Documentation/fb/api.txt

   1                         The Frame Buffer Device API
   2                         ---------------------------
   3
   4 Last revised: June 21, 2011
   5
   6
   7 0. Introduction
   8 ---------------
   9
  10 This document describes the frame buffer API used by applications to interact
  11 with frame buffer devices. In-kernel APIs between device drivers and the frame
  12 buffer core are not described.
  13
  14 Due to a lack of documentation in the original frame buffer API, drivers
  15 behaviours differ in subtle (and not so subtle) ways. This document describes
  16 the recommended API implementation, but applications should be prepared to
  17 deal with different behaviours.
  18
  19
  20 1. Capabilities
  21 ---------------
  22
  23 Device and driver capabilities are reported in the fixed screen information
  24 capabilities field.
  25
  26 struct fb_fix_screeninfo {
  27         ...
  28         __u16 capabilities;             /* see FB_CAP_*                 */
  29         ...
  30 };
  31
  32 Application should use those capabilities to find out what features they can
  33 expect from the device and driver.
  34
  35 - FB_CAP_FOURCC
  36
  37 The driver supports the four character code (FOURCC) based format setting API.
  38 When supported, formats are configured using a FOURCC instead of manually
  39 specifying color components layout.
  40
  41
  42 2. Types and visuals
  43 --------------------
  44
  45 Pixels are stored in memory in hardware-dependent formats. Applications need
  46 to be aware of the pixel storage format in order to write image data to the
  47 frame buffer memory in the format expected by the hardware.
  48
  49 Formats are described by frame buffer types and visuals. Some visuals require
  50 additional information, which are stored in the variable screen information
  51 bits_per_pixel, grayscale, red, green, blue and transp fields.
  52
  53 Visuals describe how color information is encoded and assembled to create
  54 macropixels. Types describe how macropixels are stored in memory. The following
  55 types and visuals are supported.
  56
  57 - FB_TYPE_PACKED_PIXELS
  58
  59 Macropixels are stored contiguously in a single plane. If the number of bits
  60 per macropixel is not a multiple of 8, whether macropixels are padded to the
  61 next multiple of 8 bits or packed together into bytes depends on the visual.
  62
  63 Padding at end of lines may be present and is then reported through the fixed
  64 screen information line_length field.
  65
  66 - FB_TYPE_PLANES
  67
  68 Macropixels are split across multiple planes. The number of planes is equal to
  69 the number of bits per macropixel, with plane i'th storing i'th bit from all
  70 macropixels.
  71
  72 Planes are located contiguously in memory.
  73
  74 - FB_TYPE_INTERLEAVED_PLANES
  75
  76 Macropixels are split across multiple planes. The number of planes is equal to
  77 the number of bits per macropixel, with plane i'th storing i'th bit from all
  78 macropixels.
  79
  80 Planes are interleaved in memory. The interleave factor, defined as the
  81 distance in bytes between the beginning of two consecutive interleaved blocks
  82 belonging to different planes, is stored in the fixed screen information
  83 type_aux field.
  84
  85 - FB_TYPE_FOURCC
  86
  87 Macropixels are stored in memory as described by the format FOURCC identifier
  88 stored in the variable screen information grayscale field.
  89
  90 - FB_VISUAL_MONO01
  91
  92 Pixels are black or white and stored on a number of bits (typically one)
  93 specified by the variable screen information bpp field.
  94
  95 Black pixels are represented by all bits set to 1 and white pixels by all bits
  96 set to 0. When the number of bits per pixel is smaller than 8, several pixels
  97 are packed together in a byte.
  98
  99 FB_VISUAL_MONO01 is currently used with FB_TYPE_PACKED_PIXELS only.
 100
 101 - FB_VISUAL_MONO10
 102
 103 Pixels are black or white and stored on a number of bits (typically one)
 104 specified by the variable screen information bpp field.
 105
 106 Black pixels are represented by all bits set to 0 and white pixels by all bits
 107 set to 1. When the number of bits per pixel is smaller than 8, several pixels
 108 are packed together in a byte.
 109
 110 FB_VISUAL_MONO01 is currently used with FB_TYPE_PACKED_PIXELS only.
 111
 112 - FB_VISUAL_TRUECOLOR
 113
 114 Pixels are broken into red, green and blue components, and each component
 115 indexes a read-only lookup table for the corresponding value. Lookup tables
 116 are device-dependent, and provide linear or non-linear ramps.
 117
 118 Each component is stored in a macropixel according to the variable screen
 119 information red, green, blue and transp fields.
