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- The Frame Buffer Device API
- ---------------------------
-
-Last revised: June 21, 2011
-
-
-0. Introduction
----------------
-
-This document describes the frame buffer API used by applications to interact
-with frame buffer devices. In-kernel APIs between device drivers and the frame
-buffer core are not described.
-
-Due to a lack of documentation in the original frame buffer API, drivers
-behaviours differ in subtle (and not so subtle) ways. This document describes
-the recommended API implementation, but applications should be prepared to
-deal with different behaviours.
-
-
-1. Capabilities
----------------
-
-Device and driver capabilities are reported in the fixed screen information
-capabilities field.
-
-struct fb_fix_screeninfo {
- ...
- __u16 capabilities; /* see FB_CAP_* */
- ...
-};
-
-Application should use those capabilities to find out what features they can
-expect from the device and driver.
-
-- FB_CAP_FOURCC
-
-The driver supports the four character code (FOURCC) based format setting API.
-When supported, formats are configured using a FOURCC instead of manually
-specifying color components layout.
-
-
-2. Types and visuals
---------------------
-
-Pixels are stored in memory in hardware-dependent formats. Applications need
-to be aware of the pixel storage format in order to write image data to the
-frame buffer memory in the format expected by the hardware.
-
-Formats are described by frame buffer types and visuals. Some visuals require
-additional information, which are stored in the variable screen information
-bits_per_pixel, grayscale, red, green, blue and transp fields.
-
-Visuals describe how color information is encoded and assembled to create
-macropixels. Types describe how macropixels are stored in memory. The following
-types and visuals are supported.
-
-- FB_TYPE_PACKED_PIXELS
-
-Macropixels are stored contiguously in a single plane. If the number of bits
-per macropixel is not a multiple of 8, whether macropixels are padded to the
-next multiple of 8 bits or packed together into bytes depends on the visual.
-
-Padding at end of lines may be present and is then reported through the fixed
-screen information line_length field.
-
-- FB_TYPE_PLANES
-
-Macropixels are split across multiple planes. The number of planes is equal to
-the number of bits per macropixel, with plane i'th storing i'th bit from all
-macropixels.
-
-Planes are located contiguously in memory.
-
-- FB_TYPE_INTERLEAVED_PLANES
-
-Macropixels are split across multiple planes. The number of planes is equal to
-the number of bits per macropixel, with plane i'th storing i'th bit from all
-macropixels.
-
-Planes are interleaved in memory. The interleave factor, defined as the
-distance in bytes between the beginning of two consecutive interleaved blocks
-belonging to different planes, is stored in the fixed screen information
-type_aux field.
-
-- FB_TYPE_FOURCC
-
-Macropixels are stored in memory as described by the format FOURCC identifier
-stored in the variable screen information grayscale field.
-
-- FB_VISUAL_MONO01
-
-Pixels are black or white and stored on a number of bits (typically one)
-specified by the variable screen information bpp field.
-
-Black pixels are represented by all bits set to 1 and white pixels by all bits
-set to 0. When the number of bits per pixel is smaller than 8, several pixels
-are packed together in a byte.
-
-FB_VISUAL_MONO01 is currently used with FB_TYPE_PACKED_PIXELS only.
-
-- FB_VISUAL_MONO10
-
-Pixels are black or white and stored on a number of bits (typically one)
-specified by the variable screen information bpp field.
-
-Black pixels are represented by all bits set to 0 and white pixels by all bits
-set to 1. When the number of bits per pixel is smaller than 8, several pixels
-are packed together in a byte.
-
-FB_VISUAL_MONO01 is currently used with FB_TYPE_PACKED_PIXELS only.
-
-- FB_VISUAL_TRUECOLOR
-
-Pixels are broken into red, green and blue components, and each component
-indexes a read-only lookup table for the corresponding value. Lookup tables
-are device-dependent, and provide linear or non-linear ramps.
-
-Each component is stored in a macropixel according to the variable screen
-information red, green, blue and transp fields.
