1 The Frame Buffer Device API 2 --------------------------- 3 4Last revised: June 21, 2011 5 6 70. Introduction 8--------------- 9 10This document describes the frame buffer API used by applications to interact 11with frame buffer devices. In-kernel APIs between device drivers and the frame 12buffer core are not described. 13 14Due to a lack of documentation in the original frame buffer API, drivers 15behaviours differ in subtle (and not so subtle) ways. This document describes 16the recommended API implementation, but applications should be prepared to 17deal with different behaviours. 18 19 201. Capabilities 21--------------- 22 23Device and driver capabilities are reported in the fixed screen information 24capabilities field. 25 26struct fb_fix_screeninfo { 27 ... 28 __u16 capabilities; /* see FB_CAP_* */ 29 ... 30}; 31 32Application should use those capabilities to find out what features they can 33expect from the device and driver. 34 35- FB_CAP_FOURCC 36 37The driver supports the four character code (FOURCC) based format setting API. 38When supported, formats are configured using a FOURCC instead of manually 39specifying color components layout. 40 41 422. Types and visuals 43-------------------- 44 45Pixels are stored in memory in hardware-dependent formats. Applications need 46to be aware of the pixel storage format in order to write image data to the 47frame buffer memory in the format expected by the hardware. 48 49Formats are described by frame buffer types and visuals. Some visuals require 50additional information, which are stored in the variable screen information 51bits_per_pixel, grayscale, red, green, blue and transp fields. 52 53Visuals describe how color information is encoded and assembled to create 54macropixels. Types describe how macropixels are stored in memory. The following 55types and visuals are supported. 56 57- FB_TYPE_PACKED_PIXELS 58 59Macropixels are stored contiguously in a single plane. If the number of bits 60per macropixel is not a multiple of 8, whether macropixels are padded to the 61next multiple of 8 bits or packed together into bytes depends on the visual. 62 63Padding at end of lines may be present and is then reported through the fixed 64screen information line_length field. 65 66- FB_TYPE_PLANES 67 68Macropixels are split across multiple planes. The number of planes is equal to 69the number of bits per macropixel, with plane i'th storing i'th bit from all 70macropixels. 71 72Planes are located contiguously in memory. 73 74- FB_TYPE_INTERLEAVED_PLANES 75 76Macropixels are split across multiple planes. The number of planes is equal to 77the number of bits per macropixel, with plane i'th storing i'th bit from all 78macropixels. 79 80Planes are interleaved in memory. The interleave factor, defined as the 81distance in bytes between the beginning of two consecutive interleaved blocks 82belonging to different planes, is stored in the fixed screen information 83type_aux field. 84 85- FB_TYPE_FOURCC 86 87Macropixels are stored in memory as described by the format FOURCC identifier 88stored in the variable screen information grayscale field. 89 90- FB_VISUAL_MONO01 91 92Pixels are black or white and stored on a number of bits (typically one) 93specified by the variable screen information bpp field. 94 95Black pixels are represented by all bits set to 1 and white pixels by all bits 96set to 0. When the number of bits per pixel is smaller than 8, several pixels 97are packed together in a byte. 98 99FB_VISUAL_MONO01 is currently used with FB_TYPE_PACKED_PIXELS only. 100 101- FB_VISUAL_MONO10 102 103Pixels are black or white and stored on a number of bits (typically one) 104specified by the variable screen information bpp field. 105 106Black pixels are represented by all bits set to 0 and white pixels by all bits 107set to 1. When the number of bits per pixel is smaller than 8, several pixels 108are packed together in a byte. 109 110FB_VISUAL_MONO01 is currently used with FB_TYPE_PACKED_PIXELS only. 111 112- FB_VISUAL_TRUECOLOR 113 114Pixels are broken into red, green and blue components, and each component 115indexes a read-only lookup table for the corresponding value. Lookup tables 116are device-dependent, and provide linear or non-linear ramps. 117 118Each component is stored in a macropixel according to the variable screen 119information red, green, blue and transp fields. 120 121- FB_VISUAL_PSEUDOCOLOR and FB_VISUAL_STATIC_PSEUDOCOLOR 122 123Pixel values are encoded as indices into a colormap that stores red, green and 124blue components. The colormap is read-only for FB_VISUAL_STATIC_PSEUDOCOLOR 125and read-write for FB_VISUAL_PSEUDOCOLOR. 