 120
 121 - FB_VISUAL_PSEUDOCOLOR and FB_VISUAL_STATIC_PSEUDOCOLOR
 122
 123 Pixel values are encoded as indices into a colormap that stores red, green and
 124 blue components. The colormap is read-only for FB_VISUAL_STATIC_PSEUDOCOLOR
 125 and read-write for FB_VISUAL_PSEUDOCOLOR.
 126
 127 Each pixel value is stored in the number of bits reported by the variable
 128 screen information bits_per_pixel field.
 129
 130 - FB_VISUAL_DIRECTCOLOR
 131
 132 Pixels are broken into red, green and blue components, and each component
 133 indexes a programmable lookup table for the corresponding value.
 134
 135 Each component is stored in a macropixel according to the variable screen
 136 information red, green, blue and transp fields.
 137
 138 - FB_VISUAL_FOURCC
 139
 140 Pixels are encoded and  interpreted as described by the format FOURCC
 141 identifier stored in the variable screen information grayscale field.
 142
 143
 144 3. Screen information
 145 ---------------------
 146
 147 Screen information are queried by applications using the FBIOGET_FSCREENINFO
 148 and FBIOGET_VSCREENINFO ioctls. Those ioctls take a pointer to a
 149 fb_fix_screeninfo and fb_var_screeninfo structure respectively.
 150
 151 struct fb_fix_screeninfo stores device independent unchangeable information
 152 about the frame buffer device and the current format. Those information can't
 153 be directly modified by applications, but can be changed by the driver when an
 154 application modifies the format.
 155
 156 struct fb_fix_screeninfo {
 157         char id[16];                    /* identification string eg "TT Builtin" */
 158         unsigned long smem_start;       /* Start of frame buffer mem */
 159                                         /* (physical address) */
 160         __u32 smem_len;                 /* Length of frame buffer mem */
 161         __u32 type;                     /* see FB_TYPE_*                */
 162         __u32 type_aux;                 /* Interleave for interleaved Planes */
 163         __u32 visual;                   /* see FB_VISUAL_*              */
 164         __u16 xpanstep;                 /* zero if no hardware panning  */
 165         __u16 ypanstep;                 /* zero if no hardware panning  */
 166         __u16 ywrapstep;                /* zero if no hardware ywrap    */
 167         __u32 line_length;              /* length of a line in bytes    */
 168         unsigned long mmio_start;       /* Start of Memory Mapped I/O   */
 169                                         /* (physical address) */
 170         __u32 mmio_len;                 /* Length of Memory Mapped I/O  */
 171         __u32 accel;                    /* Indicate to driver which     */
 172                                         /*  specific chip/card we have  */
 173         __u16 capabilities;             /* see FB_CAP_*                 */
 174         __u16 reserved[2];              /* Reserved for future compatibility */
 175 };
 176
 177 struct fb_var_screeninfo stores device independent changeable information
 178 about a frame buffer device, its current format and video mode, as well as
 179 other miscellaneous parameters.
 180
 181 struct fb_var_screeninfo {
 182         __u32 xres;                     /* visible resolution           */
 183         __u32 yres;
 184         __u32 xres_virtual;             /* virtual resolution           */
 185         __u32 yres_virtual;
 186         __u32 xoffset;                  /* offset from virtual to visible */
 187         __u32 yoffset;                  /* resolution                   */
 188
 189         __u32 bits_per_pixel;           /* guess what                   */
 190         __u32 grayscale;                /* 0 = color, 1 = grayscale,    */
 191                                         /* >1 = FOURCC                  */
 192         struct fb_bitfield red;         /* bitfield in fb mem if true color, */
 193         struct fb_bitfield green;       /* else only length is significant */
 194         struct fb_bitfield blue;
 195         struct fb_bitfield transp;      /* transparency                 */
 196
 197         __u32 nonstd;                   /* != 0 Non standard pixel format */
 198
 199         __u32 activate;                 /* see FB_ACTIVATE_*            */
 200
 201         __u32 height;                   /* height of picture in mm    */
 202         __u32 width;                    /* width of picture in mm     */
 203
 204         __u32 accel_flags;              /* (OBSOLETE) see fb_info.flags */
 205
 206         /* Timing: All values in pixclocks, except pixclock (of course) */
 207         __u32 pixclock;                 /* pixel clock in ps (pico seconds) */
 208         __u32 left_margin;              /* time from sync to picture    */
 209         __u32 right_margin;             /* time from picture to sync    */
 210         __u32 upper_margin;             /* time from sync to picture    */
 211         __u32 lower_margin;
 212         __u32 hsync_len;                /* length of horizontal sync    */
 213         __u32 vsync_len;                /* length of vertical sync      */
 214         __u32 sync;                     /* see FB_SYNC_*                */
 215         __u32 vmode;                    /* see FB_VMODE_*               */
 216         __u32 rotate;                   /* angle we rotate counter clockwise */
 217         __u32 colorspace;               /* colorspace for FOURCC-based modes */
 218         __u32 reserved[4];              /* Reserved for future compatibility */
 219 };
 220
 221 To modify variable information, applications call the FBIOPUT_VSCREENINFO
 222 ioctl with a pointer to a fb_var_screeninfo structure. If the call is
 223 successful, the driver will update the fixed screen information accordingly.
