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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.