126 127Each pixel value is stored in the number of bits reported by the variable 128screen information bits_per_pixel field. 129 130- FB_VISUAL_DIRECTCOLOR 131 132Pixels are broken into red, green and blue components, and each component 133indexes a programmable lookup table for the corresponding value. 134 135Each component is stored in a macropixel according to the variable screen 136information red, green, blue and transp fields. 137 138- FB_VISUAL_FOURCC 139 140Pixels are encoded and interpreted as described by the format FOURCC 141identifier stored in the variable screen information grayscale field. 142 143 1443. Screen information 145--------------------- 146 147Screen information are queried by applications using the FBIOGET_FSCREENINFO 148and FBIOGET_VSCREENINFO ioctls. Those ioctls take a pointer to a 149fb_fix_screeninfo and fb_var_screeninfo structure respectively. 150 151struct fb_fix_screeninfo stores device independent unchangeable information 152about the frame buffer device and the current format. Those information can't 153be directly modified by applications, but can be changed by the driver when an 154application modifies the format. 155 156struct 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 177struct fb_var_screeninfo stores device independent changeable information 178about a frame buffer device, its current format and video mode, as well as 179other miscellaneous parameters. 180 181struct 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 221To modify variable information, applications call the FBIOPUT_VSCREENINFO 222ioctl with a pointer to a fb_var_screeninfo structure. If the call is 223successful, the driver will update the fixed screen information accordingly. 224 225Instead of filling the complete fb_var_screeninfo structure manually, 226applications should call the FBIOGET_VSCREENINFO ioctl and modify only the 227fields they care about. 228 229 2304. Format configuration 231----------------------- 232 233Frame buffer devices offer two ways to configure the frame buffer format: the 234legacy API and the FOURCC-based API. 235 236 237The legacy API has been the only frame buffer format configuration API for a 238long time and is thus widely used by application. It is the recommended API 239for applications when using RGB and grayscale formats, as well as legacy 240non-standard formats. 241 242To select a format, applications set the fb_var_screeninfo bits_per_pixel field 243to the desired frame buffer depth. Values up to 8 will usually map to 244monochrome, 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 260struct 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 276Upon successful format configuration, drivers update the fb_fix_screeninfo 277type, visual and line_length fields depending on the selected format. 278 279 280The FOURCC-based API replaces format descriptions by four character codes 281(FOURCC). FOURCCs are abstract identifiers that uniquely define a format 282without explicitly describing it. This is the only API that supports YUV 283formats. Drivers are also encouraged to implement the FOURCC-based API for RGB 284and grayscale formats. 285 286Drivers that support the FOURCC-based API report this capability by setting 287the FB_CAP_FOURCC bit in the fb_fix_screeninfo capabilities field. 288 289FOURCC definitions are located in the linux/videodev2.h header. However, and 290despite starting with the V4L2_PIX_FMT_prefix, they are not restricted to V4L2 291and don't require usage of the V4L2 subsystem. FOURCC documentation is 292available in Documentation/DocBook/v4l/pixfmt.xml. 293 294To select a format, applications set the grayscale field to the desired FOURCC. 295For YUV formats, they should also select the appropriate colorspace by setting 296the colorspace field to one of the colorspaces listed in linux/videodev2.h and 297documented in Documentation/DocBook/v4l/colorspaces.xml. 298 299The red, green, blue and transp fields are not used with the FOURCC-based API. 300For forward compatibility reasons applications must zero those fields, and 301drivers must ignore them. Values other than 0 may get a meaning in future 302extensions. 303 304Upon successful format configuration, drivers update the fb_fix_screeninfo 305type, visual and line_length fields depending on the selected format. The type 306and visual fields are set to FB_TYPE_FOURCC and FB_VISUAL_FOURCC respectively. 307