-
-- FB_VISUAL_PSEUDOCOLOR and FB_VISUAL_STATIC_PSEUDOCOLOR
-
-Pixel values are encoded as indices into a colormap that stores red, green and
-blue components. The colormap is read-only for FB_VISUAL_STATIC_PSEUDOCOLOR
-and read-write for FB_VISUAL_PSEUDOCOLOR.
-
-Each pixel value is stored in the number of bits reported by the variable
-screen information bits_per_pixel field.
-
-- FB_VISUAL_DIRECTCOLOR
-
-Pixels are broken into red, green and blue components, and each component
-indexes a programmable lookup table for the corresponding value.
-
-Each component is stored in a macropixel according to the variable screen
-information red, green, blue and transp fields.
-
-- FB_VISUAL_FOURCC
-
-Pixels are encoded and interpreted as described by the format FOURCC
-identifier stored in the variable screen information grayscale field.
-
-
-3. Screen information
----------------------
-
-Screen information are queried by applications using the FBIOGET_FSCREENINFO
-and FBIOGET_VSCREENINFO ioctls. Those ioctls take a pointer to a
-fb_fix_screeninfo and fb_var_screeninfo structure respectively.
-
-struct fb_fix_screeninfo stores device independent unchangeable information
-about the frame buffer device and the current format. Those information can't
-be directly modified by applications, but can be changed by the driver when an
-application modifies the format.
-
-struct fb_fix_screeninfo {
- char id[16]; /* identification string eg "TT Builtin" */
- unsigned long smem_start; /* Start of frame buffer mem */
- /* (physical address) */
- __u32 smem_len; /* Length of frame buffer mem */
- __u32 type; /* see FB_TYPE_* */
- __u32 type_aux; /* Interleave for interleaved Planes */
- __u32 visual; /* see FB_VISUAL_* */
- __u16 xpanstep; /* zero if no hardware panning */
- __u16 ypanstep; /* zero if no hardware panning */
- __u16 ywrapstep; /* zero if no hardware ywrap */
- __u32 line_length; /* length of a line in bytes */
- unsigned long mmio_start; /* Start of Memory Mapped I/O */
- /* (physical address) */
- __u32 mmio_len; /* Length of Memory Mapped I/O */
- __u32 accel; /* Indicate to driver which */
- /* specific chip/card we have */
- __u16 capabilities; /* see FB_CAP_* */
- __u16 reserved[2]; /* Reserved for future compatibility */
-};
-
-struct fb_var_screeninfo stores device independent changeable information
-about a frame buffer device, its current format and video mode, as well as
-other miscellaneous parameters.
-
-struct fb_var_screeninfo {
- __u32 xres; /* visible resolution */
- __u32 yres;
- __u32 xres_virtual; /* virtual resolution */
- __u32 yres_virtual;
- __u32 xoffset; /* offset from virtual to visible */
- __u32 yoffset; /* resolution */
-
- __u32 bits_per_pixel; /* guess what */
- __u32 grayscale; /* 0 = color, 1 = grayscale, */
- /* >1 = FOURCC */
- struct fb_bitfield red; /* bitfield in fb mem if true color, */
- struct fb_bitfield green; /* else only length is significant */
- struct fb_bitfield blue;
- struct fb_bitfield transp; /* transparency */
-
- __u32 nonstd; /* != 0 Non standard pixel format */
-
- __u32 activate; /* see FB_ACTIVATE_* */
-
- __u32 height; /* height of picture in mm */
- __u32 width; /* width of picture in mm */
-
- __u32 accel_flags; /* (OBSOLETE) see fb_info.flags */
-
- /* Timing: All values in pixclocks, except pixclock (of course) */
- __u32 pixclock; /* pixel clock in ps (pico seconds) */
- __u32 left_margin; /* time from sync to picture */
- __u32 right_margin; /* time from picture to sync */
- __u32 upper_margin; /* time from sync to picture */
- __u32 lower_margin;
- __u32 hsync_len; /* length of horizontal sync */
- __u32 vsync_len; /* length of vertical sync */
- __u32 sync; /* see FB_SYNC_* */
- __u32 vmode; /* see FB_VMODE_* */
- __u32 rotate; /* angle we rotate counter clockwise */
- __u32 colorspace; /* colorspace for FOURCC-based modes */
- __u32 reserved[4]; /* Reserved for future compatibility */
-};
-
-To modify variable information, applications call the FBIOPUT_VSCREENINFO
-ioctl with a pointer to a fb_var_screeninfo structure. If the call is
-successful, the driver will update the fixed screen information accordingly.
