the case
with the pc386 VGA hardware, initialization into graphics mode may be deferred
until the device is ``open()`` ed.

The ``frame_buffer_initialize()`` function may look like this:

.. code-block:: c

    rtems_device_driver frame_buffer_initialize(
      rtems_device_major_number  major,
      rtems_device_minor_number  minor,
      void                      *arg)
    {
      rtems_status_code status;

      printk( "frame buffer driver initializing..\n" );

      /*
       * Register the device
       */
      status = rtems_io_register_name("/dev/fb0", major, 0);
      if (status != RTEMS_SUCCESSFUL)
      {
        printk("Error registering frame buffer device!\n");
        rtems_fatal_error_occurred( status );
      }

      /*
       * graphics hardware initialization goes here for non-console
       * devices
       */

      return RTEMS_SUCCESSFUL;
    }

Opening the Frame Buffer Device
-------------------------------

The ``frame_buffer_open()`` function is called whenever a frame buffer device
is opened.  If the frame buffer is registered as :file:`/dev/fb0`, the
``frame_buffer_open`` entry point will be called as the result of an
``open("/dev/fb0", mode)`` in the application.

Thread safety of the frame buffer driver is implementation-dependent.  The VGA
driver shown below uses a mutex to prevent multiple open() operations of the
frame buffer device.

The ``frame_buffer_open()`` function returns ``RTEMS_SUCCESSFUL`` when the
device driver is successfully opened, and ``RTEMS_UNSATISFIED`` if the device
is already open:

.. code-block:: c

    rtems_device_driver frame_buffer_close(
      rtems_device_major_number  major,
      rtems_device_minor_number  minor,
      void                      *arg
    )
    {
      if (pthread_mutex_unlock(&mutex) == 0) {
        /* restore previous state.  for VGA this means return to text mode.
         * leave out if graphics hardware has been initialized in
         * frame_buffer_initialize()
         */
        ega_hwterm();
        printk( "FBVGA close called.\n" );
        return RTEMS_SUCCESSFUL;
      }
      return RTEMS_UNSATISFIED;
    }

In the previous example, the function ``ega_hwinit()`` takes care of
hardware-specific initialization.

Closing the Frame Buffer Device
-------------------------------

The ``frame_buffer_close()`` is invoked when the frame buffer device is closed.
It frees up any resources allocated in ``frame_buffer_open()``, and should
restore previous hardware state.  The entry point corresponds to the device
driver close entry point.

Returns ``RTEMS_SUCCESSFUL`` when the device driver is successfully closed:

.. code-block:: c

    rtems_device_driver frame_buffer_close(
      rtems_device_major_number  major,
      rtems_device_minor_number  minor,
      void                      *arg)
    {
      pthread_mutex_unlock(&mutex);

      /* TODO check mutex return value, RTEMS_UNSATISFIED if it failed.  we
       * don't want to unconditionally call ega_hwterm()... */
      /* restore previous state.  for VGA this means return to text mode.
       * leave out if graphics hardware has been initialized in
       * frame_buffer_initialize() */
      ega_hwterm();
      printk( "frame buffer close called.\n" );
      return RTEMS_SUCCESSFUL;
    }

Reading from the Frame Buffer Device
------------------------------------

The ``frame_buffer_read()`` is invoked from a ``read()`` operation on the frame
buffer device.  Read functions should allow normal and partial reading at the
end of frame buffer memory.  This method returns ``RTEMS_SUCCESSFUL`` when the
device is successfully read from:

.. code-block:: c

    rtems_device_driver frame_buffer_read(
      rtems_device_major_number  major,
      rtems_device_minor_number  minor,
      void                      *arg
    )
    {
      rtems_libio_rw_args_t *rw_args = (rtems_libio_rw_args_t *)arg;
      rw_args->bytes_moved = ((rw_args->offset + rw_args->count) > fb_fix.smem_len ) ?
                               (fb_fix.smem_len - rw_args->offset) : rw_args->count;
      memcpy(rw_args->buffer,
             (const void *) (fb_fix.smem_start + rw_args->offset),
             rw_args->bytes_moved);
      return RTEMS_SUCCESSFUL;
    }

Writing to the Frame Buffer Device
----------------------------------

The ``frame_buffer_write()`` is invoked from a ``write()`` operation on the
frame buffer device.  The frame buffer write function is similar to the read
function, and should handle similar cases involving partial writes.

This method returns ``RTEMS_SUCCESSFUL`` when the device is successfully
written to:

.. code-block:: c

    rtems_device_driver frame_buffer_write(
      rtems_device_major_number  major,
      rtems_device_minor_number  minor,
      void                      *arg
    )
    {
      rtems_libio_rw_args_t *rw_args = (rtems_libio_rw_args_t *)arg;
      rw_args->bytes_moved = ((rw_args->offset + rw_args->count) > fb_fix.smem_len ) ?
                               (fb_fix.smem_len - rw_args->offset) : rw_args->count;
      memcpy((void *) (fb_fix.smem_start + rw_args->offset),
             rw_args->buffer,
             rw_args->bytes_moved);
      return RTEMS_SUCCESSFUL;
    }

Frame Buffer IO Control
-----------------------

The frame buffer driver allows several ioctls, partially compatible with the
Linux kernel, to obtain information about the hardware.

All ``ioctl()`` operations on the frame buffer device invoke
``frame_buffer_control()``.

Ioctls supported:

- ioctls to get the frame buffer screen info (fixed and variable).

- ioctl to set and get palette.

.. code-block:: c

    rtems_device_driver frame_buffer_control(
      rtems_device_major_number  major,
      rtems_device_minor_number  minor,
      void                      *arg
    )
    {
      rtems_libio_ioctl_args_t *args = arg;

      printk( "FBVGA ioctl called, cmd=%x\n", args->command  );

      switch( args->command ) {
        case FBIOGET_FSCREENINFO:
          args->ioctl_return =  get_fix_screen_info( ( struct fb_fix_screeninfo * ) args->buffer );
          break;
        case FBIOGET_VSCREENINFO:
          args->ioctl_return =  get_var_screen_info( ( struct fb_var_screeninfo * ) args->buffer );
          break;
        case FBIOPUT_VSCREENINFO:
          /* not implemented yet*/
          args->ioctl_return = -1;
          return RTEMS_UNSATISFIED;
        case FBIOGETCMAP:
          args->ioctl_return =  get_palette( ( struct fb_cmap * ) args->buffer );
          break;
        case FBIOPUTCMAP:
          args->ioctl_return =  set_palette( ( struct fb_cmap * ) args->buffer );
          break;
        default:
          args->ioctl_return = 0;
          break;
      }

      return RTEMS_SUCCESSFUL;
    }

See ``rtems/fb.h`` for more information on the list of ioctls and data
structures they work with.