 224
 225 Instead of filling the complete fb_var_screeninfo structure manually,
 226 applications should call the FBIOGET_VSCREENINFO ioctl and modify only the
 227 fields they care about.
 228
 229
 230 4. Format configuration
 231 -----------------------
 232
 233 Frame buffer devices offer two ways to configure the frame buffer format: the
 234 legacy API and the FOURCC-based API.
 235
 236
 237 The legacy API has been the only frame buffer format configuration API for a
 238 long time and is thus widely used by application. It is the recommended API
 239 for applications when using RGB and grayscale formats, as well as legacy
 240 non-standard formats.
 241
 242 To select a format, applications set the fb_var_screeninfo bits_per_pixel field
 243 to the desired frame buffer depth. Values up to 8 will usually map to
 244 monochrome, grayscale or pseudocolor visuals, although this is not required.
 245
 246 - For grayscale formats, applications set the grayscale field to one. The red,
 247   blue, green and transp fields must be set to 0 by applications and ignored by
 248   drivers. Drivers must fill the red, blue and green offsets to 0 and lengths
 249   to the bits_per_pixel value.
 250
 251 - For pseudocolor formats, applications set the grayscale field to zero. The
 252   red, blue, green and transp fields must be set to 0 by applications and
 253   ignored by drivers. Drivers must fill the red, blue and green offsets to 0
 254   and lengths to the bits_per_pixel value.
 255
 256 - For truecolor and directcolor formats, applications set the grayscale field
 257   to zero, and the red, blue, green and transp fields to describe the layout of
 258   color components in memory.
 259
 260 struct fb_bitfield {
 261         __u32 offset;                   /* beginning of bitfield        */
 262         __u32 length;                   /* length of bitfield           */
 263         __u32 msb_right;                /* != 0 : Most significant bit is */
 264                                         /* right */
 265 };
 266
 267   Pixel values are bits_per_pixel wide and are split in non-overlapping red,
 268   green, blue and alpha (transparency) components. Location and size of each
 269   component in the pixel value are described by the fb_bitfield offset and
 270   length fields. Offset are computed from the right.
 271
 272   Pixels are always stored in an integer number of bytes. If the number of
 273   bits per pixel is not a multiple of 8, pixel values are padded to the next
 274   multiple of 8 bits.
 275
 276 Upon successful format configuration, drivers update the fb_fix_screeninfo
 277 type, visual and line_length fields depending on the selected format.
 278
 279
 280 The FOURCC-based API replaces format descriptions by four character codes
 281 (FOURCC). FOURCCs are abstract identifiers that uniquely define a format
 282 without explicitly describing it. This is the only API that supports YUV
 283 formats. Drivers are also encouraged to implement the FOURCC-based API for RGB
 284 and grayscale formats.
 285
 286 Drivers that support the FOURCC-based API report this capability by setting
 287 the FB_CAP_FOURCC bit in the fb_fix_screeninfo capabilities field.
 288
 289 FOURCC definitions are located in the linux/videodev2.h header. However, and
 290 despite starting with the V4L2_PIX_FMT_prefix, they are not restricted to V4L2
 291 and don't require usage of the V4L2 subsystem. FOURCC documentation is
 292 available in Documentation/DocBook/v4l/pixfmt.xml.
 293
 294 To select a format, applications set the grayscale field to the desired FOURCC.
 295 For YUV formats, they should also select the appropriate colorspace by setting
 296 the colorspace field to one of the colorspaces listed in linux/videodev2.h and
 297 documented in Documentation/DocBook/v4l/colorspaces.xml.
 298
 299 The red, green, blue and transp fields are not used with the FOURCC-based API.
 300 For forward compatibility reasons applications must zero those fields, and
 301 drivers must ignore them. Values other than 0 may get a meaning in future
 302 extensions.
 303
 304 Upon successful format configuration, drivers update the fb_fix_screeninfo
 305 type, visual and line_length fields depending on the selected format. The type
 306 and visual fields are set to FB_TYPE_FOURCC and FB_VISUAL_FOURCC respectively.