-
-Instead of filling the complete fb_var_screeninfo structure manually,
-applications should call the FBIOGET_VSCREENINFO ioctl and modify only the
-fields they care about.
-
-
-4. Format configuration
------------------------
-
-Frame buffer devices offer two ways to configure the frame buffer format: the
-legacy API and the FOURCC-based API.
-
-
-The legacy API has been the only frame buffer format configuration API for a
-long time and is thus widely used by application. It is the recommended API
-for applications when using RGB and grayscale formats, as well as legacy
-non-standard formats.
-
-To select a format, applications set the fb_var_screeninfo bits_per_pixel field
-to the desired frame buffer depth. Values up to 8 will usually map to
-monochrome, grayscale or pseudocolor visuals, although this is not required.
-
-- For grayscale formats, applications set the grayscale field to one. The red,
- blue, green and transp fields must be set to 0 by applications and ignored by
- drivers. Drivers must fill the red, blue and green offsets to 0 and lengths
- to the bits_per_pixel value.
-
-- For pseudocolor formats, applications set the grayscale field to zero. The
- red, blue, green and transp fields must be set to 0 by applications and
- ignored by drivers. Drivers must fill the red, blue and green offsets to 0
- and lengths to the bits_per_pixel value.
-
-- For truecolor and directcolor formats, applications set the grayscale field
- to zero, and the red, blue, green and transp fields to describe the layout of
- color components in memory.
-
-struct fb_bitfield {
- __u32 offset; /* beginning of bitfield */
- __u32 length; /* length of bitfield */
- __u32 msb_right; /* != 0 : Most significant bit is */
- /* right */
-};
-
- Pixel values are bits_per_pixel wide and are split in non-overlapping red,
- green, blue and alpha (transparency) components. Location and size of each
- component in the pixel value are described by the fb_bitfield offset and
- length fields. Offset are computed from the right.
-
- Pixels are always stored in an integer number of bytes. If the number of
- bits per pixel is not a multiple of 8, pixel values are padded to the next
- multiple of 8 bits.
-
-Upon successful format configuration, drivers update the fb_fix_screeninfo
-type, visual and line_length fields depending on the selected format.
-
-
-The FOURCC-based API replaces format descriptions by four character codes
-(FOURCC). FOURCCs are abstract identifiers that uniquely define a format
-without explicitly describing it. This is the only API that supports YUV
-formats. Drivers are also encouraged to implement the FOURCC-based API for RGB
-and grayscale formats.
-
-Drivers that support the FOURCC-based API report this capability by setting
-the FB_CAP_FOURCC bit in the fb_fix_screeninfo capabilities field.
-
-FOURCC definitions are located in the linux/videodev2.h header. However, and
-despite starting with the V4L2_PIX_FMT_prefix, they are not restricted to V4L2
-and don't require usage of the V4L2 subsystem. FOURCC documentation is
-available in Documentation/DocBook/v4l/pixfmt.xml.
-
-To select a format, applications set the grayscale field to the desired FOURCC.
-For YUV formats, they should also select the appropriate colorspace by setting
-the colorspace field to one of the colorspaces listed in linux/videodev2.h and
-documented in Documentation/DocBook/v4l/colorspaces.xml.
-
-The red, green, blue and transp fields are not used with the FOURCC-based API.
-For forward compatibility reasons applications must zero those fields, and
-drivers must ignore them. Values other than 0 may get a meaning in future
-extensions.
-
-Upon successful format configuration, drivers update the fb_fix_screeninfo
-type, visual and line_length fields depending on the selected format. The type
-and visual fields are set to FB_TYPE_FOURCC and FB_VISUAL_FOURCC respectively.